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Electron diffraction study of In2Se3 melt grown crystals
Journal of Crystal Growth 96 (1989) 947—952 North-Holland, Amsterdam
947
ELECTRON DIFFRACTION STUDY OF In2Se3 MELT GROWN CRYSTALS C. DE BLASI, D. MANNO, G. MICOCCI and A. TEPORE Dipartimento di Scienza dei Materiali, Universith degli Studi, 1-73100 Lecce, Italy and Centro lnterunioersitario di Struttura della Materia (CISM), Universitd degli Studi, 1-73100 Lecce, Italy
Received 20 March 1989
A systematic investigation of the structure of In2Se3 crystals grown from the melt by the Bridgman—Stockbarger method has been performed by both selected area and convergent beam electron diffraction techniques. From the analysis of the diffraction patterns. two phases have been identified in all the examinated samples: one hexagonal phase with lattice parameters a 4.03 A and c = 19.1 A, and one rhombohedral phase which, referred to a hexagonal cell, has parameters a = 4.03 A and c = 28.9 A.
1. Introduction In2Se3 is a semiconducting compound of the A~’B~’family, the structures of which are defective [1] with respect to their metal atoms: only two thirds of the sites in the cation sublattice are occupied. The physical properties of this material are of considerable interest, and in recent years particular attention has been devoted to the study of the electrical and optical properties of In2Se3 with the idea of its possible applications in technology. In particular, this material is suitable for sensors of small particles and it could be used as detectors of ionizing radiation [2,3]. Moreover, In2Se3 has been shown to be a material with attractive characteristics for possible applications in electrochemical devices such as solid solution electrodes [41. The structure and phase transition of In2Se3 single crystals have been investigated by several authors by using various techniques [5—8].However, some discrepancies are evident among all published results. According to Julien et al. [9], In2Se3 exhibits at least three different crystalline modifications denoted a, fi and ~y. The more highly conductive a phase transforms to the intrinsically conductive f~ phase at a temperature of about 200 °C.This transformation is verified to be reversible. The $ phase transforms to the insulator
phase at a temperature of about 650°C. According to Kambas and Julien [10], the y phase can also be stabilized at room temperature under certain conditions of preparation. In this paper we present detailed studies of crystal structure of In2Se3 grown from the melt in our laboratories. The investigation has been carned out by means of electron diffraction. y
2. Exp~iimental The crystals investigated in this work have been grown from the melt by using the Bndgman— Stockbarger method. Details concerning the crystal growth are reported elsewhere [11]. Briefly, In2Se3 was synthesized from the elements in a silica ampoule following the Spandau and Klanberg procedure [12]. All the ampoules used were chemically and heat treated, charged, sealed off under vacuum and placed in a vertical two-zone tubular furnace. The ampoules move downward in a thermal gradient of 35°C/cm between the temperatures of 950 and 550°C, with a speed of 0.3 mm/h. The resulting ingots were polycrystalline with several large layered single crystal regions from which samples for analysis were cut. The composition of the samples was measured by
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
948
C. De Blasi ci a/.
/
Electron diffraction stuilv of 1n,Se~ melt grown crystals
Rutherford backscattering spectroscopy and was found, within the limits of experimental error, to he close to stoichiometry ([Se]/[Inj 1.47 ±0.05). An electrical analysis, carried Out at room temperature by means of the Van der Pauw method, with the current flowing along the layers, showed all the examinated crystals to be n-type. with a resistivity between 10~ and 10~2 ~Qcm and an electron Hall mobility between 100 and 300 cm2 V s1. To perform the structural analysis, the samples have been cleaved until transparency to the beam of a Philips electron microscope. operating at nominal 120 kV. The examination has been carried out by both selected area electron diffraction (SAD) and convergent beam electron diffraction (CBED) techniques. In the SAD technique, diffraction is obtained from a controlled area of the sample with diameter of about 5 ~.sm.The SAD technique has been particularly useful for observing regions where it was not possible to find zones without lattice
rings, the higher order Laue zone (HOLZ). coming from the intersection of the Ewald sphere with layers parallel to the zero order plane. The tables of Buxton et al. [14] have been used to correlate the symmetries of Tanaka and whole patterns to point and space group of the structure. To evaluate the lattice parameters of the structure, the disks of the zero layer and the HOLZ rings have been used. The radii R of the HOLZ rings are related to the spacings H of the reciprocal lattice plane perpendicular to the actual [uv.w] zone axis by the equation: H R/2AL~. L being the diffraction camera length. =
3. Results and discussion 3.1. SAD technique
defects.
