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Chemical inhomogeneities of flux grown rare-earth aluminate RAlO3 (R=La, Gd, Tb, Dy, Ho and Er) crystals
Journal of Crystal Growth 219 (2000) 40}55
Chemical inhomogeneities of #ux grown rare-earth aluminate RAlO (R"La, Gd, Tb, Dy, Ho and Er) crystals A.K. Razdan , K.K. Bamzai , V. Hangloo , P.N. Kotru *, B.M. Wanklyn Department of Physics, University of Jammu, Jammu, India Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK Received 25 November 1999; accepted 8 June 2000 Communicated by K. Nakajima
Abstract Results of scanning electron microscopy and energy dispersive X-ray analysis of the microstructures on the surfaces of #ux-grown crystals of RAlO (R"La, Gd, Tb, Dy, Ho and Er) are reported. The results of the microanalytical character isation of these crystals grown from PbO}PbF #ux (with and without additives) suggest their chemical imperfections due to crystallisation or precipitation of secondary phases in the growth of the major phase of RAlO . The secondary phases include PbAl O and Pb OF . Formation of other impurity phases mainly ROF, platinum free from or associated with #ux components (i.e., Pb OF ) and a secondary phase (i.e., PbAl O ) in the growth of major phase RAlO are evidenced and the deposition of the former on the growing crystals of the latter is explained to be taking place during eutectic solidi"cation. The nucleation mechanism proposed by Wanklyn and Watts (Mater. Res. Bull. 19 (1984) 711) applies generally to the #ux growth of RAlO crystals. The crystallisation of minor phases in the growth of a major phase (viz., RAlO ) and formation of other impurity phases in the multicomponent system and the containing crucible involved in #ux growth technique, leading to inhomogeneity of crystal surfaces, are discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.10.D; 81.10; 61.50.C Keywords: Rare-earth aluminate; Flux growth; Chemical inhomogeneities; EDAX
1. Introduction The main objective of growing crystals in the laboratory is to achieve high-quality materials for device applications and/or scienti"c investigations. After achieving growth of crystals, it is necessary to characterize them for the assessment of their physical and chemical perfection. Fluxed melt * Corresponding author. Tel.:/fax: #91-191-453079. E-mail address: [email protected] (P.N. Kotru).
technique involves the use of several chemicals in the starting composition, containing crucible and a furnace set-up for attainment of high temperatures. The technique has the advantage of producing facetted crystals which are quite suitable for microstructural investigations. Optical and scanning electron microscopy supplemented by energy dispersive X-ray analysis (EDAX) forms a very useful integrated method of investigations into the defect structures in crystals. Extensive use of this combination of techniques have led this laboratory
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 4 9 - 2
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
to provide concrete evidence of secondary crystallization of lead compounds during the growth of major phase of RFeO [1}3], RCrO [4}8] and RBO [9] crystals. It is also reported that platinum too gets precipitated and deposited as impurity in #ux-grown rare-earth orthoferrite crystals [10]. Precipitation and deposition of rare-earth oxy#uoride, rare-earth borates and lead-rich compounds leading to inhomogeneities on the crystal surfaces of rare-earth orthoferrites and chromites have also been reported [1}8]. Crystals of rare-earth aluminates are interesting materials on account of their optical and magnetic properties [11}13]. The aluminates (R"Gd}Dy) undergo low-temperature transition [11,12]. Their perovskite structure deviates very slightly from that of the cubic perovskite. TbAlO has a monoclini cally distorted perovskite unit cell. For GdAlO the angle of monoclinic unit cell is 903}27 , and thus twinning takes place very readily during the growth. Flux method used by some workers [14}16] to grow some RAlO crystals yielded mainly twinned crystals and only small single crystals. Wanklyn reported the growth of RAlO (R"Gd}Dy) crystals from a PbF }PbO #ux with B O as an additive [17] and of rare-earth alumi nates (R"Tb}Er) from the #ux containing B O and MoO as additive [18]. The addition of B O to PbO/PbF mixture is reported to have resulted in the improvement of crystal quality and size, and twinning greatly reduced. However, even these conditions of crystal growth are reported to have yielded crystals with microscopic twinned regions [19]. Gadolinium aluminate crystals are also reported to have been grown, using Czochralski technique. The crystals have been observed to be twinned, probably due to high temperatures involved and the crystals exhibiting low-temperature transition [20,21]. Pure melt technique is a single-chemical system and involves high temperatures slightly above the melting point of the material. As against this, #ux method of growth has the advantage of growing crystals at temperatures much below their melting point or transition temperature, but it is a multi-chemical system involving the use of several chemicals in the starting composition. This method of growth can be perfected by thoroughly investigating its yield, particularly RAlO crystals which
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pose problems as said above. Assessment of "nal products constitutes a very important part of any crystal growth programme. There exists a whole battery of physical and chemical techniques to help provide information on the quality of a crystal. There are two areas of interest. Firstly, the chemical composition of the crystal and secondly the crystal lattice structure and its perfection. In the former context application of scanning electron microscopy supplemented by EDAX techniques can play a signi"cant role. The objective behind this study is to investigate the defects (particularly chemical perfection) in #ux-grown rare-earth aluminate crystals as revealed by scanning electron microscopy used in combination with EDAX techniques and try to determine possible sources/causes which generate them. The results presented here demonstrate the e!ectiveness of this combination of techniques in making the assessment of chemical perfection of crystals. This paper reports microanalytical characterisation of RAlO (R"La, Gd, Tb, Dy, Ho and Er) crystals grown from PbO}PbF #ux (besides additives), yielding information concerning their chemical perfection. 2. Experimental procedure The single crystals of rare-earth aluminates, RAlO (R"La, Gd, Tb, Dy, Ho and Er) used in the present investigations were grown by the #ux technique using experimental procedure as reported in the literature [17,18]. Table 1 gives the details of the starting composition of the reactant, #ux component, temperature programming along with other relevant parameters. All the crystals were cleaned by dilute nitric acid. The crystal surfaces were examined by SEM (Cambridge Stereoscan S4-10) and EDAX spectrometer (KeVx) attached to the scanning unit. 3. Results A variety of structures are revealed by the surfaces of single crystals of the rare-earth aluminates under investigation. The microstructures, as probed by energy dispersive X-ray analytical (EDAX) techniques, evince the stoichiometric
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
Table 1 Starting composition, furnace program and results of EDAX analysis of RAlO crystals Sl. no
RAlO
Starting composition Solute
Solvent 5 g PbF 5 g PbO
Crucible volume (cm)
Soak temp. (3C) and time (h)
Rate of Separation cooling (K h\) from #ux
Identi"cation of secondary phases by EDAX
1260 12
2.5}9003C
By tapping with hammer
100
450, 2; then 950, 2; and then 1290,16
2}8003C
By tapping with hammer
PbAl O , Pb OF Pt, LaOF Pt
1
LaAlO
1 g La O 0.3 g Al O
2
GdAlO
72 g Gd O 22 g Al O
TbAlO (I Batch)
15.9 g Tb O 4 g Al O
45 g PbF , 140 g PbO 12 g PbO 2.3 g B O
100
1295, 14
1, for 96 h; 1.4, for 96 h; 2}9003C
By tapping with hammer
Pt, Pb OF , PbAl O19
(II-Batch)
15.9 g Tb O 4 g Al O
1295, 14
1, for 96 h; 1.2, for 72 h; 2}9003C
By tapping with hammer
Pt
DyAlO (I-Batch)
20 g Dy O 5 g Al O
1230, 12
1}8703C "nal cooling 3003C h\
Inverted crucible, i.e. hot pouring
PbAl O
(II-Batch)
36 g Dy O 9.8 g AI O
45 g PbF , 100 140 g PbO 129 g PbO , 2.3 g B O , 6.8 g MoO 30 g PbF , 100 180 g PbO 10 g PbO , 6g B O , 10 g MoO , 1.2 g V O 131 g PbF , 100 242 g PbO 2 g PbO
1}9003C
By tapping with hammer
Pt, DyOF, PbAl O
5
HoAIO
5.4 g Ho O 1.4 g Al O
50
1207, 144 then 1220, 144 then 1240, 96 1290, 15
3}9003C
By tapping with hammer
Pt, PbAl O
6
ErAIO
10.7 g Er O 2.7 g AI O
50
1290, 24
1 for 144 h; 1.1 for 72 h; 1.3}9003C
By tapping with hammer
Pt, ErOF
3
4
50
178 g PbF 140 g PbO 6 g PbO 6g B O
12 g PbF 40 g PbO 4 g PbO 0.6 g B O 2 g MoO 16 g PbF 91.2 g PbO 8 g PbO 1.6 g B O 4.8 g MoO
change in the elements of crystals and inhomogeneity of RAlO surfaces. The surfaces were examined "rstly by the energy spectra using electron as probe, followed by the recording of line scans of only those elements which exist in the respective spectra of RAlO crystals. The discussion of these line pro"les is given crystal-wise in the subsequent sections.
3.1. Lanthanum aluminate (LaAlO3 ) Fig. 1a is an electron micrograph showing some circular elevations hitherto called as discs of varying diameters, namely, the regions B}C, D}E and F}G. The elemental line pro"le along the route A}B}C}D}E}F}G}H of Fig. 1a as shown in
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
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Fig. 1b reveals the following: (i) The regions A}B and G}H show normal presence of the elements of RAlO as is expected of the crystal. The line pro"les due to LaL ? (A }B and G }H ) and Alk (A }B and ? G }H ) of Fig. 1b indicate the normal (stoichiometric) order of the surface. (ii) The line scan of PtM has an appreciable rise ? in the region B}C of Fig. 1a as is displayed by B }C of Fig. 1b. The pro"les of LaL (B }C ) ? and AlK (B }C ) have dropped down to their ? minima, thereby indicating their absence. The pro"le of PbM (B }C ) shows a little rise ? within the "rst half of the region B}C whereas a little fall in the concentration of Pt is observed as indicated by S of Fig. 1b. (iii) In the region C}D of Fig. 1a the line scan of LaL (C }D ) and AlK (C }D ) have been ? ? restored to their normal positions whereas PbM shows a little rise as displayed in the "rst ? half of the region (C }D ) of Fig. 1b. (iv) The pro"le of PtM has again a noticeable rise ? in the region D}E of Fig. 1a as shown by D }E in Fig. 1b. The other pro"le of LaL ? (D }E ), PbM (D }E ) and AlK (D }E ) ? ? have dropped down to their minima displaying their absence from the region D}E. This observation suggests that the disc D}E is composed of platinum only. (v) Along the region E}F of Fig. 1a, the line scan corresponding to PtM has increased as is re? vealed by E }F of Fig. 1b. The scan due to LaL (E }F ) is normal except for the central ? portion corresponding to E}F region, where PbM pro"le has risen to some extent, con? "rming its presence. The rise of line scan is marked as `S a. (vi) The line scan of LaL (F }G ) and AlK ? ? (F }G ) have dropped down to respective minima displaying their absence from the region F}G of Fig. 1a. In the "rst half of F}G the corresponding pro"le PtM (see region ? F }G ) and PbM (see region F }G ) corre ? sponding to the second half of the region F}G of Fig. 1a shows appreciable rise in Fig. 1b. The above observations suggest that the discs are mainly composed of Pt. The regions D}E and the
Fig. 1. (a) A scanning electron micrograph displaying some circular elevations on the surface of LaAlO crystal. A}B}C}D}E}F}G}H is the route of line scan recorded by EDAX spectrometer; (b) elemental line pro"le traced along the route A}H of (a) showing the distribution of La, Pt, Pb and Al. Notice appreciable rise of PtM pro"le along B }C and ? D }E and that of PbM pro"le along F }G ; these regions ? correspond to B}C, D}E and F}G of (a).
"rst half of F}G suggest precipitation of Pt alone, whereas Pb and Al are present in small concentrations along with Pt in the regions C}D and E}F. Besides the precipitation of Pt (B }C ), Pb is pres ent in small concentration in the central portion of disc B}C of Fig. 1a. The traces of Pb (C }D ) and Al (C }D ) are present in the region C}D, supporting the formula of PbAl O , but in small
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
concentration. The "rst half of F}G is composed of Pt whereas the second half is composed of Pb. 3.2. Gadolinium aluminate (GdAlO3 ) Fig. 2a is an SEM photograph showing a typical observation of an irregular growth on its surface.
