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Formation and properties of crystalline compounds in the Lu2O3-Al2O3 system
Journal of Crystal Growth 52 (1981) 556—560 © North-Holland Publishing Company
FORMATION AND PROPERTWS OF CRYSTALLINE COMPOUNDS IN THE Lu203-A1203 SYSTEM A.G. PETROSYAN, GO. SHIRINYAN, K.L. OVANESYAN and A.S. KUZANYAN Institute for Physical Research, Academy of Sciences of the Armenian SSR. Ashtarak, USSR
Supercooling of the melt is a necessary condition for the formation of lutetium orthoaluminate structure. Unless supercooled, a melt of Lu203 : A1203 solidifies yields lutetium aluminum garnet and lutetium oxide. The growth of garnets and orthoaluminates is described briefly, and some basic physical properties of the crystals are reported.
1. Introduction
materials to prepare melts in molybdenum crucibles using equipment similar to that descnbed in ref. [3]. Some of experimental procedures involved in the present investigation follow the recommendations given in ref. [2]. The components were mixed in amounts corresponding to known crystalline compounds in the Y203— AI,01 [4] and Lu201—Al203 [5, 6] systems, and raised to the melting point over a period of one hour under an atmosphere of N2(Ar) with 20 vol% of hydrogen. After melting, the charge was heated for 2 h at some higher temperature, typically by 40 to 200°C.The melts were then cooled at a rate of 4.3°C/mm. The melting, “soaking” and solidification temperatures were measured with an optical pyrometer focussed on the melt surface, while the cooling and solidification processes were visually controlled. The crucibles were set to positions A (nearly isothermal field) and B (temperature gradient field of 1—10°C/mm)of fig. la. The shapes of the crucibles (fig. ib) permitted the adjustment of heat dissipation from the crucible base and melt cooling when in contact with a seed. The latter was applied in position B to stimulate the perovskite structure upon cooling Lu203 Al203 melts. The major variable in experiments was the degree of melt undercooling in position B which was realized by appropriate choice of melt volume, crucible dimensions and the extent of heat dissipation from the crucible base. Crystal growth runs were performed in molybdenum containers using the same equip-
Several experiments have been reported which show a marked dependence of the solid products upon the solidification conditions of Y1A15012 melts [1, 2]. A study of samples obtained by cooling melts with a chemical composition Lu203 : A1203 also reveals a marked variation in their structural characteristics. In some cases Lu203 A1203 melts yield lutetium orthoaluminate with perovskite structure and in others a mixture of Lu3A15O12 garnet and lutetium oxide. In addition, during our Bndgman—Stockbarger growth runs lutetium oxide has been seen in Lu3A15O12 crystals as a second phase while yttrium orthoaluminate is known to be the second phase in Y1A15O12. This paper reports experiments which explain the varying structural composition of solidified Lu203 : A1203 melts and the reason for the occurrence of lutetium oxide and not lutetium orthoaluminate in Lu3A15O12 garnet as a second phase. Crystal growth of compounds in the Lu203— Al203 and Y203—Al203 systems is also described and some major properties of Lu1A15O12 and Y3A15O12 including thermal expansion coefficients and optical dispersion are reported. 2. Experimental techniques High purity lutetium oxide, yttrium oxide and crystalline sapphire were used as component 556
A.G. Petrosyan et a!. I Crystalline compounds in Lu
203—A1203 system
557
3. Results
&cm
20
3.1. Investigation of solidified melts 15
) J
5
Table 1 summarizes the solidification conditions and resulting structures of melts having compositions equivalent to compounds existing in Lu203—Al203 and Y2OT-A1203 systems. In an field, melts with a composition 3 Lu~O3:5 Al203 become supercooled by 200— 240°Cbelow the melting point and, upon spontaneous solidification, yield a two-phase mixture
A
isothermal ~
_—~.