CBED patterns of areas of about 40 nm in diameter have been recorded from crystal zones free of defects and of good quality in order to observe the crystal symmetries. In this way every beam is made of a superposition of electron plane waves with wave vectors distributed within a cone; the transmitted and diffracted beams give a diffraction pattern made of disks. The intensity ohserved in every point of the disks depends on the crystal symmetries, which can be deduced in a simple and direct way by analyzing the symmetry of both the transmitted disk and the whole diffraction pattern. The symmetry of the transmitted disk has been obtained according to the Tanaka technique [13]. In accordance with this technique, a 500 ~smaperture in the second condenser has given enlarging and overlapping of the disks in the diffraction pattern. The transmitted beam is selected by rising the sample and by allowing the transmitted disk to pass a 15 ~.tmaperture, inserted in the image plane of the objective lens. The whole pattern exhibits both disks of a plane perpendicular to the zone axis, the zero order Laue zone (ZOLZ), and concentric circular
The analysis of the patterns has been performed according to the procedure described elsewhere [15]. It has been checked that every spot in the diffraction pattern verified both the proper vector sums and the angular relations, in order to be sure that the pattern has been correctly interpreted. Fig. 1 shows typical electron micrographies of SAD patterns_as recorded from a In 2Se1 crystal. Fig. la is [11.31 zone axis pattern of a hexagonal phase, with lattice parameters a 4.03 A and c 18.9 A. The pattern in fig. lb belongs to a rhombohedral phase; by referring the structure to hexagonal axes, the pattern of the picture has [44.11zone axis with lattice parameters a 4.03 A and c 28.9 A. The presence of extended lattice defects in the observed regions gives rise to the splitting of the higher order spots, as it can be seen in both pictures. Similar results have been obtained in all observed samples; therefore SAD indicates that ln2Se~is made by a mixture of hexagonal and rhombohedral phases. By tilting the samples properly, diffraction patterns have been observed like that reported in fig. =
=
=
=
C. De Blasi et al.
/ Electron
diffraction study of In ,Se
949
3 melt grown crystals.
Is
Is’
Fig. 1. Typical electron micrographies of selected area diffraction patterns: (a) [11.3] zone axis of the hexagonal phase; (b) [44.1] zone axis of the rhombohedral phase referred to hexagonal axes. In both cases, only the spots necessary to assign the indices are labelled.
2, where a net of very strong reflections and a net of faint reflections are evident. The analysis of the pattern has shown the faint reflections belong to the hexagonal lattice and the strong ones have indexes fulfilling the condition:
as obtained from a good quality region of ln2Se3 crystal. An accurate inspection shows the common symmetry of both pictures is 3m. According to the tables of Buxton et a]., such symmetries can be attributed either to the 3m diffraction group, which
2h + k + j 3 which holds if the rhombohedral lattice is referred to hexagonal axes. So, we can conclude that both structures have been simultaneously observed,
corresponds to the 3m point group, or to the 6~mm~diffraction group, corresponding to the ~m point group. To resolve the question about the sign of the point group, according to Buxton et a!., we have to look for the presence of mirror reflection of the ±g vectors excited in Bragg condition. Fig. 4a reports the enlarged central part of the [00.1] zone axis CBED pattern in Bragg condition, while figs. 4b and 4c report, respectively, the (12.0) and (12.0) diffraction disks in Bragg condition. A mirror reflection between the two disks is evident, and we can conclude the point group is 3m and the structure is rhombohedric with space group R3m. The lattice parameters have been computed from fig. 3a according to the above described procedure, by using for g and H the relations of the hexagonal lattice: 2=4(h2+k2+hk)/3a2+12/c2, g H2 [3a2(u2 + v2 + uv) + c2w2] 1
3.2. CBED technique Fig. 3 reports a typical example of Tanaka (fig. 3a) and low camera length [00.1] (fig. 3b) patterns
=
Fig. 2. SAD pattern showing the [00.1]zone axis of the hexagonal phase (faint reflections) and of the rhombohedral lattice (strong reflections).