The elemental line scanning done along the route A}B}K}C}D across this structure is shown in Fig. 2b. Critical analysis of the line scan reveals the following: (i) The general surface covering the regions A}B and C}D show that the relative concentration of Gd and Al is in stoichiometric order (see GdL and AlK lines corresponding to Gd and ? ? Al, respectively, as represented by A }B , C }D and A }B , C }D of Fig. 2b). (ii) The line pro"le corresponding to PtM dis? plays certain rise in the region B}K of Fig. 2a as displayed by the curve B }K of Fig. 2b. The pro"les corresponding to GdL (B }C ) ? and AlK (B }K ) have dropped down to their ? respective minima thereby revealing their absence from the region B}K of Fig. 2a. (iii) Along the region K}C, the line scan due to AlK (see K }C ) of Fig. 2b shows signi"cant ? rise, whereas the line scan of GdL drops ? down. It may be noted that the Pt scan is displayed at its base line suggesting its absence from the region. From the above observations, it is suggested that the deposition of Pt has taken place in the region B}K of Fig. 2a. The region K}C shows the presence of Al (it could be Al O ); moreover, the stoichiometry of GdAlO breaks down along the region K}C. 3.3. Terbium aluminate (TbAlO3 ) Fig. 3a is an SEM picture revealing an almost circular disc on TbAlO crystal surface. The el emental line scan taken along the route A}B}C}D is shown in Fig. 3b. The study of the pro"le reveals the following:
Fig. 2. (a) An SEM micrograph recorded on the crystal surface of GdAlO showing an irregular deposition. A}B}C}D is the route of line scan across the deposition; (b) scan route traced by EDAX spectrometer along the route A}D of (a) displaying the detection of Gd, Pt and Al. Along B }C , Pt has increased to a maximum whereas the pro"les corresponding to Gd and Al have fallen down to their minima.
(i) The line pro"les of TbL (A }B and C }D ) ? and AlK (A }B and C }D ) are normal ? along the regions A}B and C}D, respectively, the crystal surface being homogeneous within these regions. (ii) PtM (B }C ) shows signi"cant rise in the ? region (B}C) of Fig. 3a, whereas the line scans of TbL and AlK fall to the minima indicating ? ? their absence from the disc B}C. It may be
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
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Fig. 3. (a) An electron micrograph recorded on the surface of TbAlO showing a disc. A}B}C}D}E is the scan route; (b) elemental line pro"le traced by EDAX spectrometer illustrating detection of Tb, Pt, Pb and Al from A to E of (a). The elemental pro"le indicates the presence of platinum and lead.
Fig. 4. (a) An SEM micrograph recorded on another surface of TbAlO crystal showing a cavity "lled by a growth or depos ition; (b) line scan recorded along the path of A}B}C}D of (a). Notice increase of pro"le corresponding to Pt along B}C of (a).
noted that the peak PtM (B }C ) drops down ? a little at two places (T and S ) of Fig. 3b, whereas the line scans corresponding to PbM ? (B }C ) and TbL (B }C ) show tendency to ? increase. (iii) In the region D}E of Fig. 3a there is a little drop in the concentration of Tb as displayed by D }E of Fig. 3b. The pro"le of PbM indicate ? the presence of Pb in small concentration as evidenced by D }E of Fig. 3b. (iv) The pro"le corresponding to PbM shows ? a little rise in the centre of C}D of Fig. 3a as
indicated by K of Fig. 3b, while others corre sponding to TbL and AlK do not show devi? ? ation from the normal path. From the above observation, it is understood that the disc B}C is composed of Pt. Also, the lead compounds and some traces of #ux (Pb OF ) and TbOF or Tb O at the two places of disc B}C of Fig. 3a exist. Fig. 4a is an electron micrograph of TbAlO crystal surface which was grown by using #ux PBO}PbO with MoO and B O as the additive.
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
The elemental line scan carried along the route A}B}C}D reveals the following: (i) The regions A}B and C}D of Fig. 4a show the normal presence of the elements of TbAlO crystals. (ii) In the cavity region B}C, pro"le of PtM ? shows a signi"cant rise as indicated by B }C of Fig. 4b, while the scans corresponding to TbL and AlK fall to their minimum suggest? ? ing the absence of Tb and Al from the regions B}C (see regions B }C and B }C of Fig. 4b). The above observation supposes the deposition of Pt taking place within the cavity of TbAlO crystal. 3.4. Dysprosium aluminate (DyAlO3 ) DyAlO crystal surface shows an irregular de position as viewed by SEM and shown in Fig. 5a. There are small grains sticking to this kind of structure B}C. The elemental scan recording along the route A}B}C}D is shown in Fig. 5b. One is able to "nd the following: (i) Along the regions A}B and C}D of Fig. 5a, the pro"les due to AlK and DyL (marked as ? ? A }B , C }D , and A }B , C }D ) in Fig. 5b show their normal presence as expected of DyAlO . (ii) The line scan corresponding to Vk showing ? a little rise at the centre of irregular structure B}C of Fig. 5a (marked as K), is evinced by B }K }C of Fig. 5b. (iii) The line pro"le of PbM along the region B}C ? of Fig. 5a rises as is clear from B }C of Fig. 5b. Pb is present in this region in varying concentrations as is clear from the #uctuating pro"le corresponding to PbM ) AlK (B }C ) ? ? pro"le is normal in the region B}C of Fig. 5a which is clear from the fact that the pro"le does not indicate the #uctuation. It is signi"cant to note that the line scan corresponding to DyL ? has dropped down to its minimum, thereby suggesting the absence of dysprosium along the route B}C of Fig. 5a. The above observations suggest that most of the region of structure B}C may be composed of
Fig. 5. (a) A scanning electron micrograph recorded on a DyAlO surface showing an irregular deposition. A}B}C}D is the route along which line scan has been recorded; (b) line pro"les corresponding to Dy, V, Pb and Al are displayed revealing their distribution across B}C of (a).
PbAl O ; the region at almost the end C of B}C shows some traces of either Dy O or DyOF in addition to the formation of PbAl O . The grainy structure K on the regions B}C of Fig. 5a may be composed of some traces of V O com bined with PbAl O . From the table of growth conditions it may be noted that V O and MoO were added as additive, besides the #ux composition. In this case, platinum was not detected. DyAlO crystal growth from the #ux system PbO}PbO }PbF without any additive was studied to look into the secondary crystallisation/precipitation of a foreign material. Fig. 6a is an SEM micrograph recorded on the DyAlO surface
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
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(i) DyL (A }B , C }D and E }F ) and AlK ? ? (A }B , C }D and E }F ) display their nor mal presence in the regions of A}B, C}D and E}F of Fig. 6(a) as is expected of DyAlO and shown by Fig. 6(b). (ii) There is an appreciable rise in the pro"le corresponding to the PtM (B }C ) in the region ? B}C of Fig. 6b, but for the middle portion of region B}C where the pro"le due to Pt has dropped down to almost its minimum level. The line scans of DyL (B }C ) and AlK ? ? (B }C ) have dropped down to their respective minima suggesting the absence of Dy and Al from the structure B}C of Fig. 6a. There is a little fall in the concentration of Dy as is shown by the pro"le DyL in Fig. 6b, whereas ? the pro"les due to PtM (D }E ) and PbM ? ? (D }E ) have increased in the "rst half of re gion D}E and the second half of D}E, respectively, (see Fig. 6b). The AlK (D }E ) scan has ? gone down to the minimum in the "rst half of region D}E but the pro"le becomes almost normal in the second half of region D}E (see D }E pro"le of Fig. 6b).
Fig. 6. (a) A micrograph recorded on another DyAlO surface as viewed under SEM displays the growth of platinum within the two cavities (B}C and D}E). A}B}C}D}E}F is the track of line scan; (b) elemental line pro"le of Dy, Pt, Pb and Al showing their distribution along B}C and D}E of (a).
displaying the growth of platinum crystallites within the two rectangular cavities as detected by EDAX techniques. It is noticed that the precipitation and subsequent deposition of foreign materials has almost "lled up the rectangular cavities. The line A}B}C}D}E}F is the track along which the elemental scan was traced by EDAX spectrometer to detect the elements along the cavities of Fig. 6a. The critical analysis of Figs. 6a and b lead us to the following conclusions:
Through the above observations, it is clear that the precipitation and the subsequent deposition of Pt has taken place in the region B}C (see the two halves of region B }C ) and in the "rst half of the region D}E of Fig. 6a (see pro"le D }E Fig. 6b). Also Pb is combined with Al which is suggested to be the crystallisation of PbAl O in the middle portion B}C and the second half D}E of the structure. It may be noted that the phase PbAl O may contain a few traces of either Dy O or DyOF, as the pro"le corresponding to DyL ? (D }E ) has not dropped down to minimum along the line D}E of Fig. 6b. The "rst half of the D}E (Figs. 6a and b) shows Pt in smaller concentration whereas Al is absent along this region. The presence of Dy (ignoring a little fall of its pro"le) indicates the formation of DyOF or Dy O . 3.5. Holmium aluminate (HoAlO3 ) Fig. 7a is an SEM micrograph illustrating peculiar type of grainy structure on HoAlO crys tal surface. The stoichiometric behaviour of this
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
Fig. 7. (a) An electron micrograph recorded on the crystal surface of HoAlO evincing a grainy type structure. A}B}C}D}E}F is the scan route; (b) line scans of various elements, e.g., Ho, Pt, Pb and Al are traced across the route A to F of (a) showing the distribution of these elements. The regions B }C and E }F display an appreciable rise of pro"le corre sponding to Pt.
structure as investigated by EDAX techniques along the route A}B}C}D}E}F is shown in Fig. 7b. The study reveals the following: (i) The line scans of HoL (A }B ) and AlK ? ? (A }B ) show normal presence of the corre
sponding elements in the region A}B of Fig. 7a as is expected from HoAlO surface. However, the scan due to PtM (A }B ) is positioned ? a little above its minimum level, indicating the presence of Pt as traces. (ii) AlK scan is normal throughout the path ? A}B}C}D}E}F of Fig. 7a as illustrated in A }B }C }D }E }F of Fig. 7b. (iii) In the "rst half of the region B}C of Fig. 7a, PbM pro"le (B }C ) has risen, whereas the ? trace corresponding to HoL (B }C ) has ? dropped to some extent thereby suggesting its presence in traces (see Fig. 7b). In the second half of the region B}C, only PtM pro"le ? (B }C ) has displayed its appreciable rise whereas the other line scans have dropped down to their minima. (iv) Along C}D, the pro"le AlK (C }D ) alone is ? normal whereas the other line scans are at the minimum levels (Fig. 7b), thereby suggesting the presence of only Al in the region. (v) In the "rst half of the region D}E (Fig. 7a), the pro"le due to HoL (D }E ) and AlK ? ? (D }E ) indicate the formation of the major face HoAlO . However, in the second half of the region D}E, besides the normal behaviour of HoL (D }E ) and AlK (D }E ) there is ? ? a slight rise in the line scan of PtM as shown ? by the trace D }E of Fig. 7b. (vi) The line pro"le of PtM (E }F ) shows re? markable rise in the region E}F of Fig. 7a whereas the pro"le of HoL has decreased to ? its minimum. (vii) The pro"le due to PbM has risen a little as ? shown by E }F of Fig. 7b. It may be noted that the trace suggests the presence of Pb in the region E}F of Fig. 7a in varying concentration. From the above analysis, it is clear that the precipitation of PbAl O occurs along with the traces of HoOF or Ho O in the "rst half of the region B}C of Fig. 7a. Deposition of Pt has occurred in the second half of the region B}C. The "rst half of the region D}E (Fig. 7a) is composed of HoAlO (expected normal surface), whereas the traces of Pt are present in addition to Ho and Al. It may be noted that Pt has entered into the structure of HoAlO , thereby making it inhomogeneous.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
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The precipitation and subsequent deposition of Pt has taken place in the region E}F of Fig. 7a. The combination of Pb and Al which results in the formation of PbAl O in small concentrations is also noticed. It is suggested that Pt traces have mixed up with PbAl O thereby rendering the minor phase inhomogeneous. 3.6. Erbium aluminate (ErAlO3 ) There is a peculiar structure over the surface of ErAlO crystal as viewed by SEM and shown in Fig. 8a. The elemental line scan recorded along the track A}B}C}D of Fig. 8a is shown in Fig. 8b. The analysis of this pro"le reveals the following: (i) The line pro"les of ErL (A }B and C }D ) ? and AlK (A }B and C }D ) are normal, ? thereby suggesting that the regions A}B and C}D of Fig. 8a have a composition expected of the major phase of ErAlO surface. (ii) In the region B}C of Fig. 8a, the elemental line pro"le corresponding to PtM has risen re? markably as illustrated by B }C in Fig. 8b. The other pro"les due to ErL (B }C ) and ? AlK (B }C ) have dropped down to their ? respective minima, indicating thereby the absence of these elements (Er and Al) from the elevated structure B}C. (iii) From the study of AlK pro"le in the region ? A}B (Fig. 8a), one notices its fall in the middle part of the region A}B as illustrated by A }K }B of Fig. 8b. From the above results, it follows that only Pt is deposited in the region B}C. Further, the concentration of Al falls down to some extent in the region A}B, suggesting the formation of either ErOF or Er O .