1800
1900
T~C
2000
a I
I
I
L.~J
I
L.,,,,J
L~J Fig. 1. (a) Temperature distnbution in the system (uncorrected pyrometer readings). (b) Crucible shapes.
ment. Pulling rates were within the range 0.5— 6 mm/h. The structures were determined using traditional X-ray powder diffraction techniques. Lattice spacings were measured accurately to within = ±1x 10~A over the 290—1570 K temperature range. The minimum deflection method was employed to measure the refractive indices in the range from 0.435 to 1.25 ~m (error ~Xn= ±2x 10~).
LuAIO3/Al201 similar to Y1A15012 melts which, under identical conditions, yield YAIO3/A1203 [2, 71. Upon cooling of Lu3Al5O12 and Y3A16012 melts in a temperature gradient such that they are not undercooled, garnet-type structures are formed. Melts with a composition of 2: 1 always yield the monoclinic phase. In position A they become supercooled by 30—80°C. In position A melts with Lu203 : A1203 (1: 1) chemical composition become supercooled by 80-100°C and, upon spontaneous solidification, form lutetium orthoaluminate with perovskite structure. The orthoaluminate structure is also formed upon cooling Lu203 : Al203 melts in position B only if some degree of melt undercooling is achieved. In the absence of supercooling the products of solidification of Lu203:A1203 melts in position B exhibit a mixed X-ray pattern
Table I Solidification conditions and structure of solidified melts
Melt composition
Position
Supercooling, (°C)
Structural composition at 300 K
Lu,01 : A1203
A B B A B A B A B A B A B
40-100 —25 —0 200-240 —0 —30 —0 30—80 —0 —230 —0 —30 —0
LuAIO3 LuAIO3 Lu3AlsOi2 + Lu,03 LuAIO1/A1203 Lu3A15O12 Lu4AI2O9 Lu4A12O9 YAIO3 YAIO3
3 Lu2O~:5 Al203 2Lu,03:A1203 Y203 : Al203 3 Y203 :5 A1203 2Y203:A1203
YAIO3/A1203 Y3A15O12 Y4A1209 Y4A1209
558
J~J~ AG. Petrosyan et a!. I Crystalline compounds in Lu
20,-.A1201 system
I
5
25
Single phase powdered samples of LuAIO3 and Lu4A12O9 subjected to 2 h treatment at 1600°Cin air showed complete decomposition of the initial materials, the products of phase separation being in both cases lutetium aluminum garnet and lutetium oxide. In summary, we note that cooling rate variations within the range 1—20°C/mm, changes in melt overheating by 40 to 200°Cand change of atmosphere (N2/20 vol% hydrogen or Ar) have no influence upon the products formed. 3.2. Crystal growth
9. ______________________________________ 25 Fig. 2. X-ray diffraction patterns of samples obtained on solidification of Lu203 : Al203 melts: (a) supercooled by 25— 100°C (structural composition resulted is LuAIO3); (b) without undercooling (structural composition resulted is Lu3A15O12+Lu203).