where h, k and I are the Miller indexes. In this way, we have obtained the lattice parameters a 4.03 A and c 28.6 A. We stress the fact that different zone axis CBED patterns of =
=
~)SI)
(
/1’ B/a
(1 ~I
I. Ui Iron ilI//rai lion ituili it In
‘i
inc/i 5rini 0
ii I
S
I ig
3. (a)
jUIL I
1 ,inaka and I
h con~entionat con~.ergen beam patterns recorded marked .ini]
I.
.i
rom a good qualii~ region ihret’fokl rol.ition ,iu~.is e~dent in hi tb pictures
\ mirror hue iii
o,
I
it.
Fig. 4. Ia) [DII.I] zone
:Ris diffraction pattern in Bragg condition. ftc indicated 12W and 12W reflections ore normal to the mirror line m of the ~ hole pattern: IB) and IC) sho~the (I 2.01 and (II.)) reflections in Bragg condi lions. respect iscls
,
C. Dc Blasi et al.
/ Electron
diffraction study of In Se
951
7 melt grown crystals.
Also, the CBED indicates In 2Se3 to be made of hexagonal and rhombohedral phases. The mixture of such structures is evident in fig. 6. This picture shows the strong [00.1] zone axis pattern of the rhombohedral lattice and the faint [001] pattern of the hexagonal phase.
4. Conclusions
Fig. 5. Whole CBED pattern of the hexagonal phase with [113] low symmetry zone axis,
the rhombohedral structure have been obtained by tilting the samples. In addition, CBED patterns have been observed with low symmetry zone axis belonging to a hexagonal structure. Such an example is reported in fig. 5, which shows the whole [11.3] hexagonal CBED pattern. The computation of the lattice parameters gives a 4.03 A and c 19.1 A. =
=
The structure of the compound In2Se5 has been investigated by several authors and some discrepancies are obvious among the published results. The confusion in the literature is not only on the structure, but also on its polymorphism because this compound undergoes several phase transitions. However, it is evident that the structure of the In2Se1 depends on the method of preparation. The crystals analyzed in this work at room temperature have been grown from the melt by the Bridgman—Stockbarger method and the structure has been found to be the a-phase. An accurate analysis of the crystals performed by both selected area and convergent beam electron diffraction techniques has evidenced the structure as .
prevailingly rhombohedral with equivalent hexagonal unit cell parameters a 4.03 A and c 28.9 A. These results are in fair agreement with the B-type samples analyzed by Popovic et al. [6], even if the sign of the space group is different. In addition, we have found a fraction of the volume in samples to crystallize in the hexagonal structure with crystallographic constants a 4.03 A and c 19.1 A. =
~
=
=
=
Acknowledgement This work was supported by the Ministry of Education of Italy (MPI).
Fig. 6. CBED low camera length pattern showing the strong [00.1] zone axis diffraction of the rhombohedral lattice and (indicated by arrows) the faint reflections of the zero layer and the first order Laue zone of the [00.11zone axis of the hexagonal structure.