Fig. 8. (a) An electron micrograph recorded on an ErAlO crystal surface evincing a peculiar structure grown over the surface; (b) scan routes of Er, Al and Pt across the track A}B}C}D of (a) reveal the #uctuations of elements. Pt has remarkably risen up above the normal scan route of elements of Er and Al.
4. Discussions Microanalytical studies carried out on the virgin surfaces of RAlO (R"La, Gd}Er) crystals reveal microstructures and microelevations whose surface chemical composition are not the same as expected of RAlO surfaces. The elemental line pro"les sug gest the presence of Pb, Pt and excess of rare-earth
elements `Ra on the surfaces of #ux grown RAlO crystals. The following o!ers a discussion on crystallisation or precipitation and subsequent deposition of secondary phases or minor phases in the crystal growth of the major phase RAlO .
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4.1. Precipitation of Pb-compounds 4.1.1. PbAl12 O19 The detection of energy peaks due to Pb suggests the precipitation of lead, but this possibility is ruled out since metallic lead (Pb) dissolves in HNO . Since the crystals have been cleaned by HNO , the microstructures due to lead compounds are present on the surfaces and have not been washed o! during acid cleaning. However, Pb is readily available from the #ux, PbO}PbF , used for the growth of RAlO crystals. The lead Pb> ion replaces R> of RAlO giving rise to the formation of PbAl O . The secondary crystallization of PbAl O is no ticed along the region B}C of DyAlO crystals (see Figs. 5a and 6a). The traces of other compounds in association with PbAl O are also found, as for example along the region C}D of LaAlO surface, where some Pt-traces are noticed (Figs. 1a and b). The middle portion of the region C}D of Fig. 3a (as shown by K peak in Fig. 3a), which corresponds to TbAlO surface, indicates the precipitation of TbOF associated with PbAl O as traces. The region D}E of Fig. 3a suggests the formation of PbAl O associated with TbOF as traces. This is just the converse of what is seen in the region C}D of Fig. 3a. The TbAlO crystal surfaces have been rendered inhomogeneous with the replacement of Tb> ion by Pb>; also the crystallised phase of PbAl O is associated with the traces of TbOF. The analysis of the second half of the region D}E of DyAlO (Figs. 5b and 6b) suggests the crystallisa tion of PbAl O with the traces of DyOF. Some traces of vanadium (from the source V O as addi tive in the #ux) are noticed at somewhere in the middle of region B}C (identi"ed by K in Fig. 5a and K in Fig. 5b). It follows that the region is composed of vanadium oxide combined with PbAl O instead of expected precipitation of DyVO as a secondary phase. The "rst half of the region B}C due to HoAlO surface shows the traces of Pt and the crystallised phase of PbAl O (Figs. 7a and b) which is associated with HoOF. R O was used in excess for the pre cipitation of RAlO crystals but this excess of rare earth oxide actually favours the growth of ROF or rare-earth oxide [16]. One could argue that there
may be a formation of Tb O rather than TbOF. However, the formation of Tb O in place of TbOF is ruled out since the crystals were cleaned by dilute HNO which would have dissolved Tb O , if formed. The crystal surface of DyAlO and HoAlO were cleaned repeatedly by concen trated acid, examined and then their elemental line scans studied. The line scans corresponding to Dy and Ho recorded after the repeated concentrated acid cleaning remain the same, thus suggesting the formation of ROF (R"Dy and Ho) because Dy O and Ho O are soluble in concentrated acid and therefore, would have been washed o!. The secondary crystallisation of Pb CrO and PbFe O as lead compounds have been reported to be taking place in the growth of RCrO (R"Y, Yb, La, Gd) [4}8] and RFeO (R"Dy, Er, Ho) [1}3]. These reports substantiate the replacement of rare-earth ion R> by Pb> derived from the solvent components used in the growth of rareearth orthoferrites and orthochromites and as discussed above in rare-earth aluminates. The growth process used here is #ux growth by spontaneous nucleation. The precipitation of secondary phases including PbAl O among others and their entry as inclusions or causing inhomogeneities in the crystal of major phase is largely related to limited convective mixing. In a uniform liquid heated symmetrically from above, convection is ideally absent and mixing occurs only by di!usion. In #uxed melts, a limited amount of convective mixing may occur because of asymmetric heating, as provided in a mu%e furnace such as the one used here. Most of the imperfections in the #ux-grown crystals result from a very rapid growth under conditions where convective mixing is limited and di!usion is a major controlling factor. Striations parallel to crystal faces representing compositional changes either in the major components of the crystal or in amount of impurities incorporated are due to regular variations in the growth rate. Banding in crystals is correlated to furnace temperature variations [22]. Thus, one source of inhomogeneities in crystals is correlated with the unstable growth caused due to supersaturation surges (very high supersaturation leading to instability in growth) and limited convective mixing in spontaneous nucleation method.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
In a situation as the one used here (spontaneous nucleation method), the crucible is cooler at its base, the convection is limited and dissolution as well as crystal growth occurs mainly by di!usion. The remedy to this lies in increasing convection by using gradient transfer method with the crucible base in the hot zone and a seed provided in the cooler zone, or by using `pulling from the #uxa. Many di$culties arising in spontaneous nucleation method as the one used here can be avoided by these techniques such as (1) supersaturation is ideally kept lower than that required to cause nucleation on the platinum, and thus growth takes place on the seed; (2) #ux inclusions are reduced and the quality of the grown crystals is very good, as #ux inclusions get reduced; (3) a faster growth rate is permissible and (4) solution is aided and undissolved material in the hot zone will not act as nuclei. However, in order to achieve these results, more elaborate and expensive equipment, knowledge of the relevant part of the phase diagram and very careful control of growth conditions are needed. In any case, it is "rst very essential to "nd a #ux which, besides producing the required phase, is non-volatile and thus suitable for seeded growth. Seeded growth has complications which are required to be addressed to. However, growth of crystals of a few mm in size can usually be grown from a #ux which provides good nucleating conditions by spontaneous nucleation in crucibles cooler near the base, with a very slow rate of cooling over the transition from supersaturation to initial nucleation (which in practice is less than 13C h\). In a simple mu%e furnace the rate of cooling is the parameter which must be kept low in order to diminish chances of defect formations. Asymmetric heating helps in increasing convection which is obtained by asymmetric location of the crucible with respect to the elements or by an asymmetric arrangement of elements [23]. With more sophisticated equipment, stirring is achieved by the rotation of the crucible with reversal of direction of rotation. `Accelerated crucible rotation techniquea and use of specially shaped crucible with a conical base and localised cooling at the lowest point is reported to yield better results [24]. It is realised that this technique, which if applied for the growth of rare-earth aluminate crystals, will
51
bring down the chances of inhomogeneity. Requirement of superior quality crystals of large size necessitates the use of `pulling from the #uxa technique as explained above. 4.1.2. Pb2 OF2 The signi"cant rise of line scan corresponding to PbM and the fall of pro"les corresponding to RL ? ? and AlK along a particular region indicates the ? presence of either dissolved or incompletely dissolved #ux (Pb OF ). The second half of the region F}G of LaAlO surface (Figs. 1a and b) shows the deposition of Pb OF . In certain cases, some traces of LaOF, Al (Al O ) and Pt are associated with Pb OF phase, as for example, the middle portion of the region E}F of LaAlO surface (Figs. 1a and b). The deposition of Pb OF (incompletely or completely dissolved #ux) as one of the lead compounds on the surfaces of #ux grown crystals has also been reported in the case of DyFeO [1] and rare-earth orthchromites (YCrO , LaCrO , YbCrO and GdCrO ) [4}8]. The precipitation of lead compound is also reported in the growth of LaBO crystals grown from PbO/PbO #ux [9]. 4.2. Formation of rare-earth oxyyuorides (ROF) The precipitation of rare-earth elements (R"La, Tb, Dy, Ho and Er) as sole metallic components is ruled out, since they are highly reactive. In fact, it suggests the formation of rare-earth compounds, ROF or RBO . The precipitation of RBO is, however, ruled out since the crystals were cleaned in nitric acid and RBO would have been removed by this process. Maintenance of the same peaks in the EDAX spectra (same behaviour of line pro"les) even after repeated cleaning of crystals in HNO rules out the possibility of formation of RBO in these cases. The formation of DyBO for DyAlO crystal (Figs. 6a and b) has, already, been ruled out because B O was not used as an addi tive for the growth of this crystal. It may be noted that detection of elements of Fluorine, Boron and Oxygen was not possible by the EDAX technique used in the present investigation [25]. The above observations, therefore, suggest the formation of ROF. In certain cases, formation of ROF also takes place in combination with some
52
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other compounds. For instance, the formation of LaOF (Figs. 1a and b) as traces coupled with #ux components (Pb OF ), TbOF with traces of PbAl O (Figs. 3a and 4a), DyOF along with the traces of PbAl O (Figs. 5a and b) and traces of Pt (Figs. 6a and b), and HoOF as traces with PbAl O (Figs. 7a and b) for the crystals of LaAlO , TbAlO , DyAlO and HoAlO , respec tively. However, the middle portion of region A}B corresponding to ErAlO (Figs. 8a and b) indicate the formation of ErOF as a lone rare-earth compound. Now, an important question arises as what could be the causes leading to the formation of ROF. Probably, the answer lies in the use of excess R O . As already explained under Section 4.1 it is used to avoid the formation of garnet phase, R Al O , which is a complicated aspect of the #ux growth of rare-earth aluminates, RAlO . The melt containing excess of R O shifts the equilibrium in the direc tion of the aluminate phase. Excess of R O , on the other hand, favours the growth of ROF (rare-earth oxy#uoride) or R O initially and these may inter fere with the production of good-quality aluminates [16]. One could, therefore, conclude that the use of excess of R O is a necessary evil in the #ux growth of RAlO crystals. As already discussed above, besides the major phase RAlO , there exist the possibility of the pre cipitation of other minor phases like PbAl O and oxy#ourides of lead. While the use of PbO}PbF }B O in the growth of RAlO crystals has several advantages, there exist a possibility of impurity phases, besides the major one (RAlO ). Because of the lower vapour pressure and lower eutectic temperature of PbO}PbF mixtures, they are preferable in many cases to the use of PbF alone, and much less corrosive than PbO. Considering the free energy of formation of oxides and #uorides [26,27], it is much more di$cult for PbF to get reduced to the metal. Therefore, the solution to the problem of precipitation of PbAl O and oxy#ourides lies in maintaining optimum ratio of PbF /PbO. As, for example, a high PbF /PbO ratio sometimes results in the combination with the #ux. For instance, when RAlO is the intended phase, where R> is a rare-earth ion of small ionic radius such as Er> or Dy> there is a strong
tendency for ROF to form, the rare-earth ion concentration is thus decreased and R Al O is for med in place of RAlO [21]. One can avoid it by using a high PbO/PbF ratio and excess R O , but these measures have to be balanced against a deterioation in the nucleation conditions [17]. It is observed that when the content of PbO is increased beyond about 50%, solvent power decreases, and at the same time the nucleating conditions deterioate. Alternatively, excess solvent and a high initial PbF /PbO ratio can be used in conjunction with long soak period, so that evaporation and hydrolysis a!ect the necessary reduction in #uoride ion concentration before cooling commences. In fact, in all cases, where PbO/PbF is used, there is this complication of a changing composition. It is realised that the #uxes containing lead, more particularly PbO}PbF }B O need to be more critical ly studied so far as the growth of RAlO crystals are concerned. 4.3. Crystallisation of Pt RAlO crystals (R"La, Gd}Er) have been grown in Pt crucibles. The EDAX of microelevations on the crystal surfaces detect Pt in the crystalline composition of RAlO . It, therefore, becomes important to investigate if the containing crucible itself becomes a source of foreign matter during the crystallisation of rare-earth aluminate. The crystallisation of Pt alone is observed on the RAlO surfaces. The regions B}C and D}E and the "rst half of region F}G of LaAlO surfaces (Figs. 1a and b) and the region B}C of Figs. 2a and b of GdAlO show the presence of Pt. Pt is also detec ted along the region B}C of Figs. 3a and b corresponding to TbAlO crystals. The deposition of Pt is also observed along the region B}C of Fig. 6a (Fig. 6b), second half of region B}C of Fig. 7a (Fig. 7b) and B}C of Fig. 8a (Fig. 8b) for DyAlO , HoAlO and ErAlO crystals, respectively. Wanklyn and Watts [28] proposed that there are preferred nucleation sites on the Pt surfaces, and that they are being removed by dissolution of Pt by the #ux (when molten) or possibly by recrystallization of the platinum surface. This process is considered to take place at all temperatures at which the molten #ux is present. In the present
