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Vapour growth of SbSI on a seed
Journal of Crystal Growth 41(1977)61—66 ~ North-Holland Publishing Company
VAPOUR GROWTH OF SbSI ON A SEED * L.A. ZADOROZHNAYA, V.A. LYACHOVITSKAYA, E.I. GIVARGIZOV and L.M. BELYAEV Institute of Crystallography, Academy of Sciences of the USSR, Moscow 11 7333, USSR Received 15 April 1977
3) single crystals of SbSI were grown from the vapour phase on seeds. The seeds were cut Large isometric (up to 5 X 7 X 10 mm from a thick needle and were treated to remove the disturbed surface layers. Two growing techniques were compared: the method of a constant temperature gradient in conventional ampoule and the method of the reversal gradient in Scholz’ ampoule geometry. It was found that in the former method growth in the c-direction (i.e., along the needle axis) proceeds only in whisker form, probably due to a poison impurity effect, whereas in the latter a bulk crystal was grown. This result is attributed to prevention of the poison effect during the reverse (i.e., etching) stage. Prismatic faces (hk0) grow principally by a chain-type mechanism.
1. Introduction
2.1.1. Conventional (tube) technique Experiments on vapour transport growth were made in an evacuated down to ~i04 Torr horizontal glass tube (diameter 25—30 mm, length 150—200 mm) heated by a two-zone resistance furnace. The temperature was typically about 360°Cin the source zone and 10—15°C lower in the crystallization one. Purified polycrystalline SbSI was placed in the former zone whereas a seed in the latter one. At the initial stage (approximately 10 mm), a reverse temperature gradient was put to clean the seed. Then, the normal gradient was adjusted for a period from several hours to several days.
The discovery of ferroelectricity in AVBVICVII semiconductors by Merz and co-workers [1] has drawn a great deal of attention on these materials. From among them, most extensively studied was SbSI. Preparation of large isometric crystals of this material is, however, a very complicated problem. This is due to its specific properties; as it was shown by Donges [2], SbSI has a chain-type structure and, accordingly grows principally as needle crystals. In this work, the morphology of SbSI crystals grown by different methods has been studied. As a result, a technique for growing of bulk SbSI crystals has been developed.
2.1.2. Crystallization by temperature-gradient reversal in Scholz ‘ampoule geometry This vapour transport technique [3—5]was found to be very useful for growing large isometric crystals of SbSI. Principally, the Scholz apparatus is an axially symmetrical vertical ampoule, the crystallization taking place in a zone along its axis whereas the source material is placed on the periphery, concentrically to this zone. Here, some modification of the apparatus was made. The salient features of our apparatus were the following. (a) Plane heater: a wire coil was introduced into a 10 mm quartz tube bent in a vertical plane along the upper part of the ampoule. In this case, the source
2. Experimental 2.1. Crystal gro wth apparatus Two versions of the closed ampoule method for growing of SbSI crystals, the conventional and the Scholz technique, were used.
*
Part os this paper was presented as a post-deadline report at the First European Conference on Crystal Growth, ZUrich,
1976. 61
62
L.A. Zadorozhnaya et al.
/
temperature varied in some limits, for instance from 360°Cin the hottest part to about 340°C,in the relatively cold sections; it is the latter temperature that determines the supersaturation in the crystallization zone. Owing to such a heater geometry, in situ observations of growth process in the ampoule were facilitated. (b) Thermal radiation screen was combined of a vertical quartz cylinder and a metallic dome with an internal polished surface. Thus, it was possible to eliminate parasitic crystals in the upper part of the amp oule. (c) A box with dry ice was mounted on a shaft contacting the crystallization zone to obtain high supersaturations. This apparatus was equipped with a rotating drive motor (typical rate 2 rpm) and with an auxiliary bottom heater for temperature reversing. All important poInts of the ampoule were temperature-controlled by thermocouples. Two ampoule sizes were usually used: “small” (50 mm diameter in the bottom part) and “large” (100 mm diameter). In the former, the temperature difference ~T between the zones was normally (without any special cooling or heating) about 5—10°C;in the latter, the difference was typically 20°C.With dry ice cooling, this difference came up to 40°C.If, moreover, a fan was used in combination with dry ice, the difference reached about 80°C.At the reversing stage, the seed temperature exceeded that of the source by 20°C, the growth to etching interval ratio being typically 5 : I (i.e., 60 to 12 mm).