References [1] P.C. Newman, J. Phys. Chem. Solids 23 (1961) 19. [2] V.M. Koshicin, L.P. Gal’chinetskii, V.M. Kulik and RI. Minkov, Solid State Commun. 13 (1973) 1.
LI LI
952
C. De Blasi ci a!.
/
Electron diffraction study of In Se
[3] V.M. Koshkin, L.P. Gal’chinetskii, V.M. Kulik. O.K. Gusev and V.A. Ulmanis, Soviet At. Energy 42 (1977) 321. [4] C. Julien, M. Eddrief, K. Kambas and M. Balkanski. Thin Solid Films 137 (1986) 27. [5] S.A. Semiletov, Soviet Phys.-Cryst. 5 (1961) 673. [6] S. Popovic, B. Celusika and D. Bidjin. Phys. Status Solidi (a) 6, 301 (1971). [7] J. Van Landuyt, 0. Van Tendeloo and S. Amelinckx. Phys. Status Solidi (a) 30 (1975) 299. [8] A. Likforman. P. Fourcray. M. Guittard, J. Flahaut, F. Poirier and N. Szydlo, J. Solid State Chem. 33 (1980) 91. 19] C. Julien, M. Eddrief, M. Balkanski, E. Hatzikranitis and K. Kambas, Phys. Status Solidi (a) 88 (1985) 687.
7 melt grown crystals
[10] K. Kambas and C. Julien. Mater. Res. Bull. 17 (1982) 1583. [11] C. Dc Blasi. A.V. Dngo. G. Micocci. A. Tepore and A.M. Mancini, J. Crystal Growth 94 (1989) 455. [12] N. Spandau and F. Klanberg, Z. Anorg. Allgem. Chem. 295 (1958) 300. [13] M. Tanaka, R. Saito. K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [141 B.F. Buxton, J.A. Eades. J.W. Steeds and G.M. Rackham. Phil. Trans. Roy. Soc. London A281 (1976) 171. [15] C. Dc Blasi, D. Manno, S. Mongelli and A. Rizzo. Nuovo Cimento D7 (1986) 795.
947
ELECTRON DIFFRACTION STUDY OF In2Se3 MELT GROWN CRYSTALS C. DE BLASI, D. MANNO, G. MICOCCI and A. TEPORE Dipartimento di Scienza dei Materiali, Universith degli Studi, 1-73100 Lecce, Italy and Centro lnterunioersitario di Struttura della Materia (CISM), Universitd degli Studi, 1-73100 Lecce, Italy
Received 20 March 1989
A systematic investigation of the structure of In2Se3 crystals grown from the melt by the Bridgman—Stockbarger method has been performed by both selected area and convergent beam electron diffraction techniques. From the analysis of the diffraction patterns. two phases have been identified in all the examinated samples: one hexagonal phase with lattice parameters a 4.03 A and c = 19.1 A, and one rhombohedral phase which, referred to a hexagonal cell, has parameters a = 4.03 A and c = 28.9 A.