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
investigation, platinum has crystallised probably on the face of RAlO crystal surfaces prior to the solidi"cation of #ux for GdAlO (Figs. 2a and b), TbAlO (Figs. 4a and b) and ErAlO (Figs. 8a and b). There may be at relatively high temperatures (above 12603C), slight solution of platinum which is followed by its deposition as crystallite platelets sometimes in or on a growing crystal. In one of the experiments of Wanklyn [21] at 13303C for PbO}PbF , the solution and transport of platinum resulted in the deposition of a platinum sheet. Precipitation of Pt along with traces of other elements is evinced for RAlO surfaces. The middle portion of region B}C (S of Fig. 1b) reveals Pt with traces of Pb OF (#ux) and region E}F shows traces of both in the case of LaAlO . The region B}C belonging to TbAlO (Figs. 3a and b) display the presence of #ux (Pb OF ) at S and T (Fig. 3b) while Pt is present in traces. The region E}F of HoAlO (Figs. 7a and b) reveals the crystallization of PbAl O associated with the crystallization of Pt. Both the lead compounds and platinum have rendered the HoAlO surfaces inhomogeneous. It is suggested [28] that preferred nucleation sites on the Pt surface are dissolved by melt, and that, on cooling to the eutectic temperature, Pt reprecipitates to form new nucleation sites. These sites are then e!ective in subsequent #ux growth. The nature of precipitated Pt and the size of the particles depend on the rate of cooling and the viscosity of the melt, which together limit the di!usion of Pt atoms or ions towards the coolest regions on the crucible walls. The deposition of Pt along with Pb compounds takes place by uninterrupted swift cooling to the eutectic and is followed by the precipitation of dissolved Pt accompanying solidi"cation of the #ux on the RAlO surfaces. The present paper eluci dates the precipitation of Pt along with Pb OF #ux and PbAl O (secondary crystallisation) on the crystal surfaces. This process takes place as a consequence of the formation of new nucleation sites at the eutectic temperatures. It has been observed that some regions of the surfaces of crystals illustrate the absence of R> ions from RAlO surfaces, making them in homogeneous. The region K}C of Fig. 2a belonging to GdAlO is composed of either aluminum
53
oxide, Al O , or Al (see Fig. 2b). Also, Pt is present in traces along the regions A}B and D}E of HoAlO (Figs. 7a and b) besides the expected ele ments of HoAlO , it may be noted that Pt has not displaced elements of the crystal from the surface. The region C}D and central portion of region D}E of Fig. 7a are composed of either Al O or Al. The presence of Pt in traces without disturbing the stoichiometry of crystals is displayed by HoAlO crystal (refer to "rst half of region D}E of Fig. 7a). However, the observation of Figs. 2a and b for GdAlO (region K}C) and Figs. 7a and b for HoAlO (region C}D) show the absence of rare earth R> from these regions indicating thereby the disturbance in maintaining the stoichiometry of the crystal surface. These observations could be attributed to some sort of residual complexes of the solute (may be Al O ). The progressive decomposi tion of residual complexes of the solute have been suggested by Wanklyn and Watts [28]. There is a report on dissolution of Pt during the #ux growth of crystals [28]. The observation reported here on DyAlO crystals which were separ ated from #ux by hammer tapping (Figs. 6a and b) indicate the precipitation of Pt. However, the crystals of DyAlO which were separated from #ux by hot pouring technique (i.e., inverting crucible when #ux is molten) did not o!er any evidence in support of the deposition of Pt (Figs. 5a and b). From these observations, it may be suggested that Pt deposits below 9803C may be nearly at the eutectic temperature. The above conclusion is further substantiated by the analysis carried out on the surfaces of ErAlO crystals grown from the starting composition of 11.8 g Er O , 3 g Al O , 100 g PbO, 17.6 g PbF , 8.8 g PbO , 1.8 g B O and 5.3 g MoO . The crys tals were separated when the #ux was molten (hot pouring technique). EDAX analysis did not evince the deposition of Pt in this case also. As the microstructures and microelevations formed due to secondary crystallisation do not evince any modi"cation of growth layers so, it is suggested that their deposition may be taking place after the cessation of crystal growth of rare-earth aluminates. From the above discussion, it is clear that the crucible containing starting composition can be
54
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
a source of contamination in crystals. Although other materials (graphite, molybdenum and tungsten) have been used occasionally under reducing conditions, for routine #ux growth there is no alternative to Pt. Experience has indicated that pure grade Pt crucibles are more resistant to attack by #uxes [21]. As an extreme result of such an attack, LaAlO crystals grown from PbO}PbF #ux, in crucibles of 80% Pt and 20% Rh, were reported to contain upto 18% of noble metal [29]. The major factors determining the life time of Pt ware are the type of #ux used and the length of the time spent in high temperatures. Compatibility of #uxes with Pt has been discussed in some review articles [21]. PbO attacks Pt severely. However, PbF with PbO is much less corrosive. PbF is easily less reduced than either Bi O or PbO. At relatively high temp eratures (above 12603C) there may be slight solution of Pt, followed by its (and/or in association with some #ux components) deposition as crystalline platelets sometimes in or on a growing crystal. Certain degree of vapour transport of Pt is not ruled out, and the same may be deposited on cooler parts of furnace in the form of thin platelets. To avoid this problem and avoid Pt contamination in crystals it is necessary to maintain Pt under oxidising conditions, especially during the initial heating period. This can be achieved in several ways: (1) By providing a #ow of oxygen gas into the mu%e furnace. (2) By holding the temperature at 4003C for a few hours during the heating period, when PbO is a #ux component. This will ensure an oxidising atmosphere at higher temperatures, since PbO absorbs oxygen, forming Pb O at about 4003C, and excess oxygen is released on further heating. (3) PbO may be substituted for a few percent of PbO with the main objective of having oxygen released on heating. Use of 2}3 wt% PbO in all #ux mixtures containing PbO or PbF pre vents attack on Pt crucibles to a very great extent. As already discussed, crystals of rare-earth aluminates grown from PbO}PbF }B O #ux sys
tem are not completely free from Pt deposition, although the Pt contamination is controlled to a great extent in comparison with other #ux systems. To prevent contamination by Pt one has to ensure that the growth process is carried out under oxidising conditions, especially during the initial heating period. The observations reported here clearly show contamination by Pt, inspite of the growth conditions used in certain experiments as provided under (2) and (3). Experiments performed by providing a continuous #ow of oxygen gas into the mu%e furnace may ensure a better compatibility of #ux with Pt, nearly eliminating Pt depositions on crystals. Alternatively, one may use hot pouring technique for the separation of crystals from #ux to minimise the possibility of precipitated Pt getting deposited on the crystal surfaces, as already reported in the observations. In Table 1, a summary of the results on microanalytical characterisation along with the growth conditions is given. From what is discussed above, one arrives at the following conclusions.
5. Conclusions (1) The secondary crystallization in the #ux growth of RAlO crystals by spontaneous nu cleation method has caused the inhomogeneity of their surfaces. The precipitation of lead compounds (for instance, PbAl O and Pb OF ) include the secondary phases gener ated during the growth of RAlO crystals from PbO}PbF #ux. Pb> derived from the #ux replaces rare-earth ion R> of the crystals of RAlO (R"La, Tb, Dy and Ho) giving rise to the formation of PbAl O . The in homogeneities in the crystal of major phase are largely related to limited convective mixing in growth by spontaneous nucleation method. The situation could be largely improved by encouraging convection using stirring either by the rotation of the crucible or using `Accelerated crucible rotation techniquea. Alternatively, one could avoid the di$culty by using gradient transfer method or by establishing more expensive and complicated method of `pulling from the #uxa.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
(2) Some microareas of RAlO crystal (R"La, Tb}Er) surfaces are de"cient in aluminium. The remaining excess of rare-earth element is considered to have consumed itself in the formation of rare-earth oxy#uoride (ROF). In certain cases the absence of rare-earth R> is observed. The disturbance in maintaining the stoichiometry of the crystal surfaces could be attributed to some sort of residual complexes of solute, i.e. Al O or Al. It is suggested that the #uxes containing lead, more particularly PbO}PbF }B O as applied to the growth of RAlO crystals, need to be more critically investigated. (3) Platinum free from lead compounds or platinum associated with Pb OF (#ux compo nents) and PbAl O (secondary phase) are deposited during the eutectic solidi"cation. The observations suggest that the nucleation mechanism proposed by Wanklyn and Watts [28] applies generally to the #ux growth of single crystals of RAlO crystals. Experiments performed by providing a continuous #ow of oxygen gas into the mu%e furnace may ensure a better compatibility of #ux with Pt, creating the possibility for near elimination of Pt depositions on crystals. Alternatively, one may use hot pouring technique to reduce the possibility of Pt depositions on crystal surfaces. References [1] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, J. Mater. Sci. 21 (1986) 1609. [2] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, J. Mater. Sci. 21 (1986) 3625. [3] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, B.E. Watts, J. Mater. Sci. 22 (1987) 4484. [4] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, J. Mater. Sci. 24 (1989) 2401.
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[5] A.K.Razdan, Ph.D. Thesis, Jammu University, Jammu, (1989). [6] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, J. Phys. D. 23 (1990) 1676. [7] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, Proc. Indian Natn. Sci. Acad. A 57 (1991) 351. [8] A.K. Razdan, P.N. Kotru, B.M. Wanklyn, Mater. Sci. Eng. B 15 (1992) 199. [9] P.N. Kotru, Anima Jain, A.K. Razdan, B.M. Wanklyn, J. Mater. Sci. 24 (1989) 1413. [10] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, B.E. Watts, J. Mater. Sci. Lett. 8 (1989) 186. [11] J.D. Cashion, A.H. Cooke, J.F.B. Hawkes, M.J.M. Leask, T.L. Thorp, M.R. Wells, J. Appl. Phys. 39 (1968) 1360. [12] J.D. Cashion, A.H. Cooke, M.J.M. Leask, T.L. Thorp, M.R. Wells, J. Mater. Sci. 3 (1968) 402. [13] J. Sivardiere, S.Q. Ambrunaz, C. R. (Paris) B 273 (1971) 619. [14] J.P. Remeika, J. Am. Chem. Soc. 78 (1956) 4259. [15] R.C. Linares, J. Appl. Phys. B 3 (1962) 1747. [16] G. Garton, B.M. Wanklyn, J. Crystal Growth 1 (1967) 164. [17] B.M. Wanklyn, J. Crystal Growth 5 (1969) 323. [18] B.M. Wanklyn, D. Midgley, B.K. Tanner, J. Crystal Growth 29 (1975) 281. [19] K.K. Bamzai, P.R. Dhar, P.N. Kotru, B.M. Wanklyn, Mater. Chem. Phys. 62 (2000) 214. [20] R. Maselsky, W.E. Kramer, R.H. Hopkins, J. Crystal Growth 2 (1968) 209. [21] B.M. Wanklyn, in: B.R. Pamplin (Ed.), Crystal Growth, Vol. I, Pergamon Press, Oxford and New York, 1974, pp. 217}288. [22] A.B. Chase, W.R. Wilcox, J. Am. Ceram. Soc. 50 (1967) 332. [23] E.A.D. White, (1972), unpublished. [24] H.J. Sheel, E.O. Schulz-Dubois, J. Crystal Growth 8 (1971) 304. [25] T.D. Mckilney, K.F.J. Heidenrich, D.B. Wittry (Eds.), The Electron Microprobe, Wiley, New York, 1966. [26] A. Glassner, ANL-5750 US Govt. Printing O$ce, 1959. [27] Levin, Robbins, MC Murdie Phase Diagrams for Ceramists, Am. Ceram. Soc. Inc., 1964. [28] B.M. Wanklyn, B.E. Watts, Mater. Res. Bull. 19 (1984) 711. [29] Z.N. Zonn, V.A. Io!e, in: Sheftal (Ed.), Growth of Crystals Vol. 8, Consultant Bureau, New York, 1969, 63.