Vapour growth of SbSIon a seed
2.3. Crystal examination As-grown crystal faces were examined by scanning electron microscopy. In addition, the dielectric constant of the crystals as a function of temperature was measured.
3. Results and discussion 3.1. Morphology and growth mechanisms A nearly-equilibrium growth form of SbSI was previously determined on the basis of goniometric examination of the crystals which were grown from the vapour phase at very low supersaturations [6]. It was found that the habitus (“macromorphology”) of the SbSI crystals represents a combination of simple forms belonging to two zones: vertical ~hk0} and ho rizontal {hol}, see fig. 1. Here, a micromorphological study of these faces in relation to growth conditions is made. 3.1.1. Vertical zone faces Twelve faces, of rhombic prisms (1 lO}, [120} and of pynacoids (lOO}, {0l0}, are characteristic of SbSI crystals. Of them, four faces U 10} are most extended; these are formed even at relatively higl~ supersaturations at which other faces of the vertica zone do not practically develop.
2.2. Seed preparation Two kinds of SbSI crystals were used as seeds. First, small (mm sizes) single crystals with natural faces grown preliminarily in the conventional tube ampoules at low (5—10°C) temperature difference [6]. Second, single crystals with cross-sections ~-~1 X 1 .5 mm2 and hights —~2mm cut from needle crystals perpendicularly to their c-axis; cut faces of these seeds were chemically—mechanically polished and then chemically polished in special solutions. These faces had the orientation (001) which never appears in the growth habit of SbSI crystals (see below, sec~ tion 3.1).
120
100 (/0
0/0
____________
C
a
Fig. 1. The nearly-equilibrium form of the SbSI crystab (from the goniometric data).
L.A. Zadorozhnaya et al.
/
Vapour growth of SbS! on a seed
63
a5um
H
~
Ii-5pm
_____________
•
__
_______
11g. 2. Various growth stages of the vertical zone (110) face. Conventional (tube) technique. The crystallization tcmperaturc T~= 345°C.~T = 15°C; (~, b) process duration 3 h (c, d) 7 ii (a) 2411.
64
L.A. Zadorozhnaya eta!.
/
Vapour growth of SbSI on a seed
,ot~J
__
H
Fig. 3. The niicromorphology of the vertical zone (110) face grown by the temperature-gradient reversal technique at 345°C,~T 15°C.Process duration 24 h.
Fig. 4. Island-mode gross th on th~cut section (001) h~tube technique. Growth conditions as in fig. 2, process duration 24 Ii.
IUpml ~
________
~:
~
15Pm, Fig. 5. Quasicontinuous and continuous growth on the cut seLtion (001) by the temperature-gradient reversal technique; (a) T~= 345°C,i~T 10°C;(b) 340°C, 15°C; (c) 320°C,40°C.Process duration 24 h in all cases.
L.A. Zadorozhnaya et al.
/
Vapour growth of SbSIon a seed
65
In fig. 2 are illustrated various stages of material overgrowing on the {l lO} faces during experiments in the tube ampoules. Dark lines in fig. 2a are grooves (related probably to imperfections) which have been formed due to vapour etching at the 10 mm reversal temperature cleaning stage, see section 2.1.1; bright elongated spots are hillocks formed at the initial growth stage. As is seen, these growth figures are preferentially developed at the end of the grooves; sometimes, however, they are formed irrelatively to the grooves (fig. 2b). Subsequently, these growth figures are expanded but their elongated character remains (fig. 2c). In addition, the neighbour islands intergrow (fig. 2d) and finally form a continuous layer (fig. 2e).