showing Lu3A15O12 garnet and lutetium oxide (fig. 2). Under the same conditions melts of Y203 : Al,O3 do not reveal such a marked difference in structural composition. In the latter case X-ray powder diffraction spectra show yttrium orthoaluminate irrespective of the solidification conditions, It seems important to mention two extra experiments (not included in table 1) in which seed ‘crystals or higher cooling rates were employed. First, a Lu203 : Al203 melt upon cooling in contact with a seed crystal (either a YA1O3 crystal, or LuAlO1 ceramic) solidified in position B as a mixture of Lu3Al~O1, and Lu4Al,O9. Second, a Lu201 : Al201 melt upon cooling at rates 50—150°Cin position B also solidified as a mixture of Lu3Al~O12and Lu4A12O9. In both runs the melts were not undercooled to any appreciable value,
Optical quality Lu3A15O12 and Y3A15O12 garnets, 14—25 mm in diameter and 150 mm in length were produced along [001], [210] and [221] crystallographic axes. These directions have earlier been reported for growth of3~)ions laser quality [8]. A garnets doped with rare earth (TR difference is observed between the crystallization character of Lu 3A15O12 and Y3A15O12 garnets. If a Lu3Al~O12melt is depleted in aluminum oxide, the result is that the excessive lutetium oxide is deposited on the crystal surface, while in the case of Y3A15O12 garnet, yttrium orthoaluminate is being formed. The latter may be produced both on crystal surface and in the bulk. Melts with a composition Lu203 : A1203 crystallizing under typical growth conditions (pulling rate within 0.5—6 mm/h, temperature gradient within 1—10°C/mm, melt overheating by 40 to 200°C) yield garnet and lutetium oxide. Small crystals of lutetium orthoaluminate were produced only by crystallizing undercooled melts. Yttrium orthoaluminate crystals were obtained under usual growth conditions. 3.3. Crystal properties 3.3.1. Cracking of garnets Observations have shown that, in addition to the irregular cracking of strained crystals along random directions, garnets grown along the [001] axis show some tendency to crack so that flat fissures or clefts are formed along and/or at right angles to the crystal axis. Due to the cubic structure of garnets (O~°—Ia3d) the transverse fissures are related to (100) planes, while the longitudinal
AG. Petrosyan et a!. I Crystalline compounds in Lu, O—Al~O system
ones may, in general, be aligned in (100), (110), (210), (310), etc. planes. The surfaces of flat clefts were related most often with (110) and (100) planes, but, on single occasions, with (310) and (410) planes. To a certain extent, the observed pattern of cracking may be due to the assumed existence of {100} type cleavage in garnets [7]. Our tentative data indicate that in garnets the cracks propagate more easily along (110) than (100) planes. We note here that due to facets in the central parts of the crystals, the surfaces of clefts may become distorted. When garnets are grown along a fourth-order axis, the condition of facet-free growth [8] becomes ~max = arcsin(D/2R) < 35°, where D is the crystal diameter and R is the radius of curvature of the convex spherical interface, so that {211} and {110} facets are generated at peripheral regions of the crystals. Dislocation densities (estimated in etch pits) varied in different samples from 0 to 50 cm2. These conditions, along with high homogeneity, provide flat clefts with almost mirror surfaces suitable for study. An example of crystal cracking is given in fig. 3 where Iongitudinal fissures are seen along (010) and (100) planes in a garnet crystal grown along the [001] axis. 3.3.2. Lattice spacings and thermal expansion coefficients The lattice parameters of garnets at 290 K are 12.004 A for Y,A15O,2 and 11.908—11.917 A for Lu3A1,O,2. Some scattering in the second case can be attributed to variation of the Lu/Al ratio in the crystals studied. Table 2 lists the constants in the equation of ref. [9] used to calculate the
Fig. 3. Longitudinal fissures along (011)) and (1(X)) planes in garnet grown parallel to [001] axis. Crystal diameter 18mm.