Chemical inhomogeneities of #ux grown rare-earth aluminate RAlO (R"La, Gd, Tb, Dy, Ho and Er) crystals A.K. Razdan , K.K. Bamzai , V. Hangloo , P.N. Kotru *, B.M. Wanklyn Department of Physics, University of Jammu, Jammu, India Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK Received 25 November 1999; accepted 8 June 2000 Communicated by K. Nakajima
Abstract Results of scanning electron microscopy and energy dispersive X-ray analysis of the microstructures on the surfaces of #ux-grown crystals of RAlO (R"La, Gd, Tb, Dy, Ho and Er) are reported. The results of the microanalytical character isation of these crystals grown from PbO}PbF #ux (with and without additives) suggest their chemical imperfections due to crystallisation or precipitation of secondary phases in the growth of the major phase of RAlO . The secondary phases include PbAl O and Pb OF . Formation of other impurity phases mainly ROF, platinum free from or associated with #ux components (i.e., Pb OF ) and a secondary phase (i.e., PbAl O ) in the growth of major phase RAlO are evidenced and the deposition of the former on the growing crystals of the latter is explained to be taking place during eutectic solidi"cation. The nucleation mechanism proposed by Wanklyn and Watts (Mater. Res. Bull. 19 (1984) 711) applies generally to the #ux growth of RAlO crystals. The crystallisation of minor phases in the growth of a major phase (viz., RAlO ) and formation of other impurity phases in the multicomponent system and the containing crucible involved in #ux growth technique, leading to inhomogeneity of crystal surfaces, are discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.10.D; 81.10; 61.50.C Keywords: Rare-earth aluminate; Flux growth; Chemical inhomogeneities; EDAX
1. Introduction The main objective of growing crystals in the laboratory is to achieve high-quality materials for device applications and/or scienti"c investigations. After achieving growth of crystals, it is necessary to characterize them for the assessment of their physical and chemical perfection. Fluxed melt * Corresponding author. Tel.:/fax: #91-191-453079. E-mail address: [email protected] (P.N. Kotru).
technique involves the use of several chemicals in the starting composition, containing crucible and a furnace set-up for attainment of high temperatures. The technique has the advantage of producing facetted crystals which are quite suitable for microstructural investigations. Optical and scanning electron microscopy supplemented by energy dispersive X-ray analysis (EDAX) forms a very useful integrated method of investigations into the defect structures in crystals. Extensive use of this combination of techniques have led this laboratory
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 5 4 9 - 2
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to provide concrete evidence of secondary crystallization of lead compounds during the growth of major phase of RFeO [1}3], RCrO [4}8] and RBO [9] crystals. It is also reported that platinum too gets precipitated and deposited as impurity in #ux-grown rare-earth orthoferrite crystals [10]. Precipitation and deposition of rare-earth oxy#uoride, rare-earth borates and lead-rich compounds leading to inhomogeneities on the crystal surfaces of rare-earth orthoferrites and chromites have also been reported [1}8]. Crystals of rare-earth aluminates are interesting materials on account of their optical and magnetic properties [11}13]. The aluminates (R"Gd}Dy) undergo low-temperature transition [11,12]. Their perovskite structure deviates very slightly from that of the cubic perovskite. TbAlO has a monoclini cally distorted perovskite unit cell. For GdAlO the angle of monoclinic unit cell is 903}27 , and thus twinning takes place very readily during the growth. Flux method used by some workers [14}16] to grow some RAlO crystals yielded mainly twinned crystals and only small single crystals. Wanklyn reported the growth of RAlO (R"Gd}Dy) crystals from a PbF }PbO #ux with B O as an additive [17] and of rare-earth alumi nates (R"Tb}Er) from the #ux containing B O and MoO as additive [18]. The addition of B O to PbO/PbF mixture is reported to have resulted in the improvement of crystal quality and size, and twinning greatly reduced. However, even these conditions of crystal growth are reported to have yielded crystals with microscopic twinned regions [19]. Gadolinium aluminate crystals are also reported to have been grown, using Czochralski technique. The crystals have been observed to be twinned, probably due to high temperatures involved and the crystals exhibiting low-temperature transition [20,21]. Pure melt technique is a single-chemical system and involves high temperatures slightly above the melting point of the material. As against this, #ux method of growth has the advantage of growing crystals at temperatures much below their melting point or transition temperature, but it is a multi-chemical system involving the use of several chemicals in the starting composition. This method of growth can be perfected by thoroughly investigating its yield, particularly RAlO crystals which
41
pose problems as said above. Assessment of "nal products constitutes a very important part of any crystal growth programme. There exists a whole battery of physical and chemical techniques to help provide information on the quality of a crystal. There are two areas of interest. Firstly, the chemical composition of the crystal and secondly the crystal lattice structure and its perfection. In the former context application of scanning electron microscopy supplemented by EDAX techniques can play a signi"cant role. The objective behind this study is to investigate the defects (particularly chemical perfection) in #ux-grown rare-earth aluminate crystals as revealed by scanning electron microscopy used in combination with EDAX techniques and try to determine possible sources/causes which generate them. The results presented here demonstrate the e!ectiveness of this combination of techniques in making the assessment of chemical perfection of crystals. This paper reports microanalytical characterisation of RAlO (R"La, Gd, Tb, Dy, Ho and Er) crystals grown from PbO}PbF #ux (besides additives), yielding information concerning their chemical perfection. 2. Experimental procedure The single crystals of rare-earth aluminates, RAlO (R"La, Gd, Tb, Dy, Ho and Er) used in the present investigations were grown by the #ux technique using experimental procedure as reported in the literature [17,18]. Table 1 gives the details of the starting composition of the reactant, #ux component, temperature programming along with other relevant parameters. All the crystals were cleaned by dilute nitric acid. The crystal surfaces were examined by SEM (Cambridge Stereoscan S4-10) and EDAX spectrometer (KeVx) attached to the scanning unit. 3. Results A variety of structures are revealed by the surfaces of single crystals of the rare-earth aluminates under investigation. The microstructures, as probed by energy dispersive X-ray analytical (EDAX) techniques, evince the stoichiometric
42
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
Table 1 Starting composition, furnace program and results of EDAX analysis of RAlO crystals Sl. no
RAlO
Starting composition Solute
Solvent 5 g PbF 5 g PbO
Crucible volume (cm)
Soak temp. (3C) and time (h)
Rate of Separation cooling (K h\) from #ux
Identi"cation of secondary phases by EDAX
1260 12
2.5}9003C
By tapping with hammer
100
450, 2; then 950, 2; and then 1290,16
2}8003C
By tapping with hammer
PbAl O , Pb OF Pt, LaOF Pt
1
LaAlO
1 g La O 0.3 g Al O
2
GdAlO
72 g Gd O 22 g Al O
TbAlO (I Batch)
15.9 g Tb O 4 g Al O
45 g PbF , 140 g PbO 12 g PbO 2.3 g B O
100
1295, 14
1, for 96 h; 1.4, for 96 h; 2}9003C
By tapping with hammer
Pt, Pb OF , PbAl O19
(II-Batch)
15.9 g Tb O 4 g Al O
1295, 14
1, for 96 h; 1.2, for 72 h; 2}9003C
By tapping with hammer
Pt
DyAlO (I-Batch)
20 g Dy O 5 g Al O
1230, 12
1}8703C "nal cooling 3003C h\
Inverted crucible, i.e. hot pouring
PbAl O
(II-Batch)
36 g Dy O 9.8 g AI O
45 g PbF , 100 140 g PbO 129 g PbO , 2.3 g B O , 6.8 g MoO 30 g PbF , 100 180 g PbO 10 g PbO , 6g B O , 10 g MoO , 1.2 g V O 131 g PbF , 100 242 g PbO 2 g PbO
1}9003C
By tapping with hammer
Pt, DyOF, PbAl O
5
HoAIO
5.4 g Ho O 1.4 g Al O
50
1207, 144 then 1220, 144 then 1240, 96 1290, 15
3}9003C
By tapping with hammer
Pt, PbAl O
6
ErAIO
10.7 g Er O 2.7 g AI O
50
1290, 24
1 for 144 h; 1.1 for 72 h; 1.3}9003C
By tapping with hammer
Pt, ErOF
3
4
50
178 g PbF 140 g PbO 6 g PbO 6g B O
12 g PbF 40 g PbO 4 g PbO 0.6 g B O 2 g MoO 16 g PbF 91.2 g PbO 8 g PbO 1.6 g B O 4.8 g MoO
change in the elements of crystals and inhomogeneity of RAlO surfaces. The surfaces were examined "rstly by the energy spectra using electron as probe, followed by the recording of line scans of only those elements which exist in the respective spectra of RAlO crystals. The discussion of these line pro"les is given crystal-wise in the subsequent sections.
3.1. Lanthanum aluminate (LaAlO3 ) Fig. 1a is an electron micrograph showing some circular elevations hitherto called as discs of varying diameters, namely, the regions B}C, D}E and F}G. The elemental line pro"le along the route A}B}C}D}E}F}G}H of Fig. 1a as shown in
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
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Fig. 1b reveals the following: (i) The regions A}B and G}H show normal presence of the elements of RAlO as is expected of the crystal. The line pro"les due to LaL ? (A }B and G }H ) and Alk (A }B and ? G }H ) of Fig. 1b indicate the normal (stoichiometric) order of the surface. (ii) The line scan of PtM has an appreciable rise ? in the region B}C of Fig. 1a as is displayed by B }C of Fig. 1b. The pro"les of LaL (B }C ) ? and AlK (B }C ) have dropped down to their ? minima, thereby indicating their absence. The pro"le of PbM (B }C ) shows a little rise ? within the "rst half of the region B}C whereas a little fall in the concentration of Pt is observed as indicated by S of Fig. 1b. (iii) In the region C}D of Fig. 1a the line scan of LaL (C }D ) and AlK (C }D ) have been ? ? restored to their normal positions whereas PbM shows a little rise as displayed in the "rst ? half of the region (C }D ) of Fig. 1b. (iv) The pro"le of PtM has again a noticeable rise ? in the region D}E of Fig. 1a as shown by D }E in Fig. 1b. The other pro"le of LaL ? (D }E ), PbM (D }E ) and AlK (D }E ) ? ? have dropped down to their minima displaying their absence from the region D}E. This observation suggests that the disc D}E is composed of platinum only. (v) Along the region E}F of Fig. 1a, the line scan corresponding to PtM has increased as is re? vealed by E }F of Fig. 1b. The scan due to LaL (E }F ) is normal except for the central ? portion corresponding to E}F region, where PbM pro"le has risen to some extent, con? "rming its presence. The rise of line scan is marked as `S a. (vi) The line scan of LaL (F }G ) and AlK ? ? (F }G ) have dropped down to respective minima displaying their absence from the region F}G of Fig. 1a. In the "rst half of F}G the corresponding pro"le PtM (see region ? F }G ) and PbM (see region F }G ) corre ? sponding to the second half of the region F}G of Fig. 1a shows appreciable rise in Fig. 1b. The above observations suggest that the discs are mainly composed of Pt. The regions D}E and the
Fig. 1. (a) A scanning electron micrograph displaying some circular elevations on the surface of LaAlO crystal. A}B}C}D}E}F}G}H is the route of line scan recorded by EDAX spectrometer; (b) elemental line pro"le traced along the route A}H of (a) showing the distribution of La, Pt, Pb and Al. Notice appreciable rise of PtM pro"le along B }C and ? D }E and that of PbM pro"le along F }G ; these regions ? correspond to B}C, D}E and F}G of (a).
"rst half of F}G suggest precipitation of Pt alone, whereas Pb and Al are present in small concentrations along with Pt in the regions C}D and E}F. Besides the precipitation of Pt (B }C ), Pb is pres ent in small concentration in the central portion of disc B}C of Fig. 1a. The traces of Pb (C }D ) and Al (C }D ) are present in the region C}D, supporting the formula of PbAl O , but in small
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
concentration. The "rst half of F}G is composed of Pt whereas the second half is composed of Pb. 3.2. Gadolinium aluminate (GdAlO3 ) Fig. 2a is an SEM photograph showing a typical observation of an irregular growth on its surface.