In this respect, the Scholz technique provides greater possibilities. Owing to differences of growth and dissolution rates between perfect and imperfect areas on the seed surfaces, it was possible to use higher supersaturations, and, thus, achieve the continuous growth on this face. This is demonstrated in fig. 5. Even at low supersaturations (ST’-’ 10—15°C) a quasicontinuous growth was observed (figs. 5a, b), whereas at sufficiently high ones (~T-~ 40°C) the continuous layer was formed (fig. 5c). One possible explanation for this effect is the following. The island growth on the (001) cut section is due to poison impurities such as the non-volatile Sb2S3 component of SbSI or any residual oxides
It is interesting to note that these islands often have a form quite similar to the macrocrystal habitus (cf. fig. 1 and the island indicated by the double arrow in fig. 2c). Quite similar pictures were observed-on all other faces of the vertical zone. This micromorphology is also characteristic of SbSI crystals grown by the Scholz method (fig. 3). It is concluded that all these faces grow by a nucleation mechanism, the nuclei having the elongated form as a consequence of the molecular structure of this material. 3.1.2. Horizontal zone faces Only the rhombic prism {10l} was found in the horizontal zone of SbSI crystals [6]. Moreover, only two faces (101) and (101) were, in fact, observed on the crystal tip, although the other two faces could be developed, in principle, because SbSI crystals grow here in a temperature range in which the non-polar phase is stable. Some interesting results were obtained in experiments in which the material was overgrown on the cut sections with overall orientation (001). Such a face was never observed on the free-grown SbSI crystals and seems to be improbable at all. During growth, the plane (001) was covered with a lot of column crystals or “whiskers”, see fig. 4. The tips of these columns were formed by two faces {101} making a roof, i.e., regeneration of the original rhombic prism in microscale occurs. Such an island-mode growth was obtained in the conventional (tube) ampoule at the temperature difference z~T~’ 15°C.At lower supersaturations, this face failed to grow at all, whereas at higher supersaturations polycrystalline growth occurred.
which were highly adsorbed on the starting seed surface. At sufficiently high supersaturations, these impurities are buried at initial growth stages so that the continuous layer is consequently formed. 3.2. Bulk crystal growing Using the temperature reversing in the Scholz’ ampoule geometry, bulk SbSI single crystals were vapour grown for the first time. particular, large isometric 3) Insingle crystals were grown (up toa cut 5 X seed 7 X 10with mminitial sizes of about 1 X 1 .5 X 2 from mm3. An example of such a crystal is shown in fig. 6: this specimen was prepared during a 72 h run at 290 C and i.~T= 80 C. Its habitus includes both the
-
-
-
~
Fig. 6. Bulk crystal grown by the temperature-gradient reversal technique in Scholz’ ampoule geometry. T~= 290°C, 1~T = 80°C,process duration 72 h.
L.A. Zadorozhnaya etal.
66
/ Vapour
growth of SbSIon a seed
4. Conclusions 5 4 -~
3
2
0
\ 10
20
30
40
Fig. 7. Dielectric constant e of a bulk SbSI crystal as a function of temperature.
In view of the specific internal structure of SbSI, it is difficult to grow isometric crystals by spontaneous growth in a closed ampoule. So, the only way to obtam bulk crystals of this material seems to be overgrowth on a seed. This approach, however, usually fails because of the island-mode growth in the c-direction. It was found that the problem is principally solved by exploiting the temperature-gradient reversal technique. Large isometric (up to 5 X7 X 10mm3 single crystals of SbSI were grown from the vapour phase on seeds. These crystals are homogeneous and monolithic. Acknowledgements
vertical zone faces (1 l0}, {lOO}, -[120} and the horizontal zone faces {lol} the mean growth rates in various directions being approximately 0.05—0.1 mm/h (15 X l0~—3X 10_6 cm/see). ,
The authors wish to thank G.B. Netesov for experimental assistance and T.R. Volk for the dielectric constant measurements.
3.3. Crystal properties References The quality of the grown crystals (in particular, their homogeneity) was estimated from the measurements of the dielectric constant as a function of ternperature. These results for a bulk specimen are presented in fig. 7. As is seen the constant is very high (=~5 10 4 ) at the phase transition point, the transition occuring in a narrow temperature interval. In addi~tion,it was found that all crystals grown on seeds by the temperature-gradient reversal technique had practically the same transition point (near 20°C).These facts evidence the grown crystals are homogeneous and monolithic. .
.
.
.
.
.
Ill
E. Fatuzzo, G. Harbeke, W.I. Merz, R. Nitsche, H.Roetschi andW. Ruppel, Phys. Rev. 127 (1962) 2036.
[21E.
Donges, Z. Anorg. Ailgem. Chem. 263 (1950) 112, 250. [31H. Scholz and R. Kluckow, in: Crystal Peiser (Pergamon, Oxford 1967) p. 475. Growth, Ed. H.S.
[41H. Scholz, Philips Tech. Rev. [5]H. Scholz, Acts Electron. 17 [6] L.A. Zadorozhnaya, V.A.
28 (1967) 316. (1974) 69. Lyachovitskaya and L.M. Belyaev, Soviet Phys..Cryst. 18 (1973) 363.