1. Introduction In2Se3 is a semiconducting compound of the A~’B~’family, the structures of which are defective [1] with respect to their metal atoms: only two thirds of the sites in the cation sublattice are occupied. The physical properties of this material are of considerable interest, and in recent years particular attention has been devoted to the study of the electrical and optical properties of In2Se3 with the idea of its possible applications in technology. In particular, this material is suitable for sensors of small particles and it could be used as detectors of ionizing radiation [2,3]. Moreover, In2Se3 has been shown to be a material with attractive characteristics for possible applications in electrochemical devices such as solid solution electrodes [41. The structure and phase transition of In2Se3 single crystals have been investigated by several authors by using various techniques [5—8].However, some discrepancies are evident among all published results. According to Julien et al. [9], In2Se3 exhibits at least three different crystalline modifications denoted a, fi and ~y. The more highly conductive a phase transforms to the intrinsically conductive f~ phase at a temperature of about 200 °C.This transformation is verified to be reversible. The $ phase transforms to the insulator
phase at a temperature of about 650°C. According to Kambas and Julien [10], the y phase can also be stabilized at room temperature under certain conditions of preparation. In this paper we present detailed studies of crystal structure of In2Se3 grown from the melt in our laboratories. The investigation has been carned out by means of electron diffraction. y
2. Exp~iimental The crystals investigated in this work have been grown from the melt by using the Bndgman— Stockbarger method. Details concerning the crystal growth are reported elsewhere [11]. Briefly, In2Se3 was synthesized from the elements in a silica ampoule following the Spandau and Klanberg procedure [12]. All the ampoules used were chemically and heat treated, charged, sealed off under vacuum and placed in a vertical two-zone tubular furnace. The ampoules move downward in a thermal gradient of 35°C/cm between the temperatures of 950 and 550°C, with a speed of 0.3 mm/h. The resulting ingots were polycrystalline with several large layered single crystal regions from which samples for analysis were cut. The composition of the samples was measured by
0022-0248/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
948
C. De Blasi ci a/.
/
Electron diffraction stuilv of 1n,Se~ melt grown crystals
Rutherford backscattering spectroscopy and was found, within the limits of experimental error, to he close to stoichiometry ([Se]/[Inj 1.47 ±0.05). An electrical analysis, carried Out at room temperature by means of the Van der Pauw method, with the current flowing along the layers, showed all the examinated crystals to be n-type. with a resistivity between 10~ and 10~2 ~Qcm and an electron Hall mobility between 100 and 300 cm2 V s1. To perform the structural analysis, the samples have been cleaved until transparency to the beam of a Philips electron microscope. operating at nominal 120 kV. The examination has been carried out by both selected area electron diffraction (SAD) and convergent beam electron diffraction (CBED) techniques. In the SAD technique, diffraction is obtained from a controlled area of the sample with diameter of about 5 ~.sm.The SAD technique has been particularly useful for observing regions where it was not possible to find zones without lattice
rings, the higher order Laue zone (HOLZ). coming from the intersection of the Ewald sphere with layers parallel to the zero order plane. The tables of Buxton et al. [14] have been used to correlate the symmetries of Tanaka and whole patterns to point and space group of the structure. To evaluate the lattice parameters of the structure, the disks of the zero layer and the HOLZ rings have been used. The radii R of the HOLZ rings are related to the spacings H of the reciprocal lattice plane perpendicular to the actual [uv.w] zone axis by the equation: H R/2AL~. L being the diffraction camera length. =
3. Results and discussion 3.1. SAD technique
defects.
CBED patterns of areas of about 40 nm in diameter have been recorded from crystal zones free of defects and of good quality in order to observe the crystal symmetries. In this way every beam is made of a superposition of electron plane waves with wave vectors distributed within a cone; the transmitted and diffracted beams give a diffraction pattern made of disks. The intensity ohserved in every point of the disks depends on the crystal symmetries, which can be deduced in a simple and direct way by analyzing the symmetry of both the transmitted disk and the whole diffraction pattern. The symmetry of the transmitted disk has been obtained according to the Tanaka technique [13]. In accordance with this technique, a 500 ~smaperture in the second condenser has given enlarging and overlapping of the disks in the diffraction pattern. The transmitted beam is selected by rising the sample and by allowing the transmitted disk to pass a 15 ~.tmaperture, inserted in the image plane of the objective lens. The whole pattern exhibits both disks of a plane perpendicular to the zone axis, the zero order Laue zone (ZOLZ), and concentric circular
The analysis of the patterns has been performed according to the procedure described elsewhere [15]. It has been checked that every spot in the diffraction pattern verified both the proper vector sums and the angular relations, in order to be sure that the pattern has been correctly interpreted. Fig. 1 shows typical electron micrographies of SAD patterns_as recorded from a In 2Se1 crystal. Fig. la is [11.31 zone axis pattern of a hexagonal phase, with lattice parameters a 4.03 A and c 18.9 A. The pattern in fig. lb belongs to a rhombohedral phase; by referring the structure to hexagonal axes, the pattern of the picture has [44.11zone axis with lattice parameters a 4.03 A and c 28.9 A. The presence of extended lattice defects in the observed regions gives rise to the splitting of the higher order spots, as it can be seen in both pictures. Similar results have been obtained in all observed samples; therefore SAD indicates that ln2Se~is made by a mixture of hexagonal and rhombohedral phases. By tilting the samples properly, diffraction patterns have been observed like that reported in fig. =