The elemental line scanning done along the route A}B}K}C}D across this structure is shown in Fig. 2b. Critical analysis of the line scan reveals the following: (i) The general surface covering the regions A}B and C}D show that the relative concentration of Gd and Al is in stoichiometric order (see GdL and AlK lines corresponding to Gd and ? ? Al, respectively, as represented by A }B , C }D and A }B , C }D of Fig. 2b). (ii) The line pro"le corresponding to PtM dis? plays certain rise in the region B}K of Fig. 2a as displayed by the curve B }K of Fig. 2b. The pro"les corresponding to GdL (B }C ) ? and AlK (B }K ) have dropped down to their ? respective minima thereby revealing their absence from the region B}K of Fig. 2a. (iii) Along the region K}C, the line scan due to AlK (see K }C ) of Fig. 2b shows signi"cant ? rise, whereas the line scan of GdL drops ? down. It may be noted that the Pt scan is displayed at its base line suggesting its absence from the region. From the above observations, it is suggested that the deposition of Pt has taken place in the region B}K of Fig. 2a. The region K}C shows the presence of Al (it could be Al O ); moreover, the stoichiometry of GdAlO breaks down along the region K}C. 3.3. Terbium aluminate (TbAlO3 ) Fig. 3a is an SEM picture revealing an almost circular disc on TbAlO crystal surface. The el emental line scan taken along the route A}B}C}D is shown in Fig. 3b. The study of the pro"le reveals the following:
Fig. 2. (a) An SEM micrograph recorded on the crystal surface of GdAlO showing an irregular deposition. A}B}C}D is the route of line scan across the deposition; (b) scan route traced by EDAX spectrometer along the route A}D of (a) displaying the detection of Gd, Pt and Al. Along B }C , Pt has increased to a maximum whereas the pro"les corresponding to Gd and Al have fallen down to their minima.
(i) The line pro"les of TbL (A }B and C }D ) ? and AlK (A }B and C }D ) are normal ? along the regions A}B and C}D, respectively, the crystal surface being homogeneous within these regions. (ii) PtM (B }C ) shows signi"cant rise in the ? region (B}C) of Fig. 3a, whereas the line scans of TbL and AlK fall to the minima indicating ? ? their absence from the disc B}C. It may be
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
45
Fig. 3. (a) An electron micrograph recorded on the surface of TbAlO showing a disc. A}B}C}D}E is the scan route; (b) elemental line pro"le traced by EDAX spectrometer illustrating detection of Tb, Pt, Pb and Al from A to E of (a). The elemental pro"le indicates the presence of platinum and lead.
Fig. 4. (a) An SEM micrograph recorded on another surface of TbAlO crystal showing a cavity "lled by a growth or depos ition; (b) line scan recorded along the path of A}B}C}D of (a). Notice increase of pro"le corresponding to Pt along B}C of (a).
noted that the peak PtM (B }C ) drops down ? a little at two places (T and S ) of Fig. 3b, whereas the line scans corresponding to PbM ? (B }C ) and TbL (B }C ) show tendency to ? increase. (iii) In the region D}E of Fig. 3a there is a little drop in the concentration of Tb as displayed by D }E of Fig. 3b. The pro"le of PbM indicate ? the presence of Pb in small concentration as evidenced by D }E of Fig. 3b. (iv) The pro"le corresponding to PbM shows ? a little rise in the centre of C}D of Fig. 3a as
indicated by K of Fig. 3b, while others corre sponding to TbL and AlK do not show devi? ? ation from the normal path. From the above observation, it is understood that the disc B}C is composed of Pt. Also, the lead compounds and some traces of #ux (Pb OF ) and TbOF or Tb O at the two places of disc B}C of Fig. 3a exist. Fig. 4a is an electron micrograph of TbAlO crystal surface which was grown by using #ux PBO}PbO with MoO and B O as the additive.
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
The elemental line scan carried along the route A}B}C}D reveals the following: (i) The regions A}B and C}D of Fig. 4a show the normal presence of the elements of TbAlO crystals. (ii) In the cavity region B}C, pro"le of PtM ? shows a signi"cant rise as indicated by B }C of Fig. 4b, while the scans corresponding to TbL and AlK fall to their minimum suggest? ? ing the absence of Tb and Al from the regions B}C (see regions B }C and B }C of Fig. 4b). The above observation supposes the deposition of Pt taking place within the cavity of TbAlO crystal. 3.4. Dysprosium aluminate (DyAlO3 ) DyAlO crystal surface shows an irregular de position as viewed by SEM and shown in Fig. 5a. There are small grains sticking to this kind of structure B}C. The elemental scan recording along the route A}B}C}D is shown in Fig. 5b. One is able to "nd the following: (i) Along the regions A}B and C}D of Fig. 5a, the pro"les due to AlK and DyL (marked as ? ? A }B , C }D , and A }B , C }D ) in Fig. 5b show their normal presence as expected of DyAlO . (ii) The line scan corresponding to Vk showing ? a little rise at the centre of irregular structure B}C of Fig. 5a (marked as K), is evinced by B }K }C of Fig. 5b. (iii) The line pro"le of PbM along the region B}C ? of Fig. 5a rises as is clear from B }C of Fig. 5b. Pb is present in this region in varying concentrations as is clear from the #uctuating pro"le corresponding to PbM ) AlK (B }C ) ? ? pro"le is normal in the region B}C of Fig. 5a which is clear from the fact that the pro"le does not indicate the #uctuation. It is signi"cant to note that the line scan corresponding to DyL ? has dropped down to its minimum, thereby suggesting the absence of dysprosium along the route B}C of Fig. 5a. The above observations suggest that most of the region of structure B}C may be composed of
Fig. 5. (a) A scanning electron micrograph recorded on a DyAlO surface showing an irregular deposition. A}B}C}D is the route along which line scan has been recorded; (b) line pro"les corresponding to Dy, V, Pb and Al are displayed revealing their distribution across B}C of (a).
PbAl O ; the region at almost the end C of B}C shows some traces of either Dy O or DyOF in addition to the formation of PbAl O . The grainy structure K on the regions B}C of Fig. 5a may be composed of some traces of V O com bined with PbAl O . From the table of growth conditions it may be noted that V O and MoO were added as additive, besides the #ux composition. In this case, platinum was not detected. DyAlO crystal growth from the #ux system PbO}PbO }PbF without any additive was studied to look into the secondary crystallisation/precipitation of a foreign material. Fig. 6a is an SEM micrograph recorded on the DyAlO surface
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
47
(i) DyL (A }B , C }D and E }F ) and AlK ? ? (A }B , C }D and E }F ) display their nor mal presence in the regions of A}B, C}D and E}F of Fig. 6(a) as is expected of DyAlO and shown by Fig. 6(b). (ii) There is an appreciable rise in the pro"le corresponding to the PtM (B }C ) in the region ? B}C of Fig. 6b, but for the middle portion of region B}C where the pro"le due to Pt has dropped down to almost its minimum level. The line scans of DyL (B }C ) and AlK ? ? (B }C ) have dropped down to their respective minima suggesting the absence of Dy and Al from the structure B}C of Fig. 6a. There is a little fall in the concentration of Dy as is shown by the pro"le DyL in Fig. 6b, whereas ? the pro"les due to PtM (D }E ) and PbM ? ? (D }E ) have increased in the "rst half of re gion D}E and the second half of D}E, respectively, (see Fig. 6b). The AlK (D }E ) scan has ? gone down to the minimum in the "rst half of region D}E but the pro"le becomes almost normal in the second half of region D}E (see D }E pro"le of Fig. 6b).
Fig. 6. (a) A micrograph recorded on another DyAlO surface as viewed under SEM displays the growth of platinum within the two cavities (B}C and D}E). A}B}C}D}E}F is the track of line scan; (b) elemental line pro"le of Dy, Pt, Pb and Al showing their distribution along B}C and D}E of (a).
displaying the growth of platinum crystallites within the two rectangular cavities as detected by EDAX techniques. It is noticed that the precipitation and subsequent deposition of foreign materials has almost "lled up the rectangular cavities. The line A}B}C}D}E}F is the track along which the elemental scan was traced by EDAX spectrometer to detect the elements along the cavities of Fig. 6a. The critical analysis of Figs. 6a and b lead us to the following conclusions:
Through the above observations, it is clear that the precipitation and the subsequent deposition of Pt has taken place in the region B}C (see the two halves of region B }C ) and in the "rst half of the region D}E of Fig. 6a (see pro"le D }E Fig. 6b). Also Pb is combined with Al which is suggested to be the crystallisation of PbAl O in the middle portion B}C and the second half D}E of the structure. It may be noted that the phase PbAl O may contain a few traces of either Dy O or DyOF, as the pro"le corresponding to DyL ? (D }E ) has not dropped down to minimum along the line D}E of Fig. 6b. The "rst half of the D}E (Figs. 6a and b) shows Pt in smaller concentration whereas Al is absent along this region. The presence of Dy (ignoring a little fall of its pro"le) indicates the formation of DyOF or Dy O . 3.5. Holmium aluminate (HoAlO3 ) Fig. 7a is an SEM micrograph illustrating peculiar type of grainy structure on HoAlO crys tal surface. The stoichiometric behaviour of this
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
Fig. 7. (a) An electron micrograph recorded on the crystal surface of HoAlO evincing a grainy type structure. A}B}C}D}E}F is the scan route; (b) line scans of various elements, e.g., Ho, Pt, Pb and Al are traced across the route A to F of (a) showing the distribution of these elements. The regions B }C and E }F display an appreciable rise of pro"le corre sponding to Pt.
structure as investigated by EDAX techniques along the route A}B}C}D}E}F is shown in Fig. 7b. The study reveals the following: (i) The line scans of HoL (A }B ) and AlK ? ? (A }B ) show normal presence of the corre
sponding elements in the region A}B of Fig. 7a as is expected from HoAlO surface. However, the scan due to PtM (A }B ) is positioned ? a little above its minimum level, indicating the presence of Pt as traces. (ii) AlK scan is normal throughout the path ? A}B}C}D}E}F of Fig. 7a as illustrated in A }B }C }D }E }F of Fig. 7b. (iii) In the "rst half of the region B}C of Fig. 7a, PbM pro"le (B }C ) has risen, whereas the ? trace corresponding to HoL (B }C ) has ? dropped to some extent thereby suggesting its presence in traces (see Fig. 7b). In the second half of the region B}C, only PtM pro"le ? (B }C ) has displayed its appreciable rise whereas the other line scans have dropped down to their minima. (iv) Along C}D, the pro"le AlK (C }D ) alone is ? normal whereas the other line scans are at the minimum levels (Fig. 7b), thereby suggesting the presence of only Al in the region. (v) In the "rst half of the region D}E (Fig. 7a), the pro"le due to HoL (D }E ) and AlK ? ? (D }E ) indicate the formation of the major face HoAlO . However, in the second half of the region D}E, besides the normal behaviour of HoL (D }E ) and AlK (D }E ) there is ? ? a slight rise in the line scan of PtM as shown ? by the trace D }E of Fig. 7b. (vi) The line pro"le of PtM (E }F ) shows re? markable rise in the region E}F of Fig. 7a whereas the pro"le of HoL has decreased to ? its minimum. (vii) The pro"le due to PbM has risen a little as ? shown by E }F of Fig. 7b. It may be noted that the trace suggests the presence of Pb in the region E}F of Fig. 7a in varying concentration. From the above analysis, it is clear that the precipitation of PbAl O occurs along with the traces of HoOF or Ho O in the "rst half of the region B}C of Fig. 7a. Deposition of Pt has occurred in the second half of the region B}C. The "rst half of the region D}E (Fig. 7a) is composed of HoAlO (expected normal surface), whereas the traces of Pt are present in addition to Ho and Al. It may be noted that Pt has entered into the structure of HoAlO , thereby making it inhomogeneous.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
49
The precipitation and subsequent deposition of Pt has taken place in the region E}F of Fig. 7a. The combination of Pb and Al which results in the formation of PbAl O in small concentrations is also noticed. It is suggested that Pt traces have mixed up with PbAl O thereby rendering the minor phase inhomogeneous. 3.6. Erbium aluminate (ErAlO3 ) There is a peculiar structure over the surface of ErAlO crystal as viewed by SEM and shown in Fig. 8a. The elemental line scan recorded along the track A}B}C}D of Fig. 8a is shown in Fig. 8b. The analysis of this pro"le reveals the following: (i) The line pro"les of ErL (A }B and C }D ) ? and AlK (A }B and C }D ) are normal, ? thereby suggesting that the regions A}B and C}D of Fig. 8a have a composition expected of the major phase of ErAlO surface. (ii) In the region B}C of Fig. 8a, the elemental line pro"le corresponding to PtM has risen re? markably as illustrated by B }C in Fig. 8b. The other pro"les due to ErL (B }C ) and ? AlK (B }C ) have dropped down to their ? respective minima, indicating thereby the absence of these elements (Er and Al) from the elevated structure B}C. (iii) From the study of AlK pro"le in the region ? A}B (Fig. 8a), one notices its fall in the middle part of the region A}B as illustrated by A }K }B of Fig. 8b. From the above results, it follows that only Pt is deposited in the region B}C. Further, the concentration of Al falls down to some extent in the region A}B, suggesting the formation of either ErOF or Er O .
