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Volatility of potassium trimolybdate melt and solubility of yttrium-aluminium borate in it
Journal of Crystal Growth 49 (1980) 141—144 © North-Holland Publishing Company
VOLATILITY OF POTASSIUM TRIMOLYBDATE MELT AND SOLUBILITY OF YTI’RIUM-ALUMINIUM BORATE IN IT N.I. LEONYUK, A.V. PASHKOVA and L.Z. GOKHMAN Center of C,ystallography and Crystal Chemistry, Geological Faculty, Moscow University, Leninskiye Gory, Moscow, 1] 7234, USSR Received 26 April 1979; accepted for publication 2 August 1979
The volatility and thermal stability of potassium trimolybdate melt was studied with the temperature range from 900 to 1150°C. The dependence of the melt volatilization rate on temperature has been found, the composition of the products of thermal melt dissociation has been determined, and the volatilization activation energies have been evaluated. The solubility in the melt of yttrium—aluminium borate has been investigated, with the volatility taken into account. All borate crystals have been produced on an inoculation.
Along with various polymolybdates of alkaline metal, potassium trimolybdate has useful applications as a solvent during high-temperature solidification from a solution in a melt. Particularly, it is used to
that the overall removal of volatile phases both from the open crucible and from a crucible closed with a capt having a 5—10 mm id outlet tube (conditions of YA13[B03J4 crystal grown on an inoculation from a
produce rare earth—aluminium borate crystals [1]. The volatilization of K2Mo3O10 melt from open crucibles is relatively unimportant even at 1150°C, but with long test duration the losses can be tangible. Judging from the smooth maximum for K2Mo3O10 in the K2MoO4—Mo03 phase diagram [2], potassium trimolybdate melts with partial decomposition. However, the volatility and thermal stability of its melt have not been a subject of a systematic investigation. Since the composition of a solvent affects the composition and equality of growing crystals, it is important to know the behaviour of a solvent melt at high temperatures, especially under conditions approaching those of the crystal production. And even more so because the compositon of the solution! melt changes as a result of the volatilization, and this, as a rule, changes the stability boundaries of the phase being solidified. Therefore, prior to the investigation of the YA13 [B03]4 solubiity in potassium trimolybdate melt, we have studied the process of isothermal solvent volatilization in air atmosphere in the range from 900 to 1150°Cand the composition of the products of thermal melt dissociation, Preliminary solvent volatilization tests indicated
solution in K2Mo3O10 melt) were approximately the same. Acicular yellowish-coloured crystals are formed in a cooler furnace zone, precipitating on the inner walls of the furnace working chamber, and inside the tube when the crucible is provided with a cap having an outlet tube. The temperature in the zone of “needles” precipitation was 650—700°C. Potassium trimolybdate containing 81.52 wt% MoO3 and 18.46 wt% K20 was used for the quantitative determination of the volatilization rate. The volatilization tests were run at 900, 1000, 1100 and 1150°Cin parallel in two platinum crucibles 32 mm in diameter each. The initial weighed quantity was 50 g. The crucibles were closed with quartz caps having 10 mm id tubes. The entire free space in the furnace working chamber was filled with a refractory material to reduce heat transfer by convection. The temperature gradient across the volumes of the crucibles did not exceed 2—3°C,and the accuracy of the tempera. ture control was not worse than ±0.5°. The temperature was measured with a Pt—lO%Rh/Pt thermocouple directly at the melt surface. The melt was held at a prescribed constant temperature, after the volatilization rate reached a constant volume, for a 141
142
N.J. Leonyuk et al.
/
Volatility of K
2Mo3O10 and solubility of Y—Al borate in it
tially
increases
with
rising
temperature.
Above
of the melt weight loss values greatly widens. 0,10
1100°C
~y o,os 0,12
1150 C
H
~
0,06
~1
0C 0,04
1000
‘
To identify the vapour ‘phase, temperature was measured in the zone of its condensation; the condensate was chemically analysed and studied by X-ray structural technique. The chemical analysis (99.98 1150°Cit becomes difficult to control as the scatter wt% MoO3) and reading of powder X-ray traces showed that the condensate was molybdenum anhydride. At the same time it is obvious from table I that
there is a trend to an increase of the MoO
3 content in 0,02 900°C
0
25
50
75
100
the residual melt compared to the initial substance inspite of MoO3 volatilization. After the volatilization the specimens acquire an yellowish and greenish colour due to the enrichment with molybdenum anhyd ride.