=
=
=
C. De Blasi et al.
/ Electron
diffraction study of In ,Se
949
3 melt grown crystals.
Is
Is’
Fig. 1. Typical electron micrographies of selected area diffraction patterns: (a) [11.3] zone axis of the hexagonal phase; (b) [44.1] zone axis of the rhombohedral phase referred to hexagonal axes. In both cases, only the spots necessary to assign the indices are labelled.
2, where a net of very strong reflections and a net of faint reflections are evident. The analysis of the pattern has shown the faint reflections belong to the hexagonal lattice and the strong ones have indexes fulfilling the condition:
as obtained from a good quality region of ln2Se3 crystal. An accurate inspection shows the common symmetry of both pictures is 3m. According to the tables of Buxton et a]., such symmetries can be attributed either to the 3m diffraction group, which
2h + k + j 3 which holds if the rhombohedral lattice is referred to hexagonal axes. So, we can conclude that both structures have been simultaneously observed,
corresponds to the 3m point group, or to the 6~mm~diffraction group, corresponding to the ~m point group. To resolve the question about the sign of the point group, according to Buxton et a!., we have to look for the presence of mirror reflection of the ±g vectors excited in Bragg condition. Fig. 4a reports the enlarged central part of the [00.1] zone axis CBED pattern in Bragg condition, while figs. 4b and 4c report, respectively, the (12.0) and (12.0) diffraction disks in Bragg condition. A mirror reflection between the two disks is evident, and we can conclude the point group is 3m and the structure is rhombohedric with space group R3m. The lattice parameters have been computed from fig. 3a according to the above described procedure, by using for g and H the relations of the hexagonal lattice: 2=4(h2+k2+hk)/3a2+12/c2, g H2 [3a2(u2 + v2 + uv) + c2w2] 1
3.2. CBED technique Fig. 3 reports a typical example of Tanaka (fig. 3a) and low camera length [00.1] (fig. 3b) patterns
=
Fig. 2. SAD pattern showing the [00.1]zone axis of the hexagonal phase (faint reflections) and of the rhombohedral lattice (strong reflections).
where h, k and I are the Miller indexes. In this way, we have obtained the lattice parameters a 4.03 A and c 28.6 A. We stress the fact that different zone axis CBED patterns of =
=
~)SI)
(
/1’ B/a
(1 ~I
I. Ui Iron ilI//rai lion ituili it In
‘i
inc/i 5rini 0
ii I
S
I ig
3. (a)
jUIL I
1 ,inaka and I
h con~entionat con~.ergen beam patterns recorded marked .ini]
I.
.i
rom a good qualii~ region ihret’fokl rol.ition ,iu~.is e~dent in hi tb pictures
\ mirror hue iii
o,
I
it.
Fig. 4. Ia) [DII.I] zone
:Ris diffraction pattern in Bragg condition. ftc indicated 12W and 12W reflections ore normal to the mirror line m of the ~ hole pattern: IB) and IC) sho~the (I 2.01 and (II.)) reflections in Bragg condi lions. respect iscls
,
C. Dc Blasi et al.
/ Electron
diffraction study of In Se
951
7 melt grown crystals.
Also, the CBED indicates In 2Se3 to be made of hexagonal and rhombohedral phases. The mixture of such structures is evident in fig. 6. This picture shows the strong [00.1] zone axis pattern of the rhombohedral lattice and the faint [001] pattern of the hexagonal phase.