linear (aa) and volume (a~)coefficients of expansion, along with relevant data on molybdenum which is used as a container material. The linear coefficients of expansion calculated for the temperature range of 290—1275 K amount to 8.8 X 106 K-’ (Lu 3Al5O,2), 9.1 X 10-6 K-’ (Y1Al~O,2) and 6.8 x 106 K-’ (Mo). The error in the values of aa is ±0.3X 106 K’. Diffraction patterns of LuA1O3 and YAIO3 were projected in the orthorhombic system (D~— Pnma). The lattice parameter values obtained are: a = 5.181 A, b = 5.312 A, c = 7.358 A for YAIO1, and a = 5.11 A, b = 5.31 A, c = 7.29 A for LuAIO,. 3.3.3. Light dispersion The refractive indices n(A) of crystals in the wavelength range from 0.435 to 1.25 ~tm were interpolated using the equation of ref. [10]. The following dispersion formulae were obtained for
Table 2 Constants for calculating expansion coefficients
Material Lu,A1,0 1, Y,A1~O 12 Mo
I 3 x10
b x1O~5
a~—2.2447 6.5649 ab— a~—3.7736 ab — 10.9536 4.874l ab— a~—14.5663
0.7118 2.0958 0.9328 2.7192 0.9564 2.8525
559
c xlO9
d
Temperatuj~e (K)
2.27 7.56
0.0137 0.0162
290—1575
0.81 3.66 —1.39 —3.89
0.2944 0.8281 0.6179 1.8470
290—1275 290—1575
561)
AG. Petrosyan et a!. I Crystalline compounds in Lu
20,—A120, system
Lu,Al~O,2(1) and Y,A150,2 (2) garnets: 2(A)= 3.3275151 0.0149248A2+ 0.0178355A2 n 6 + 0.0046614A 8, ~ + 0.0009334A (1) + 0.0000737A n2(A) = 3.296823 0.0166197A2 + 0.0126503A-2 + 0.0069986A 0.0013968A6 + 0.0001088A8. (2) —
—
“—
4. Summary and conclusions The structures of solidified melts with chemical composition equivalent to garnet and orthoaluminate phases in the Lu 2O,—Al20, system are found to be governed by the degree of melt supercooling, a factor which has earlier been found to be the major one in governing the formation of either Y,A15O12 or a two-phase mixture YA1O,/Al20, in solidified melts with composition equivalent to Y,Al5012, as discussed in ref. [2]. The major factor of interest is that melts of Lu20,: AI2O, produce upon cooling the lutetium orthoaluminate phase only if they are undercooled by some 25—100°C. If no appreciable supercooling is present at the solidification ternperature, melts yield lutetium aluminum garnet and lutetium oxide although orthoaluminate seeds are expected to stimulate the perovskite structure. Melts with Y20, : A12O3 composition failed to yield such a result and produce upon cooling the YA1O, perovskite irrespective of supercooling. The contrasting behaviour of these melts has not been clarified in this paper as it presents a separate subject of investigation. The appearance of lutetium oxide in Lu,Al,012 garnets as a second phase in preference to yttrium orthoaluminate in Y,A15O12 can be understood in terms of results of the present investigation. It is notable that lutetium oxide is deposited on the external surface along the whole length of the crystal forming a fine “shirt”, whilst YA1O, appearing in Y,A150,2 is dis-
tributed randomly both on the crystal surface and in bulk. The appearance of the Lu 4A12O0 phase in Lu 20, : A12O, melts in cases when crystals with the perovskite structure were seed used or high cooling rates applied, presents another question for consideration and it is felt that some additional experiments are needed to explain the situation. However, close structural radicals in the first case and time restrictions for lutetium oxide to become separated in the second may be the possible factors involved. Our measurements show that during crystal growth in molybdenum containers, the difference between thermal coefficients causes some tensile stress upon the crystals. It would be interesting to study whether the “shirt” of lutetium oxide mentioned above could be used to prevent additional strain developing in the crystals.
References [11 Kh.S.
Bagdasarov, in: Crystal Growth and Crystal Structure, part 2, 4th All-Union Conf. on Crystal
Growth (Acad. Sci. Armenian SSR, Yerevan, 1972). [2] B. Cockayne and B. Lent, J. Crystal Growth 46 (1979) 371. [31Kh.S. Bagdasarov, S.B. Dokhnovskii, VS. Papkov and IV. Smolentsev, Pribory i Tekh. Expenm. 4 (1966) 229. [4] M. Mizano and T. Noguchi, Rept. Govt. Ind. Res. Inst. Nagoya 16 (1967) 171. [5] AK. Shirvinskaya and V.F. Popova, DokI. Akad. Nauk SSSR 233 (1977) 1110. [6] A.O. Ivanov, L.G. Morozova, IV. Mochalov and PP. Feofilov, Opt. i Spektrosk. 38 (1975) 405.