VAPOUR GROWTH OF SbSI ON A SEED * L.A. ZADOROZHNAYA, V.A. LYACHOVITSKAYA, E.I. GIVARGIZOV and L.M. BELYAEV Institute of Crystallography, Academy of Sciences of the USSR, Moscow 11 7333, USSR Received 15 April 1977
3) single crystals of SbSI were grown from the vapour phase on seeds. The seeds were cut Large isometric (up to 5 X 7 X 10 mm from a thick needle and were treated to remove the disturbed surface layers. Two growing techniques were compared: the method of a constant temperature gradient in conventional ampoule and the method of the reversal gradient in Scholz’ ampoule geometry. It was found that in the former method growth in the c-direction (i.e., along the needle axis) proceeds only in whisker form, probably due to a poison impurity effect, whereas in the latter a bulk crystal was grown. This result is attributed to prevention of the poison effect during the reverse (i.e., etching) stage. Prismatic faces (hk0) grow principally by a chain-type mechanism.
1. Introduction
2.1.1. Conventional (tube) technique Experiments on vapour transport growth were made in an evacuated down to ~i04 Torr horizontal glass tube (diameter 25—30 mm, length 150—200 mm) heated by a two-zone resistance furnace. The temperature was typically about 360°Cin the source zone and 10—15°C lower in the crystallization one. Purified polycrystalline SbSI was placed in the former zone whereas a seed in the latter one. At the initial stage (approximately 10 mm), a reverse temperature gradient was put to clean the seed. Then, the normal gradient was adjusted for a period from several hours to several days.
The discovery of ferroelectricity in AVBVICVII semiconductors by Merz and co-workers [1] has drawn a great deal of attention on these materials. From among them, most extensively studied was SbSI. Preparation of large isometric crystals of this material is, however, a very complicated problem. This is due to its specific properties; as it was shown by Donges [2], SbSI has a chain-type structure and, accordingly grows principally as needle crystals. In this work, the morphology of SbSI crystals grown by different methods has been studied. As a result, a technique for growing of bulk SbSI crystals has been developed.
2.1.2. Crystallization by temperature-gradient reversal in Scholz ‘ampoule geometry This vapour transport technique [3—5]was found to be very useful for growing large isometric crystals of SbSI. Principally, the Scholz apparatus is an axially symmetrical vertical ampoule, the crystallization taking place in a zone along its axis whereas the source material is placed on the periphery, concentrically to this zone. Here, some modification of the apparatus was made. The salient features of our apparatus were the following. (a) Plane heater: a wire coil was introduced into a 10 mm quartz tube bent in a vertical plane along the upper part of the ampoule. In this case, the source
2. Experimental 2.1. Crystal gro wth apparatus Two versions of the closed ampoule method for growing of SbSI crystals, the conventional and the Scholz technique, were used.
*
Part os this paper was presented as a post-deadline report at the First European Conference on Crystal Growth, ZUrich,
1976. 61
62
L.A. Zadorozhnaya et al.
/
temperature varied in some limits, for instance from 360°Cin the hottest part to about 340°C,in the relatively cold sections; it is the latter temperature that determines the supersaturation in the crystallization zone. Owing to such a heater geometry, in situ observations of growth process in the ampoule were facilitated. (b) Thermal radiation screen was combined of a vertical quartz cylinder and a metallic dome with an internal polished surface. Thus, it was possible to eliminate parasitic crystals in the upper part of the amp oule. (c) A box with dry ice was mounted on a shaft contacting the crystallization zone to obtain high supersaturations. This apparatus was equipped with a rotating drive motor (typical rate 2 rpm) and with an auxiliary bottom heater for temperature reversing. All important poInts of the ampoule were temperature-controlled by thermocouples. Two ampoule sizes were usually used: “small” (50 mm diameter in the bottom part) and “large” (100 mm diameter). In the former, the temperature difference ~T between the zones was normally (without any special cooling or heating) about 5—10°C;in the latter, the difference was typically 20°C.With dry ice cooling, this difference came up to 40°C.If, moreover, a fan was used in combination with dry ice, the difference reached about 80°C.At the reversing stage, the seed temperature exceeded that of the source by 20°C, the growth to etching interval ratio being typically 5 : I (i.e., 60 to 12 mm).