Fig. 8. (a) An electron micrograph recorded on an ErAlO crystal surface evincing a peculiar structure grown over the surface; (b) scan routes of Er, Al and Pt across the track A}B}C}D of (a) reveal the #uctuations of elements. Pt has remarkably risen up above the normal scan route of elements of Er and Al.
4. Discussions Microanalytical studies carried out on the virgin surfaces of RAlO (R"La, Gd}Er) crystals reveal microstructures and microelevations whose surface chemical composition are not the same as expected of RAlO surfaces. The elemental line pro"les sug gest the presence of Pb, Pt and excess of rare-earth
elements `Ra on the surfaces of #ux grown RAlO crystals. The following o!ers a discussion on crystallisation or precipitation and subsequent deposition of secondary phases or minor phases in the crystal growth of the major phase RAlO .
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A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
4.1. Precipitation of Pb-compounds 4.1.1. PbAl12 O19 The detection of energy peaks due to Pb suggests the precipitation of lead, but this possibility is ruled out since metallic lead (Pb) dissolves in HNO . Since the crystals have been cleaned by HNO , the microstructures due to lead compounds are present on the surfaces and have not been washed o! during acid cleaning. However, Pb is readily available from the #ux, PbO}PbF , used for the growth of RAlO crystals. The lead Pb> ion replaces R> of RAlO giving rise to the formation of PbAl O . The secondary crystallization of PbAl O is no ticed along the region B}C of DyAlO crystals (see Figs. 5a and 6a). The traces of other compounds in association with PbAl O are also found, as for example along the region C}D of LaAlO surface, where some Pt-traces are noticed (Figs. 1a and b). The middle portion of the region C}D of Fig. 3a (as shown by K peak in Fig. 3a), which corresponds to TbAlO surface, indicates the precipitation of TbOF associated with PbAl O as traces. The region D}E of Fig. 3a suggests the formation of PbAl O associated with TbOF as traces. This is just the converse of what is seen in the region C}D of Fig. 3a. The TbAlO crystal surfaces have been rendered inhomogeneous with the replacement of Tb> ion by Pb>; also the crystallised phase of PbAl O is associated with the traces of TbOF. The analysis of the second half of the region D}E of DyAlO (Figs. 5b and 6b) suggests the crystallisa tion of PbAl O with the traces of DyOF. Some traces of vanadium (from the source V O as addi tive in the #ux) are noticed at somewhere in the middle of region B}C (identi"ed by K in Fig. 5a and K in Fig. 5b). It follows that the region is composed of vanadium oxide combined with PbAl O instead of expected precipitation of DyVO as a secondary phase. The "rst half of the region B}C due to HoAlO surface shows the traces of Pt and the crystallised phase of PbAl O (Figs. 7a and b) which is associated with HoOF. R O was used in excess for the pre cipitation of RAlO crystals but this excess of rare earth oxide actually favours the growth of ROF or rare-earth oxide [16]. One could argue that there
may be a formation of Tb O rather than TbOF. However, the formation of Tb O in place of TbOF is ruled out since the crystals were cleaned by dilute HNO which would have dissolved Tb O , if formed. The crystal surface of DyAlO and HoAlO were cleaned repeatedly by concen trated acid, examined and then their elemental line scans studied. The line scans corresponding to Dy and Ho recorded after the repeated concentrated acid cleaning remain the same, thus suggesting the formation of ROF (R"Dy and Ho) because Dy O and Ho O are soluble in concentrated acid and therefore, would have been washed o!. The secondary crystallisation of Pb CrO and PbFe O as lead compounds have been reported to be taking place in the growth of RCrO (R"Y, Yb, La, Gd) [4}8] and RFeO (R"Dy, Er, Ho) [1}3]. These reports substantiate the replacement of rare-earth ion R> by Pb> derived from the solvent components used in the growth of rareearth orthoferrites and orthochromites and as discussed above in rare-earth aluminates. The growth process used here is #ux growth by spontaneous nucleation. The precipitation of secondary phases including PbAl O among others and their entry as inclusions or causing inhomogeneities in the crystal of major phase is largely related to limited convective mixing. In a uniform liquid heated symmetrically from above, convection is ideally absent and mixing occurs only by di!usion. In #uxed melts, a limited amount of convective mixing may occur because of asymmetric heating, as provided in a mu%e furnace such as the one used here. Most of the imperfections in the #ux-grown crystals result from a very rapid growth under conditions where convective mixing is limited and di!usion is a major controlling factor. Striations parallel to crystal faces representing compositional changes either in the major components of the crystal or in amount of impurities incorporated are due to regular variations in the growth rate. Banding in crystals is correlated to furnace temperature variations [22]. Thus, one source of inhomogeneities in crystals is correlated with the unstable growth caused due to supersaturation surges (very high supersaturation leading to instability in growth) and limited convective mixing in spontaneous nucleation method.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
In a situation as the one used here (spontaneous nucleation method), the crucible is cooler at its base, the convection is limited and dissolution as well as crystal growth occurs mainly by di!usion. The remedy to this lies in increasing convection by using gradient transfer method with the crucible base in the hot zone and a seed provided in the cooler zone, or by using `pulling from the #uxa. Many di$culties arising in spontaneous nucleation method as the one used here can be avoided by these techniques such as (1) supersaturation is ideally kept lower than that required to cause nucleation on the platinum, and thus growth takes place on the seed; (2) #ux inclusions are reduced and the quality of the grown crystals is very good, as #ux inclusions get reduced; (3) a faster growth rate is permissible and (4) solution is aided and undissolved material in the hot zone will not act as nuclei. However, in order to achieve these results, more elaborate and expensive equipment, knowledge of the relevant part of the phase diagram and very careful control of growth conditions are needed. In any case, it is "rst very essential to "nd a #ux which, besides producing the required phase, is non-volatile and thus suitable for seeded growth. Seeded growth has complications which are required to be addressed to. However, growth of crystals of a few mm in size can usually be grown from a #ux which provides good nucleating conditions by spontaneous nucleation in crucibles cooler near the base, with a very slow rate of cooling over the transition from supersaturation to initial nucleation (which in practice is less than 13C h\). In a simple mu%e furnace the rate of cooling is the parameter which must be kept low in order to diminish chances of defect formations. Asymmetric heating helps in increasing convection which is obtained by asymmetric location of the crucible with respect to the elements or by an asymmetric arrangement of elements [23]. With more sophisticated equipment, stirring is achieved by the rotation of the crucible with reversal of direction of rotation. `Accelerated crucible rotation techniquea and use of specially shaped crucible with a conical base and localised cooling at the lowest point is reported to yield better results [24]. It is realised that this technique, which if applied for the growth of rare-earth aluminate crystals, will
51
bring down the chances of inhomogeneity. Requirement of superior quality crystals of large size necessitates the use of `pulling from the #uxa technique as explained above. 4.1.2. Pb2 OF2 The signi"cant rise of line scan corresponding to PbM and the fall of pro"les corresponding to RL ? ? and AlK along a particular region indicates the ? presence of either dissolved or incompletely dissolved #ux (Pb OF ). The second half of the region F}G of LaAlO surface (Figs. 1a and b) shows the deposition of Pb OF . In certain cases, some traces of LaOF, Al (Al O ) and Pt are associated with Pb OF phase, as for example, the middle portion of the region E}F of LaAlO surface (Figs. 1a and b). The deposition of Pb OF (incompletely or completely dissolved #ux) as one of the lead compounds on the surfaces of #ux grown crystals has also been reported in the case of DyFeO [1] and rare-earth orthchromites (YCrO , LaCrO , YbCrO and GdCrO ) [4}8]. The precipitation of lead compound is also reported in the growth of LaBO crystals grown from PbO/PbO #ux [9]. 4.2. Formation of rare-earth oxyyuorides (ROF) The precipitation of rare-earth elements (R"La, Tb, Dy, Ho and Er) as sole metallic components is ruled out, since they are highly reactive. In fact, it suggests the formation of rare-earth compounds, ROF or RBO . The precipitation of RBO is, however, ruled out since the crystals were cleaned in nitric acid and RBO would have been removed by this process. Maintenance of the same peaks in the EDAX spectra (same behaviour of line pro"les) even after repeated cleaning of crystals in HNO rules out the possibility of formation of RBO in these cases. The formation of DyBO for DyAlO crystal (Figs. 6a and b) has, already, been ruled out because B O was not used as an addi tive for the growth of this crystal. It may be noted that detection of elements of Fluorine, Boron and Oxygen was not possible by the EDAX technique used in the present investigation [25]. The above observations, therefore, suggest the formation of ROF. In certain cases, formation of ROF also takes place in combination with some
52
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
other compounds. For instance, the formation of LaOF (Figs. 1a and b) as traces coupled with #ux components (Pb OF ), TbOF with traces of PbAl O (Figs. 3a and 4a), DyOF along with the traces of PbAl O (Figs. 5a and b) and traces of Pt (Figs. 6a and b), and HoOF as traces with PbAl O (Figs. 7a and b) for the crystals of LaAlO , TbAlO , DyAlO and HoAlO , respec tively. However, the middle portion of region A}B corresponding to ErAlO (Figs. 8a and b) indicate the formation of ErOF as a lone rare-earth compound. Now, an important question arises as what could be the causes leading to the formation of ROF. Probably, the answer lies in the use of excess R O . As already explained under Section 4.1 it is used to avoid the formation of garnet phase, R Al O , which is a complicated aspect of the #ux growth of rare-earth aluminates, RAlO . The melt containing excess of R O shifts the equilibrium in the direc tion of the aluminate phase. Excess of R O , on the other hand, favours the growth of ROF (rare-earth oxy#uoride) or R O initially and these may inter fere with the production of good-quality aluminates [16]. One could, therefore, conclude that the use of excess of R O is a necessary evil in the #ux growth of RAlO crystals. As already discussed above, besides the major phase RAlO , there exist the possibility of the pre cipitation of other minor phases like PbAl O and oxy#ourides of lead. While the use of PbO}PbF }B O in the growth of RAlO crystals has several advantages, there exist a possibility of impurity phases, besides the major one (RAlO ). Because of the lower vapour pressure and lower eutectic temperature of PbO}PbF mixtures, they are preferable in many cases to the use of PbF alone, and much less corrosive than PbO. Considering the free energy of formation of oxides and #uorides [26,27], it is much more di$cult for PbF to get reduced to the metal. Therefore, the solution to the problem of precipitation of PbAl O and oxy#ourides lies in maintaining optimum ratio of PbF /PbO. As, for example, a high PbF /PbO ratio sometimes results in the combination with the #ux. For instance, when RAlO is the intended phase, where R> is a rare-earth ion of small ionic radius such as Er> or Dy> there is a strong
tendency for ROF to form, the rare-earth ion concentration is thus decreased and R Al O is for med in place of RAlO [21]. One can avoid it by using a high PbO/PbF ratio and excess R O , but these measures have to be balanced against a deterioation in the nucleation conditions [17]. It is observed that when the content of PbO is increased beyond about 50%, solvent power decreases, and at the same time the nucleating conditions deterioate. Alternatively, excess solvent and a high initial PbF /PbO ratio can be used in conjunction with long soak period, so that evaporation and hydrolysis a!ect the necessary reduction in #uoride ion concentration before cooling commences. In fact, in all cases, where PbO/PbF is used, there is this complication of a changing composition. It is realised that the #uxes containing lead, more particularly PbO}PbF }B O need to be more critical ly studied so far as the growth of RAlO crystals are concerned. 4.3. Crystallisation of Pt RAlO crystals (R"La, Gd}Er) have been grown in Pt crucibles. The EDAX of microelevations on the crystal surfaces detect Pt in the crystalline composition of RAlO . It, therefore, becomes important to investigate if the containing crucible itself becomes a source of foreign matter during the crystallisation of rare-earth aluminate. The crystallisation of Pt alone is observed on the RAlO surfaces. The regions B}C and D}E and the "rst half of region F}G of LaAlO surfaces (Figs. 1a and b) and the region B}C of Figs. 2a and b of GdAlO show the presence of Pt. Pt is also detec ted along the region B}C of Figs. 3a and b corresponding to TbAlO crystals. The deposition of Pt is also observed along the region B}C of Fig. 6a (Fig. 6b), second half of region B}C of Fig. 7a (Fig. 7b) and B}C of Fig. 8a (Fig. 8b) for DyAlO , HoAlO and ErAlO crystals, respectively. Wanklyn and Watts [28] proposed that there are preferred nucleation sites on the Pt surfaces, and that they are being removed by dissolution of Pt by the #ux (when molten) or possibly by recrystallization of the platinum surface. This process is considered to take place at all temperatures at which the molten #ux is present. In the present