[71 J. [8] [9]
[10] [11] [12]
Caslavsky and D. Viechnicki, presented at 4th Am. Conf. on Crystal Growth, NBS, Gaithersburg, MD, 1978. A.A. Kaminskii and A.G. Petrosyan, Doki. Akad. Nauk SSSR 246 (1979) 63. E.A. Stepantsov, V.G. Govorkov, G.V. Berezhkova, Kh.S. Bagdasarov and G.I. Rogov, Kristallografiya 21 (1977) 142. A.G. Petrosyan, GO. Shirinyan, K.L. Ovanesyan and A.A. Avetisyan, Kristall Tech. 13 (1978) 43. B.J. Skinner, Am. Mineralogist 41(1956)428. A. Barwolff, I. Grzanna and H. Zschaeck, Jemna Mech. a Opt. (Czech) 44 (1977) 92.
FORMATION AND PROPERTWS OF CRYSTALLINE COMPOUNDS IN THE Lu203-A1203 SYSTEM A.G. PETROSYAN, GO. SHIRINYAN, K.L. OVANESYAN and A.S. KUZANYAN Institute for Physical Research, Academy of Sciences of the Armenian SSR. Ashtarak, USSR
Supercooling of the melt is a necessary condition for the formation of lutetium orthoaluminate structure. Unless supercooled, a melt of Lu203 : A1203 solidifies yields lutetium aluminum garnet and lutetium oxide. The growth of garnets and orthoaluminates is described briefly, and some basic physical properties of the crystals are reported.
1. Introduction
materials to prepare melts in molybdenum crucibles using equipment similar to that descnbed in ref. [3]. Some of experimental procedures involved in the present investigation follow the recommendations given in ref. [2]. The components were mixed in amounts corresponding to known crystalline compounds in the Y203— AI,01 [4] and Lu201—Al203 [5, 6] systems, and raised to the melting point over a period of one hour under an atmosphere of N2(Ar) with 20 vol% of hydrogen. After melting, the charge was heated for 2 h at some higher temperature, typically by 40 to 200°C.The melts were then cooled at a rate of 4.3°C/mm. The melting, “soaking” and solidification temperatures were measured with an optical pyrometer focussed on the melt surface, while the cooling and solidification processes were visually controlled. The crucibles were set to positions A (nearly isothermal field) and B (temperature gradient field of 1—10°C/mm)of fig. la. The shapes of the crucibles (fig. ib) permitted the adjustment of heat dissipation from the crucible base and melt cooling when in contact with a seed. The latter was applied in position B to stimulate the perovskite structure upon cooling Lu203 Al203 melts. The major variable in experiments was the degree of melt undercooling in position B which was realized by appropriate choice of melt volume, crucible dimensions and the extent of heat dissipation from the crucible base. Crystal growth runs were performed in molybdenum containers using the same equip-
Several experiments have been reported which show a marked dependence of the solid products upon the solidification conditions of Y1A15012 melts [1, 2]. A study of samples obtained by cooling melts with a chemical composition Lu203 : A1203 also reveals a marked variation in their structural characteristics. In some cases Lu203 A1203 melts yield lutetium orthoaluminate with perovskite structure and in others a mixture of Lu3A15O12 garnet and lutetium oxide. In addition, during our Bndgman—Stockbarger growth runs lutetium oxide has been seen in Lu3A15O12 crystals as a second phase while yttrium orthoaluminate is known to be the second phase in Y1A15O12. This paper reports experiments which explain the varying structural composition of solidified Lu203 : A1203 melts and the reason for the occurrence of lutetium oxide and not lutetium orthoaluminate in Lu3A15O12 garnet as a second phase. Crystal growth of compounds in the Lu203— Al203 and Y203—Al203 systems is also described and some major properties of Lu1A15O12 and Y3A15O12 including thermal expansion coefficients and optical dispersion are reported. 2. Experimental techniques High purity lutetium oxide, yttrium oxide and crystalline sapphire were used as component 556
A.G. Petrosyan et a!. I Crystalline compounds in Lu
203—A1203 system
557
3. Results
&cm
20
3.1. Investigation of solidified melts 15
) J
5
Table 1 summarizes the solidification conditions and resulting structures of melts having compositions equivalent to compounds existing in Lu203—Al203 and Y2OT-A1203 systems. In an field, melts with a composition 3 Lu~O3:5 Al203 become supercooled by 200— 240°Cbelow the melting point and, upon spontaneous solidification, yield a two-phase mixture
A
isothermal ~
_—~.