TIme (hour)
Fig. 1. The volatilization isotherms of potassium trimolybdate
.melt.
total time of 100 h. The crucibles were weighed every 25 h. ‘ Fig. I shows the volatilizations isotherms of the K2Mo3O10 melt and table 1 summarizes the volatili-
The comparison of the weight losses of all specimens with the results of their chemical analysis before and after the tests shows that both MoO3 and potassium oxide evaporate, with K20 volatilization being faster. Potassium trimolybdate probably undergoes partial decay proceeding according to the equation: 2 K2Mo3O10
=
K20 + 2 MoO3 + K2Mo4O13.
zation rates presented as weight loss per unit time per
Some quantity of molybdenum anhydride dis-
unit surface. Each of the calculated rates is a mean of eight measurements (four measurements each in every 25 h for two parallel tests). The extreme fluctuations of the individual rate values did not exceed 8% of the mean values.
solves in the melt, and some evaporates condensating on the tube walls due to overcooling at 650—700°C. When exposed to low temperatures, the evaporating potassium oxide probably interacts with water vapours resulting in its complete removal.
The volatilization of the volatile phase substan-
The volatilization rate data can be transformed into MoO3 and K20 partial vapour pressures by using the corresponding formula
Table 1 The rates of volatilization of potassium trirnolybdate melt; the results of its analyses after 100 h volatilization and ternperature values in the solidification zone of the volatile phase
—
17.14W cr/iT
\M
from ref. [3], as has been done in calculating the Temperature of volatilization
(°C)
Rate of volatilization U(10~g 2 h’) cm
MoO3 content in residual melt
Temperature in the vapour zone condensation
(wt%)
(°C) 695 680
±5
± 0.3
81.66 81.93
± 0.5
82.39
655
±5
± 0.8
82.56
665
±~
PbF2 partial vapour pressure above the PbO—PbF2—B203 By assuming in this formula a = I , M to be
[41.
the grammolecular weight of MoO 3, and W/Ar + UMOO3 toit beis easy the volatilization rate of MoO3 in g 2 s~ to calculate ~Moo cm
900 1000 1100 1150
1.3 3.8 9.2 12.4
± 0.1
±~
3 at T
1173,
1273, 1373 and 1423 K. A similar calculation has also been done for K20. The values of volatilization
of UMOO3 and UK2O have been determined from the general volatilization rate U and the results of the
N.J. Leonyuk et al.
/ Volatility of K2Mo3O10 and
solubility of Y—Al borate in it
143
JI 2—3°
—ig U
H
2,9
/ 90u
—~
1000
1100
T(°C)
—
r—2-3° p
p
thermocouple
Fig. 2. The temperature dependence of the logarithm of potassium trimolybdate melt volatilization rate.
Fig. 3. Scheme of tests on yttrium—aluminium borate solubility.
melts (table I). The values of the volatilization activation energies of MoO3 and K20 have been evaluated
moment of volatilization. This enables us in building solubility curves to introduce corrections for solvent
from the graphs of the logarithm of the partial pressure (ln P) versus the inverse temperature (K) for MoO3 and K20 which slightly differ from the linear ones. They are 115 and 120 kJ/mole for MoO3 and K20, respectively.
volatilization as well as to control the process of YA13 [B04J4 crystal growth with regard to solution concentration changes because of the K2Mo3O10 volatilization.