Vapour growth of SbSIon a seed
2.3. Crystal examination As-grown crystal faces were examined by scanning electron microscopy. In addition, the dielectric constant of the crystals as a function of temperature was measured.
3. Results and discussion 3.1. Morphology and growth mechanisms A nearly-equilibrium growth form of SbSI was previously determined on the basis of goniometric examination of the crystals which were grown from the vapour phase at very low supersaturations [6]. It was found that the habitus (“macromorphology”) of the SbSI crystals represents a combination of simple forms belonging to two zones: vertical ~hk0} and ho rizontal {hol}, see fig. 1. Here, a micromorphological study of these faces in relation to growth conditions is made. 3.1.1. Vertical zone faces Twelve faces, of rhombic prisms (1 lO}, [120} and of pynacoids (lOO}, {0l0}, are characteristic of SbSI crystals. Of them, four faces U 10} are most extended; these are formed even at relatively higl~ supersaturations at which other faces of the vertica zone do not practically develop.
2.2. Seed preparation Two kinds of SbSI crystals were used as seeds. First, small (mm sizes) single crystals with natural faces grown preliminarily in the conventional tube ampoules at low (5—10°C) temperature difference [6]. Second, single crystals with cross-sections ~-~1 X 1 .5 mm2 and hights —~2mm cut from needle crystals perpendicularly to their c-axis; cut faces of these seeds were chemically—mechanically polished and then chemically polished in special solutions. These faces had the orientation (001) which never appears in the growth habit of SbSI crystals (see below, sec~ tion 3.1).
120
100 (/0
0/0
____________
C
a
Fig. 1. The nearly-equilibrium form of the SbSI crystab (from the goniometric data).
L.A. Zadorozhnaya et al.
/
Vapour growth of SbS! on a seed
63
a5um
H
~
Ii-5pm
_____________
•
__
_______
11g. 2. Various growth stages of the vertical zone (110) face. Conventional (tube) technique. The crystallization tcmperaturc T~= 345°C.~T = 15°C; (~, b) process duration 3 h (c, d) 7 ii (a) 2411.
64
L.A. Zadorozhnaya eta!.
/
Vapour growth of SbSI on a seed
,ot~J
__
H
Fig. 3. The niicromorphology of the vertical zone (110) face grown by the temperature-gradient reversal technique at 345°C,~T 15°C.Process duration 24 h.
Fig. 4. Island-mode gross th on th~cut section (001) h~tube technique. Growth conditions as in fig. 2, process duration 24 Ii.
IUpml ~
________
~:
~
15Pm, Fig. 5. Quasicontinuous and continuous growth on the cut seLtion (001) by the temperature-gradient reversal technique; (a) T~= 345°C,i~T 10°C;(b) 340°C, 15°C; (c) 320°C,40°C.Process duration 24 h in all cases.
L.A. Zadorozhnaya et al.
/
Vapour growth of SbSIon a seed
65
In fig. 2 are illustrated various stages of material overgrowing on the {l lO} faces during experiments in the tube ampoules. Dark lines in fig. 2a are grooves (related probably to imperfections) which have been formed due to vapour etching at the 10 mm reversal temperature cleaning stage, see section 2.1.1; bright elongated spots are hillocks formed at the initial growth stage. As is seen, these growth figures are preferentially developed at the end of the grooves; sometimes, however, they are formed irrelatively to the grooves (fig. 2b). Subsequently, these growth figures are expanded but their elongated character remains (fig. 2c). In addition, the neighbour islands intergrow (fig. 2d) and finally form a continuous layer (fig. 2e).