1800
1900
T~C
2000
a I
I
I
L.~J
I
L.,,,,J
L~J Fig. 1. (a) Temperature distnbution in the system (uncorrected pyrometer readings). (b) Crucible shapes.
ment. Pulling rates were within the range 0.5— 6 mm/h. The structures were determined using traditional X-ray powder diffraction techniques. Lattice spacings were measured accurately to within = ±1x 10~A over the 290—1570 K temperature range. The minimum deflection method was employed to measure the refractive indices in the range from 0.435 to 1.25 ~m (error ~Xn= ±2x 10~).
LuAIO3/Al201 similar to Y1A15012 melts which, under identical conditions, yield YAIO3/A1203 [2, 71. Upon cooling of Lu3Al5O12 and Y3A16012 melts in a temperature gradient such that they are not undercooled, garnet-type structures are formed. Melts with a composition of 2: 1 always yield the monoclinic phase. In position A they become supercooled by 30—80°C. In position A melts with Lu203 : A1203 (1: 1) chemical composition become supercooled by 80-100°C and, upon spontaneous solidification, form lutetium orthoaluminate with perovskite structure. The orthoaluminate structure is also formed upon cooling Lu203 : Al203 melts in position B only if some degree of melt undercooling is achieved. In the absence of supercooling the products of solidification of Lu203:A1203 melts in position B exhibit a mixed X-ray pattern
Table I Solidification conditions and structure of solidified melts
Melt composition
Position
Supercooling, (°C)
Structural composition at 300 K
Lu,01 : A1203
A B B A B A B A B A B A B
40-100 —25 —0 200-240 —0 —30 —0 30—80 —0 —230 —0 —30 —0
LuAIO3 LuAIO3 Lu3AlsOi2 + Lu,03 LuAIO1/A1203 Lu3A15O12 Lu4AI2O9 Lu4A12O9 YAIO3 YAIO3
3 Lu2O~:5 Al203 2Lu,03:A1203 Y203 : Al203 3 Y203 :5 A1203 2Y203:A1203
YAIO3/A1203 Y3A15O12 Y4A1209 Y4A1209
558
J~J~ AG. Petrosyan et a!. I Crystalline compounds in Lu
20,-.A1201 system
I
5
25
Single phase powdered samples of LuAIO3 and Lu4A12O9 subjected to 2 h treatment at 1600°Cin air showed complete decomposition of the initial materials, the products of phase separation being in both cases lutetium aluminum garnet and lutetium oxide. In summary, we note that cooling rate variations within the range 1—20°C/mm, changes in melt overheating by 40 to 200°Cand change of atmosphere (N2/20 vol% hydrogen or Ar) have no influence upon the products formed. 3.2. Crystal growth
9. ______________________________________ 25 Fig. 2. X-ray diffraction patterns of samples obtained on solidification of Lu203 : Al203 melts: (a) supercooled by 25— 100°C (structural composition resulted is LuAIO3); (b) without undercooling (structural composition resulted is Lu3A15O12+Lu203).