increases with temperature. This may be confirmed by the temperature dependence of the K2Mo3O10 melt volatilization rate which differs somewhat from the exponential one (fig. 2), as well as by an increase
The method of determining the solution saturation points using tentative crystal inoculation was found to be the most reliable one for a study of the solubility of3 YA13 [B03]4, solution supercooling crucibles may asreach 120°Cor more. in 20— 50 cm A mix of a prescribed composition of K 2Mo3O10 and borate, about 80 g in total weight, was put in the platinum crucible. The crucible was located in a furnace zone with such axial temparture gradient as to ensure that the crucible bottom at test temperature
in the activation energy of MoO3 with rising tempera-
was 2—3°C hotter than the melt surface, and was
ture. The rates of volatilization of the K2Mo3O10 melt are much lower than those of the most common lead fluoride-based solvents. Thus, they aretrimolybdate 1.3 X i0~ and 2 h1 for potassium at 3.8 Xand l0~ g cm 900 1000°C,respectively, which is 3—4 orders of
closed with the quartz cap having the outlet tube for feeding a tentative crystal and an additional check-up thermocouple (fig. 3). Similar to the volatilization tests, the entire space in the working was filled with a refractory material. Prior tochamber determining the solution/melt saturation, a crystal of YAI
magnitude lower at the same temperatures than those of the most effective PbO—PbF2—B203 [41. The experimental data on the vaporization of the potassium trimolybdate melt make it pssible to evaluate the extent of its decay in the range from 900 to 1150°C as well as to obtain not only isotherms, but also polytherms of its weight loss for any time
0.03—0.04 g in weight, and the check-up thermocouple were fed into the upper furnace zone (hotter relative to the melt surface) through the quartz tube (fig. 3a). After the thermal cycle of the furnace, disturbed by the crystal holder (normally, at 1—3°C), has been restored the crystal was immersed into the melt (fig. 3b). In this process, the hot thermocouple
More stable molybdate complexes, as compared to the initial ones, originate in the process of vaporiza-
tion, with these complexes probably being block-andchain formations with molybdenum coordination [5].preferred The rateoctahedral of their formation
3 [B03]4,
144
N.J. Leonyuk et al.
/
Volatility of K
2Mo3O10 and solubility of Y—Al borate in it
end contacted the melt surface. The extent of satura-
tion was established by weight changes and the character of the inoculation surface. The solution/melt was considered to be saturated if the weight of the crystal remained constant with an accuracy down to 0.0002 g during a 10 h exposure. A mix with a different borate concentration was prepared to determine a next point. The growth and dissolution of an inoculation of 0.03—0.04 g in weight virtually did not change the solution concentration, since the weight of the inoculation changed only in the order of l0-~g. By numerous repetition of these steps it was possible to establish the saturation points with an accuracy of ±1.5°C. The temperature of the inoculations being immersed was somewhat higher than the temperature of the solution surface, which made it possible to avoid the formation of additional solidification centers. A small superheating of the lower crucible section prevented the formation of the YAI3 [B03J4 precipitate when the inoculation’ was dissolving in the upper section (the density of YAI borate is higher than the density of potassium trimolybdate). comparative evaluation of volatilization rates of theApure K 2Mo3O10 melt and of a solution with 20— 25 wt% YA13 [B03]4 has shown that the dissolution of such small amounts of borate has an insignificant effect on the volatilization rate, and at 1100—1150°C the differences between these rates are within the accuracy of the measurements. Therefore, the corrections for the solubiity values found were calculated from the volatilization rates of the solvent. The corrections were found to be small due to relatively short test times. The points so obtained were used to build the solubiity curve in the range of 20—25 wt% YA13[B03]4 (fig. 4). These data made it possible to grow the crystals of yttrium—aluminium borate on an inoculation. Crystals of 10—13 g in weight have been grown when the relative solution supersaturation was of an order of 0.02 in crucibles with a capacity of
150cm
.
1100
1020
900 _______________________________
~9
21
23
25 wt.% 1A13(ao3)4
The curve trimolybdate melt. Fig. 4.
of YAI3 [B03J4
solubility
in
potassium
The crystals of YA1 borate produced from the solution in the melt on the basis of potassium trimolybdate with excessive MoO3 have a typical blueish colour. They were spectroscopically analysed to have molybdenum impurities concentrations of lower oxidation degrees 5and Mo~3)whose in the crystals (Mo~ increase with increasing content of the molybdenum anhydride in the solution melt. The occurrence of these impurities in crystals of YA13 [B03]4 was earlier discussed in ref. [6].