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
investigation, platinum has crystallised probably on the face of RAlO crystal surfaces prior to the solidi"cation of #ux for GdAlO (Figs. 2a and b), TbAlO (Figs. 4a and b) and ErAlO (Figs. 8a and b). There may be at relatively high temperatures (above 12603C), slight solution of platinum which is followed by its deposition as crystallite platelets sometimes in or on a growing crystal. In one of the experiments of Wanklyn [21] at 13303C for PbO}PbF , the solution and transport of platinum resulted in the deposition of a platinum sheet. Precipitation of Pt along with traces of other elements is evinced for RAlO surfaces. The middle portion of region B}C (S of Fig. 1b) reveals Pt with traces of Pb OF (#ux) and region E}F shows traces of both in the case of LaAlO . The region B}C belonging to TbAlO (Figs. 3a and b) display the presence of #ux (Pb OF ) at S and T (Fig. 3b) while Pt is present in traces. The region E}F of HoAlO (Figs. 7a and b) reveals the crystallization of PbAl O associated with the crystallization of Pt. Both the lead compounds and platinum have rendered the HoAlO surfaces inhomogeneous. It is suggested [28] that preferred nucleation sites on the Pt surface are dissolved by melt, and that, on cooling to the eutectic temperature, Pt reprecipitates to form new nucleation sites. These sites are then e!ective in subsequent #ux growth. The nature of precipitated Pt and the size of the particles depend on the rate of cooling and the viscosity of the melt, which together limit the di!usion of Pt atoms or ions towards the coolest regions on the crucible walls. The deposition of Pt along with Pb compounds takes place by uninterrupted swift cooling to the eutectic and is followed by the precipitation of dissolved Pt accompanying solidi"cation of the #ux on the RAlO surfaces. The present paper eluci dates the precipitation of Pt along with Pb OF #ux and PbAl O (secondary crystallisation) on the crystal surfaces. This process takes place as a consequence of the formation of new nucleation sites at the eutectic temperatures. It has been observed that some regions of the surfaces of crystals illustrate the absence of R> ions from RAlO surfaces, making them in homogeneous. The region K}C of Fig. 2a belonging to GdAlO is composed of either aluminum
53
oxide, Al O , or Al (see Fig. 2b). Also, Pt is present in traces along the regions A}B and D}E of HoAlO (Figs. 7a and b) besides the expected ele ments of HoAlO , it may be noted that Pt has not displaced elements of the crystal from the surface. The region C}D and central portion of region D}E of Fig. 7a are composed of either Al O or Al. The presence of Pt in traces without disturbing the stoichiometry of crystals is displayed by HoAlO crystal (refer to "rst half of region D}E of Fig. 7a). However, the observation of Figs. 2a and b for GdAlO (region K}C) and Figs. 7a and b for HoAlO (region C}D) show the absence of rare earth R> from these regions indicating thereby the disturbance in maintaining the stoichiometry of the crystal surface. These observations could be attributed to some sort of residual complexes of the solute (may be Al O ). The progressive decomposi tion of residual complexes of the solute have been suggested by Wanklyn and Watts [28]. There is a report on dissolution of Pt during the #ux growth of crystals [28]. The observation reported here on DyAlO crystals which were separ ated from #ux by hammer tapping (Figs. 6a and b) indicate the precipitation of Pt. However, the crystals of DyAlO which were separated from #ux by hot pouring technique (i.e., inverting crucible when #ux is molten) did not o!er any evidence in support of the deposition of Pt (Figs. 5a and b). From these observations, it may be suggested that Pt deposits below 9803C may be nearly at the eutectic temperature. The above conclusion is further substantiated by the analysis carried out on the surfaces of ErAlO crystals grown from the starting composition of 11.8 g Er O , 3 g Al O , 100 g PbO, 17.6 g PbF , 8.8 g PbO , 1.8 g B O and 5.3 g MoO . The crys tals were separated when the #ux was molten (hot pouring technique). EDAX analysis did not evince the deposition of Pt in this case also. As the microstructures and microelevations formed due to secondary crystallisation do not evince any modi"cation of growth layers so, it is suggested that their deposition may be taking place after the cessation of crystal growth of rare-earth aluminates. From the above discussion, it is clear that the crucible containing starting composition can be
54
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
a source of contamination in crystals. Although other materials (graphite, molybdenum and tungsten) have been used occasionally under reducing conditions, for routine #ux growth there is no alternative to Pt. Experience has indicated that pure grade Pt crucibles are more resistant to attack by #uxes [21]. As an extreme result of such an attack, LaAlO crystals grown from PbO}PbF #ux, in crucibles of 80% Pt and 20% Rh, were reported to contain upto 18% of noble metal [29]. The major factors determining the life time of Pt ware are the type of #ux used and the length of the time spent in high temperatures. Compatibility of #uxes with Pt has been discussed in some review articles [21]. PbO attacks Pt severely. However, PbF with PbO is much less corrosive. PbF is easily less reduced than either Bi O or PbO. At relatively high temp eratures (above 12603C) there may be slight solution of Pt, followed by its (and/or in association with some #ux components) deposition as crystalline platelets sometimes in or on a growing crystal. Certain degree of vapour transport of Pt is not ruled out, and the same may be deposited on cooler parts of furnace in the form of thin platelets. To avoid this problem and avoid Pt contamination in crystals it is necessary to maintain Pt under oxidising conditions, especially during the initial heating period. This can be achieved in several ways: (1) By providing a #ow of oxygen gas into the mu%e furnace. (2) By holding the temperature at 4003C for a few hours during the heating period, when PbO is a #ux component. This will ensure an oxidising atmosphere at higher temperatures, since PbO absorbs oxygen, forming Pb O at about 4003C, and excess oxygen is released on further heating. (3) PbO may be substituted for a few percent of PbO with the main objective of having oxygen released on heating. Use of 2}3 wt% PbO in all #ux mixtures containing PbO or PbF pre vents attack on Pt crucibles to a very great extent. As already discussed, crystals of rare-earth aluminates grown from PbO}PbF }B O #ux sys
tem are not completely free from Pt deposition, although the Pt contamination is controlled to a great extent in comparison with other #ux systems. To prevent contamination by Pt one has to ensure that the growth process is carried out under oxidising conditions, especially during the initial heating period. The observations reported here clearly show contamination by Pt, inspite of the growth conditions used in certain experiments as provided under (2) and (3). Experiments performed by providing a continuous #ow of oxygen gas into the mu%e furnace may ensure a better compatibility of #ux with Pt, nearly eliminating Pt depositions on crystals. Alternatively, one may use hot pouring technique for the separation of crystals from #ux to minimise the possibility of precipitated Pt getting deposited on the crystal surfaces, as already reported in the observations. In Table 1, a summary of the results on microanalytical characterisation along with the growth conditions is given. From what is discussed above, one arrives at the following conclusions.
5. Conclusions (1) The secondary crystallization in the #ux growth of RAlO crystals by spontaneous nu cleation method has caused the inhomogeneity of their surfaces. The precipitation of lead compounds (for instance, PbAl O and Pb OF ) include the secondary phases gener ated during the growth of RAlO crystals from PbO}PbF #ux. Pb> derived from the #ux replaces rare-earth ion R> of the crystals of RAlO (R"La, Tb, Dy and Ho) giving rise to the formation of PbAl O . The in homogeneities in the crystal of major phase are largely related to limited convective mixing in growth by spontaneous nucleation method. The situation could be largely improved by encouraging convection using stirring either by the rotation of the crucible or using `Accelerated crucible rotation techniquea. Alternatively, one could avoid the di$culty by using gradient transfer method or by establishing more expensive and complicated method of `pulling from the #uxa.
A.K. Razdan et al. / Journal of Crystal Growth 219 (2000) 40}55
(2) Some microareas of RAlO crystal (R"La, Tb}Er) surfaces are de"cient in aluminium. The remaining excess of rare-earth element is considered to have consumed itself in the formation of rare-earth oxy#uoride (ROF). In certain cases the absence of rare-earth R> is observed. The disturbance in maintaining the stoichiometry of the crystal surfaces could be attributed to some sort of residual complexes of solute, i.e. Al O or Al. It is suggested that the #uxes containing lead, more particularly PbO}PbF }B O as applied to the growth of RAlO crystals, need to be more critically investigated. (3) Platinum free from lead compounds or platinum associated with Pb OF (#ux compo nents) and PbAl O (secondary phase) are deposited during the eutectic solidi"cation. The observations suggest that the nucleation mechanism proposed by Wanklyn and Watts [28] applies generally to the #ux growth of single crystals of RAlO crystals. Experiments performed by providing a continuous #ow of oxygen gas into the mu%e furnace may ensure a better compatibility of #ux with Pt, creating the possibility for near elimination of Pt depositions on crystals. Alternatively, one may use hot pouring technique to reduce the possibility of Pt depositions on crystal surfaces. References [1] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, J. Mater. Sci. 21 (1986) 1609. [2] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, J. Mater. Sci. 21 (1986) 3625. [3] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, B.E. Watts, J. Mater. Sci. 22 (1987) 4484. [4] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, J. Mater. Sci. 24 (1989) 2401.
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[5] A.K.Razdan, Ph.D. Thesis, Jammu University, Jammu, (1989). [6] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, J. Phys. D. 23 (1990) 1676. [7] P.N. Kotru, A.K. Razdan, B.M. Wanklyn, Proc. Indian Natn. Sci. Acad. A 57 (1991) 351. [8] A.K. Razdan, P.N. Kotru, B.M. Wanklyn, Mater. Sci. Eng. B 15 (1992) 199. [9] P.N. Kotru, Anima Jain, A.K. Razdan, B.M. Wanklyn, J. Mater. Sci. 24 (1989) 1413. [10] P.N. Kotru, S.K. Kachroo, B.M. Wanklyn, B.E. Watts, J. Mater. Sci. Lett. 8 (1989) 186. [11] J.D. Cashion, A.H. Cooke, J.F.B. Hawkes, M.J.M. Leask, T.L. Thorp, M.R. Wells, J. Appl. Phys. 39 (1968) 1360. [12] J.D. Cashion, A.H. Cooke, M.J.M. Leask, T.L. Thorp, M.R. Wells, J. Mater. Sci. 3 (1968) 402. [13] J. Sivardiere, S.Q. Ambrunaz, C. R. (Paris) B 273 (1971) 619. [14] J.P. Remeika, J. Am. Chem. Soc. 78 (1956) 4259. [15] R.C. Linares, J. Appl. Phys. B 3 (1962) 1747. [16] G. Garton, B.M. Wanklyn, J. Crystal Growth 1 (1967) 164. [17] B.M. Wanklyn, J. Crystal Growth 5 (1969) 323. [18] B.M. Wanklyn, D. Midgley, B.K. Tanner, J. Crystal Growth 29 (1975) 281. [19] K.K. Bamzai, P.R. Dhar, P.N. Kotru, B.M. Wanklyn, Mater. Chem. Phys. 62 (2000) 214. [20] R. Maselsky, W.E. Kramer, R.H. Hopkins, J. Crystal Growth 2 (1968) 209. [21] B.M. Wanklyn, in: B.R. Pamplin (Ed.), Crystal Growth, Vol. I, Pergamon Press, Oxford and New York, 1974, pp. 217}288. [22] A.B. Chase, W.R. Wilcox, J. Am. Ceram. Soc. 50 (1967) 332. [23] E.A.D. White, (1972), unpublished. [24] H.J. Sheel, E.O. Schulz-Dubois, J. Crystal Growth 8 (1971) 304. [25] T.D. Mckilney, K.F.J. Heidenrich, D.B. Wittry (Eds.), The Electron Microprobe, Wiley, New York, 1966. [26] A. Glassner, ANL-5750 US Govt. Printing O$ce, 1959. [27] Levin, Robbins, MC Murdie Phase Diagrams for Ceramists, Am. Ceram. Soc. Inc., 1964. [28] B.M. Wanklyn, B.E. Watts, Mater. Res. Bull. 19 (1984) 711. [29] Z.N. Zonn, V.A. Io!e, in: Sheftal (Ed.), Growth of Crystals Vol. 8, Consultant Bureau, New York, 1969, 63.