showing Lu3A15O12 garnet and lutetium oxide (fig. 2). Under the same conditions melts of Y203 : Al,O3 do not reveal such a marked difference in structural composition. In the latter case X-ray powder diffraction spectra show yttrium orthoaluminate irrespective of the solidification conditions, It seems important to mention two extra experiments (not included in table 1) in which seed ‘crystals or higher cooling rates were employed. First, a Lu203 : Al203 melt upon cooling in contact with a seed crystal (either a YA1O3 crystal, or LuAlO1 ceramic) solidified in position B as a mixture of Lu3Al~O1, and Lu4Al,O9. Second, a Lu201 : Al201 melt upon cooling at rates 50—150°Cin position B also solidified as a mixture of Lu3Al~O12and Lu4A12O9. In both runs the melts were not undercooled to any appreciable value,
Optical quality Lu3A15O12 and Y3A15O12 garnets, 14—25 mm in diameter and 150 mm in length were produced along [001], [210] and [221] crystallographic axes. These directions have earlier been reported for growth of3~)ions laser quality [8]. A garnets doped with rare earth (TR difference is observed between the crystallization character of Lu 3A15O12 and Y3A15O12 garnets. If a Lu3Al~O12melt is depleted in aluminum oxide, the result is that the excessive lutetium oxide is deposited on the crystal surface, while in the case of Y3A15O12 garnet, yttrium orthoaluminate is being formed. The latter may be produced both on crystal surface and in the bulk. Melts with a composition Lu203 : A1203 crystallizing under typical growth conditions (pulling rate within 0.5—6 mm/h, temperature gradient within 1—10°C/mm, melt overheating by 40 to 200°C) yield garnet and lutetium oxide. Small crystals of lutetium orthoaluminate were produced only by crystallizing undercooled melts. Yttrium orthoaluminate crystals were obtained under usual growth conditions. 3.3. Crystal properties 3.3.1. Cracking of garnets Observations have shown that, in addition to the irregular cracking of strained crystals along random directions, garnets grown along the [001] axis show some tendency to crack so that flat fissures or clefts are formed along and/or at right angles to the crystal axis. Due to the cubic structure of garnets (O~°—Ia3d) the transverse fissures are related to (100) planes, while the longitudinal
AG. Petrosyan et a!. I Crystalline compounds in Lu, O—Al~O system
ones may, in general, be aligned in (100), (110), (210), (310), etc. planes. The surfaces of flat clefts were related most often with (110) and (100) planes, but, on single occasions, with (310) and (410) planes. To a certain extent, the observed pattern of cracking may be due to the assumed existence of {100} type cleavage in garnets [7]. Our tentative data indicate that in garnets the cracks propagate more easily along (110) than (100) planes. We note here that due to facets in the central parts of the crystals, the surfaces of clefts may become distorted. When garnets are grown along a fourth-order axis, the condition of facet-free growth [8] becomes ~max = arcsin(D/2R) < 35°, where D is the crystal diameter and R is the radius of curvature of the convex spherical interface, so that {211} and {110} facets are generated at peripheral regions of the crystals. Dislocation densities (estimated in etch pits) varied in different samples from 0 to 50 cm2. These conditions, along with high homogeneity, provide flat clefts with almost mirror surfaces suitable for study. An example of crystal cracking is given in fig. 3 where Iongitudinal fissures are seen along (010) and (100) planes in a garnet crystal grown along the [001] axis. 3.3.2. Lattice spacings and thermal expansion coefficients The lattice parameters of garnets at 290 K are 12.004 A for Y,A15O,2 and 11.908—11.917 A for Lu3A1,O,2. Some scattering in the second case can be attributed to variation of the Lu/Al ratio in the crystals studied. Table 2 lists the constants in the equation of ref. [9] used to calculate the
Fig. 3. Longitudinal fissures along (011)) and (1(X)) planes in garnet grown parallel to [001] axis. Crystal diameter 18mm.