References [1] NJ. Leonyuk, A.V. Pashkova and T.D. Semenova, Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 11(1975)181. [2] V.I.Spitsyn, I.M. Kuleshov, Zh. Obshch. Khim. 21 [31 R.L. Seifert,in: High-Temperature Technology, Ed. J.E. Campbell (Wiley, New York, 1957) p. 421. [41J.M. Coe and D. Elwell, J. CrystalGrowth 33 (1976) 155. [51 N.!. Leonyuk and A.V. Pashkova, in: Problemy Kristallologli (MGU Publishers, Moscow, 1976) p. 305 (in Rus[6] N.LLeonyuk, N.!. Eremin and A.V. Pashkova, Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 7(1976)1249.
VOLATILITY OF POTASSIUM TRIMOLYBDATE MELT AND SOLUBILITY OF YTI’RIUM-ALUMINIUM BORATE IN IT N.I. LEONYUK, A.V. PASHKOVA and L.Z. GOKHMAN Center of C,ystallography and Crystal Chemistry, Geological Faculty, Moscow University, Leninskiye Gory, Moscow, 1] 7234, USSR Received 26 April 1979; accepted for publication 2 August 1979
The volatility and thermal stability of potassium trimolybdate melt was studied with the temperature range from 900 to 1150°C. The dependence of the melt volatilization rate on temperature has been found, the composition of the products of thermal melt dissociation has been determined, and the volatilization activation energies have been evaluated. The solubility in the melt of yttrium—aluminium borate has been investigated, with the volatility taken into account. All borate crystals have been produced on an inoculation.
Along with various polymolybdates of alkaline metal, potassium trimolybdate has useful applications as a solvent during high-temperature solidification from a solution in a melt. Particularly, it is used to
that the overall removal of volatile phases both from the open crucible and from a crucible closed with a capt having a 5—10 mm id outlet tube (conditions of YA13[B03J4 crystal grown on an inoculation from a
produce rare earth—aluminium borate crystals [1]. The volatilization of K2Mo3O10 melt from open crucibles is relatively unimportant even at 1150°C, but with long test duration the losses can be tangible. Judging from the smooth maximum for K2Mo3O10 in the K2MoO4—Mo03 phase diagram [2], potassium trimolybdate melts with partial decomposition. However, the volatility and thermal stability of its melt have not been a subject of a systematic investigation. Since the composition of a solvent affects the composition and equality of growing crystals, it is important to know the behaviour of a solvent melt at high temperatures, especially under conditions approaching those of the crystal production. And even more so because the compositon of the solution! melt changes as a result of the volatilization, and this, as a rule, changes the stability boundaries of the phase being solidified. Therefore, prior to the investigation of the YA13 [B03]4 solubiity in potassium trimolybdate melt, we have studied the process of isothermal solvent volatilization in air atmosphere in the range from 900 to 1150°Cand the composition of the products of thermal melt dissociation, Preliminary solvent volatilization tests indicated
solution in K2Mo3O10 melt) were approximately the same. Acicular yellowish-coloured crystals are formed in a cooler furnace zone, precipitating on the inner walls of the furnace working chamber, and inside the tube when the crucible is provided with a cap having an outlet tube. The temperature in the zone of “needles” precipitation was 650—700°C. Potassium trimolybdate containing 81.52 wt% MoO3 and 18.46 wt% K20 was used for the quantitative determination of the volatilization rate. The volatilization tests were run at 900, 1000, 1100 and 1150°Cin parallel in two platinum crucibles 32 mm in diameter each. The initial weighed quantity was 50 g. The crucibles were closed with quartz caps having 10 mm id tubes. The entire free space in the furnace working chamber was filled with a refractory material to reduce heat transfer by convection. The temperature gradient across the volumes of the crucibles did not exceed 2—3°C,and the accuracy of the tempera. ture control was not worse than ±0.5°. The temperature was measured with a Pt—lO%Rh/Pt thermocouple directly at the melt surface. The melt was held at a prescribed constant temperature, after the volatilization rate reached a constant volume, for a 141
142
N.J. Leonyuk et al.
/
Volatility of K
2Mo3O10 and solubility of Y—Al borate in it
tially
increases
with
rising
temperature.