4. Conclusions
Fig. 5. Whole CBED pattern of the hexagonal phase with [113] low symmetry zone axis,
the rhombohedral structure have been obtained by tilting the samples. In addition, CBED patterns have been observed with low symmetry zone axis belonging to a hexagonal structure. Such an example is reported in fig. 5, which shows the whole [11.3] hexagonal CBED pattern. The computation of the lattice parameters gives a 4.03 A and c 19.1 A. =
=
The structure of the compound In2Se5 has been investigated by several authors and some discrepancies are obvious among the published results. The confusion in the literature is not only on the structure, but also on its polymorphism because this compound undergoes several phase transitions. However, it is evident that the structure of the In2Se1 depends on the method of preparation. The crystals analyzed in this work at room temperature have been grown from the melt by the Bridgman—Stockbarger method and the structure has been found to be the a-phase. An accurate analysis of the crystals performed by both selected area and convergent beam electron diffraction techniques has evidenced the structure as .
prevailingly rhombohedral with equivalent hexagonal unit cell parameters a 4.03 A and c 28.9 A. These results are in fair agreement with the B-type samples analyzed by Popovic et al. [6], even if the sign of the space group is different. In addition, we have found a fraction of the volume in samples to crystallize in the hexagonal structure with crystallographic constants a 4.03 A and c 19.1 A. =
~
=
=
=
Acknowledgement This work was supported by the Ministry of Education of Italy (MPI).
Fig. 6. CBED low camera length pattern showing the strong [00.1] zone axis diffraction of the rhombohedral lattice and (indicated by arrows) the faint reflections of the zero layer and the first order Laue zone of the [00.11zone axis of the hexagonal structure.
References [1] P.C. Newman, J. Phys. Chem. Solids 23 (1961) 19. [2] V.M. Koshicin, L.P. Gal’chinetskii, V.M. Kulik and RI. Minkov, Solid State Commun. 13 (1973) 1.
LI LI
952
C. De Blasi ci a!.
/
Electron diffraction study of In Se
[3] V.M. Koshkin, L.P. Gal’chinetskii, V.M. Kulik. O.K. Gusev and V.A. Ulmanis, Soviet At. Energy 42 (1977) 321. [4] C. Julien, M. Eddrief, K. Kambas and M. Balkanski. Thin Solid Films 137 (1986) 27. [5] S.A. Semiletov, Soviet Phys.-Cryst. 5 (1961) 673. [6] S. Popovic, B. Celusika and D. Bidjin. Phys. Status Solidi (a) 6, 301 (1971). [7] J. Van Landuyt, 0. Van Tendeloo and S. Amelinckx. Phys. Status Solidi (a) 30 (1975) 299. [8] A. Likforman. P. Fourcray. M. Guittard, J. Flahaut, F. Poirier and N. Szydlo, J. Solid State Chem. 33 (1980) 91. 19] C. Julien, M. Eddrief, M. Balkanski, E. Hatzikranitis and K. Kambas, Phys. Status Solidi (a) 88 (1985) 687.
7 melt grown crystals
[10] K. Kambas and C. Julien. Mater. Res. Bull. 17 (1982) 1583. [11] C. Dc Blasi. A.V. Dngo. G. Micocci. A. Tepore and A.M. Mancini, J. Crystal Growth 94 (1989) 455. [12] N. Spandau and F. Klanberg, Z. Anorg. Allgem. Chem. 295 (1958) 300. [13] M. Tanaka, R. Saito. K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [141 B.F. Buxton, J.A. Eades. J.W. Steeds and G.M. Rackham. Phil. Trans. Roy. Soc. London A281 (1976) 171. [15] C. Dc Blasi, D. Manno, S. Mongelli and A. Rizzo. Nuovo Cimento D7 (1986) 795.