linear (aa) and volume (a~)coefficients of expansion, along with relevant data on molybdenum which is used as a container material. The linear coefficients of expansion calculated for the temperature range of 290—1275 K amount to 8.8 X 106 K-’ (Lu 3Al5O,2), 9.1 X 10-6 K-’ (Y1Al~O,2) and 6.8 x 106 K-’ (Mo). The error in the values of aa is ±0.3X 106 K’. Diffraction patterns of LuA1O3 and YAIO3 were projected in the orthorhombic system (D~— Pnma). The lattice parameter values obtained are: a = 5.181 A, b = 5.312 A, c = 7.358 A for YAIO1, and a = 5.11 A, b = 5.31 A, c = 7.29 A for LuAIO,. 3.3.3. Light dispersion The refractive indices n(A) of crystals in the wavelength range from 0.435 to 1.25 ~tm were interpolated using the equation of ref. [10]. The following dispersion formulae were obtained for
Table 2 Constants for calculating expansion coefficients
Material Lu,A1,0 1, Y,A1~O 12 Mo
I 3 x10
b x1O~5
a~—2.2447 6.5649 ab— a~—3.7736 ab — 10.9536 4.874l ab— a~—14.5663
0.7118 2.0958 0.9328 2.7192 0.9564 2.8525
559
c xlO9
d
Temperatuj~e (K)
2.27 7.56
0.0137 0.0162
290—1575
0.81 3.66 —1.39 —3.89
0.2944 0.8281 0.6179 1.8470
290—1275 290—1575
561)
AG. Petrosyan et a!. I Crystalline compounds in Lu
20,—A120, system
Lu,Al~O,2(1) and Y,A150,2 (2) garnets: 2(A)= 3.3275151 0.0149248A2+ 0.0178355A2 n 6 + 0.0046614A 8, ~ + 0.0009334A (1) + 0.0000737A n2(A) = 3.296823 0.0166197A2 + 0.0126503A-2 + 0.0069986A 0.0013968A6 + 0.0001088A8. (2) —
—
“—
4. Summary and conclusions The structures of solidified melts with chemical composition equivalent to garnet and orthoaluminate phases in the Lu 2O,—Al20, system are found to be governed by the degree of melt supercooling, a factor which has earlier been found to be the major one in governing the formation of either Y,A15O12 or a two-phase mixture YA1O,/Al20, in solidified melts with composition equivalent to Y,Al5012, as discussed in ref. [2]. The major factor of interest is that melts of Lu20,: AI2O, produce upon cooling the lutetium orthoaluminate phase only if they are undercooled by some 25—100°C. If no appreciable supercooling is present at the solidification ternperature, melts yield lutetium aluminum garnet and lutetium oxide although orthoaluminate seeds are expected to stimulate the perovskite structure. Melts with Y20, : A12O3 composition failed to yield such a result and produce upon cooling the YA1O, perovskite irrespective of supercooling. The contrasting behaviour of these melts has not been clarified in this paper as it presents a separate subject of investigation. The appearance of lutetium oxide in Lu,Al,012 garnets as a second phase in preference to yttrium orthoaluminate in Y,A15O12 can be understood in terms of results of the present investigation. It is notable that lutetium oxide is deposited on the external surface along the whole length of the crystal forming a fine “shirt”, whilst YA1O, appearing in Y,A150,2 is dis-
tributed randomly both on the crystal surface and in bulk. The appearance of the Lu 4A12O0 phase in Lu 20, : A12O, melts in cases when crystals with the perovskite structure were seed used or high cooling rates applied, presents another question for consideration and it is felt that some additional experiments are needed to explain the situation. However, close structural radicals in the first case and time restrictions for lutetium oxide to become separated in the second may be the possible factors involved. Our measurements show that during crystal growth in molybdenum containers, the difference between thermal coefficients causes some tensile stress upon the crystals. It would be interesting to study whether the “shirt” of lutetium oxide mentioned above could be used to prevent additional strain developing in the crystals.
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Bagdasarov, in: Crystal Growth and Crystal Structure, part 2, 4th All-Union Conf. on Crystal
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