Above
of the melt weight loss values greatly widens. 0,10
1100°C
~y o,os 0,12
1150 C
H
~
0,06
~1
0C 0,04
1000
‘
To identify the vapour ‘phase, temperature was measured in the zone of its condensation; the condensate was chemically analysed and studied by X-ray structural technique. The chemical analysis (99.98 1150°Cit becomes difficult to control as the scatter wt% MoO3) and reading of powder X-ray traces showed that the condensate was molybdenum anhydride. At the same time it is obvious from table I that
there is a trend to an increase of the MoO
3 content in 0,02 900°C
0
25
50
75
100
the residual melt compared to the initial substance inspite of MoO3 volatilization. After the volatilization the specimens acquire an yellowish and greenish colour due to the enrichment with molybdenum anhyd ride.
TIme (hour)
Fig. 1. The volatilization isotherms of potassium trimolybdate
.melt.
total time of 100 h. The crucibles were weighed every 25 h. ‘ Fig. I shows the volatilizations isotherms of the K2Mo3O10 melt and table 1 summarizes the volatili-
The comparison of the weight losses of all specimens with the results of their chemical analysis before and after the tests shows that both MoO3 and potassium oxide evaporate, with K20 volatilization being faster. Potassium trimolybdate probably undergoes partial decay proceeding according to the equation: 2 K2Mo3O10
=
K20 + 2 MoO3 + K2Mo4O13.
zation rates presented as weight loss per unit time per
Some quantity of molybdenum anhydride dis-
unit surface. Each of the calculated rates is a mean of eight measurements (four measurements each in every 25 h for two parallel tests). The extreme fluctuations of the individual rate values did not exceed 8% of the mean values.
solves in the melt, and some evaporates condensating on the tube walls due to overcooling at 650—700°C. When exposed to low temperatures, the evaporating potassium oxide probably interacts with water vapours resulting in its complete removal.
The volatilization of the volatile phase substan-
The volatilization rate data can be transformed into MoO3 and K20 partial vapour pressures by using the corresponding formula
Table 1 The rates of volatilization of potassium trirnolybdate melt; the results of its analyses after 100 h volatilization and ternperature values in the solidification zone of the volatile phase
—
17.14W cr/iT
\M
from ref. [3], as has been done in calculating the Temperature of volatilization
(°C)
Rate of volatilization U(10~g 2 h’) cm
MoO3 content in residual melt
Temperature in the vapour zone condensation
(wt%)
(°C) 695 680
±5
± 0.3
81.66 81.93
± 0.5
82.39
655
±5
± 0.8
82.56
665
±~
PbF2 partial vapour pressure above the PbO—PbF2—B203 By assuming in this formula a = I , M to be
[41.
the grammolecular weight of MoO 3, and W/Ar + UMOO3 toit beis easy the volatilization rate of MoO3 in g 2 s~ to calculate ~Moo cm
900 1000 1100 1150
1.3 3.8 9.2 12.4
± 0.1
±~
3 at T
1173,
1273, 1373 and 1423 K. A similar calculation has also been done for K20. The values of volatilization
of UMOO3 and UK2O have been determined from the general volatilization rate U and the results of the
N.J. Leonyuk et al.
/ Volatility of K2Mo3O10 and
solubility of Y—Al borate in it
143
JI 2—3°
—ig U
H
2,9
/ 90u
—~
1000
1100
T(°C)
—
r—2-3° p
p
thermocouple
Fig. 2. The temperature dependence of the logarithm of potassium trimolybdate melt volatilization rate.
Fig. 3. Scheme of tests on yttrium—aluminium borate solubility.
melts (table I). The values of the volatilization activation energies of MoO3 and K20 have been evaluated
moment of volatilization. This enables us in building solubility curves to introduce corrections for solvent
from the graphs of the logarithm of the partial pressure (ln P) versus the inverse temperature (K) for MoO3 and K20 which slightly differ from the linear ones. They are 115 and 120 kJ/mole for MoO3 and K20, respectively.
volatilization as well as to control the process of YA13 [B04J4 crystal growth with regard to solution concentration changes because of the K2Mo3O10 volatilization.