In this respect, the Scholz technique provides greater possibilities. Owing to differences of growth and dissolution rates between perfect and imperfect areas on the seed surfaces, it was possible to use higher supersaturations, and, thus, achieve the continuous growth on this face. This is demonstrated in fig. 5. Even at low supersaturations (ST’-’ 10—15°C) a quasicontinuous growth was observed (figs. 5a, b), whereas at sufficiently high ones (~T-~ 40°C) the continuous layer was formed (fig. 5c). One possible explanation for this effect is the following. The island growth on the (001) cut section is due to poison impurities such as the non-volatile Sb2S3 component of SbSI or any residual oxides
It is interesting to note that these islands often have a form quite similar to the macrocrystal habitus (cf. fig. 1 and the island indicated by the double arrow in fig. 2c). Quite similar pictures were observed-on all other faces of the vertical zone. This micromorphology is also characteristic of SbSI crystals grown by the Scholz method (fig. 3). It is concluded that all these faces grow by a nucleation mechanism, the nuclei having the elongated form as a consequence of the molecular structure of this material. 3.1.2. Horizontal zone faces Only the rhombic prism {10l} was found in the horizontal zone of SbSI crystals [6]. Moreover, only two faces (101) and (101) were, in fact, observed on the crystal tip, although the other two faces could be developed, in principle, because SbSI crystals grow here in a temperature range in which the non-polar phase is stable. Some interesting results were obtained in experiments in which the material was overgrown on the cut sections with overall orientation (001). Such a face was never observed on the free-grown SbSI crystals and seems to be improbable at all. During growth, the plane (001) was covered with a lot of column crystals or “whiskers”, see fig. 4. The tips of these columns were formed by two faces {101} making a roof, i.e., regeneration of the original rhombic prism in microscale occurs. Such an island-mode growth was obtained in the conventional (tube) ampoule at the temperature difference z~T~’ 15°C.At lower supersaturations, this face failed to grow at all, whereas at higher supersaturations polycrystalline growth occurred.
which were highly adsorbed on the starting seed surface. At sufficiently high supersaturations, these impurities are buried at initial growth stages so that the continuous layer is consequently formed. 3.2. Bulk crystal growing Using the temperature reversing in the Scholz’ ampoule geometry, bulk SbSI single crystals were vapour grown for the first time. particular, large isometric 3) Insingle crystals were grown (up toa cut 5 X seed 7 X 10with mminitial sizes of about 1 X 1 .5 X 2 from mm3. An example of such a crystal is shown in fig. 6: this specimen was prepared during a 72 h run at 290 C and i.~T= 80 C. Its habitus includes both the
-
-
-
~
Fig. 6. Bulk crystal grown by the temperature-gradient reversal technique in Scholz’ ampoule geometry. T~= 290°C, 1~T = 80°C,process duration 72 h.
L.A. Zadorozhnaya etal.
66
/ Vapour
growth of SbSIon a seed
4. Conclusions 5 4 -~
3
2
0
\ 10
20
30
40
Fig. 7. Dielectric constant e of a bulk SbSI crystal as a function of temperature.
In view of the specific internal structure of SbSI, it is difficult to grow isometric crystals by spontaneous growth in a closed ampoule. So, the only way to obtam bulk crystals of this material seems to be overgrowth on a seed. This approach, however, usually fails because of the island-mode growth in the c-direction. It was found that the problem is principally solved by exploiting the temperature-gradient reversal technique. Large isometric (up to 5 X7 X 10mm3 single crystals of SbSI were grown from the vapour phase on seeds. These crystals are homogeneous and monolithic. Acknowledgements
vertical zone faces (1 l0}, {lOO}, -[120} and the horizontal zone faces {lol} the mean growth rates in various directions being approximately 0.05—0.1 mm/h (15 X l0~—3X 10_6 cm/see). ,
The authors wish to thank G.B. Netesov for experimental assistance and T.R. Volk for the dielectric constant measurements.
3.3. Crystal properties References The quality of the grown crystals (in particular, their homogeneity) was estimated from the measurements of the dielectric constant as a function of ternperature. These results for a bulk specimen are presented in fig. 7. As is seen the constant is very high (=~5 10 4 ) at the phase transition point, the transition occuring in a narrow temperature interval. In addi~tion,it was found that all crystals grown on seeds by the temperature-gradient reversal technique had practically the same transition point (near 20°C).These facts evidence the grown crystals are homogeneous and monolithic. .
.
.
.
.
.
Ill
E. Fatuzzo, G. Harbeke, W.I. Merz, R. Nitsche, H.Roetschi andW. Ruppel, Phys. Rev. 127 (1962) 2036.
[21E.
Donges, Z. Anorg. Ailgem. Chem. 263 (1950) 112, 250. [31H. Scholz and R. Kluckow, in: Crystal Peiser (Pergamon, Oxford 1967) p. 475. Growth, Ed. H.S.
[41H. Scholz, Philips Tech. Rev. [5]H. Scholz, Acts Electron. 17 [6] L.A. Zadorozhnaya, V.A.
28 (1967) 316. (1974) 69. Lyachovitskaya and L.M. Belyaev, Soviet Phys..Cryst. 18 (1973) 363.