increases with temperature. This may be confirmed by the temperature dependence of the K2Mo3O10 melt volatilization rate which differs somewhat from the exponential one (fig. 2), as well as by an increase
The method of determining the solution saturation points using tentative crystal inoculation was found to be the most reliable one for a study of the solubility of3 YA13 [B03]4, solution supercooling crucibles may asreach 120°Cor more. in 20— 50 cm A mix of a prescribed composition of K 2Mo3O10 and borate, about 80 g in total weight, was put in the platinum crucible. The crucible was located in a furnace zone with such axial temparture gradient as to ensure that the crucible bottom at test temperature
in the activation energy of MoO3 with rising tempera-
was 2—3°C hotter than the melt surface, and was
ture. The rates of volatilization of the K2Mo3O10 melt are much lower than those of the most common lead fluoride-based solvents. Thus, they aretrimolybdate 1.3 X i0~ and 2 h1 for potassium at 3.8 Xand l0~ g cm 900 1000°C,respectively, which is 3—4 orders of
closed with the quartz cap having the outlet tube for feeding a tentative crystal and an additional check-up thermocouple (fig. 3). Similar to the volatilization tests, the entire space in the working was filled with a refractory material. Prior tochamber determining the solution/melt saturation, a crystal of YAI
magnitude lower at the same temperatures than those of the most effective PbO—PbF2—B203 [41. The experimental data on the vaporization of the potassium trimolybdate melt make it pssible to evaluate the extent of its decay in the range from 900 to 1150°C as well as to obtain not only isotherms, but also polytherms of its weight loss for any time
0.03—0.04 g in weight, and the check-up thermocouple were fed into the upper furnace zone (hotter relative to the melt surface) through the quartz tube (fig. 3a). After the thermal cycle of the furnace, disturbed by the crystal holder (normally, at 1—3°C), has been restored the crystal was immersed into the melt (fig. 3b). In this process, the hot thermocouple
More stable molybdate complexes, as compared to the initial ones, originate in the process of vaporiza-
tion, with these complexes probably being block-andchain formations with molybdenum coordination [5].preferred The rateoctahedral of their formation
3 [B03]4,
144
N.J. Leonyuk et al.
/
Volatility of K
2Mo3O10 and solubility of Y—Al borate in it
end contacted the melt surface. The extent of satura-
tion was established by weight changes and the character of the inoculation surface. The solution/melt was considered to be saturated if the weight of the crystal remained constant with an accuracy down to 0.0002 g during a 10 h exposure. A mix with a different borate concentration was prepared to determine a next point. The growth and dissolution of an inoculation of 0.03—0.04 g in weight virtually did not change the solution concentration, since the weight of the inoculation changed only in the order of l0-~g. By numerous repetition of these steps it was possible to establish the saturation points with an accuracy of ±1.5°C. The temperature of the inoculations being immersed was somewhat higher than the temperature of the solution surface, which made it possible to avoid the formation of additional solidification centers. A small superheating of the lower crucible section prevented the formation of the YAI3 [B03J4 precipitate when the inoculation’ was dissolving in the upper section (the density of YAI borate is higher than the density of potassium trimolybdate). comparative evaluation of volatilization rates of theApure K 2Mo3O10 melt and of a solution with 20— 25 wt% YA13 [B03]4 has shown that the dissolution of such small amounts of borate has an insignificant effect on the volatilization rate, and at 1100—1150°C the differences between these rates are within the accuracy of the measurements. Therefore, the corrections for the solubiity values found were calculated from the volatilization rates of the solvent. The corrections were found to be small due to relatively short test times. The points so obtained were used to build the solubiity curve in the range of 20—25 wt% YA13[B03]4 (fig. 4). These data made it possible to grow the crystals of yttrium—aluminium borate on an inoculation. Crystals of 10—13 g in weight have been grown when the relative solution supersaturation was of an order of 0.02 in crucibles with a capacity of
150cm
.
1100
1020
900 _______________________________
~9
21
23
25 wt.% 1A13(ao3)4
The curve trimolybdate melt. Fig. 4.
of YAI3 [B03J4
solubility
in
potassium
The crystals of YA1 borate produced from the solution in the melt on the basis of potassium trimolybdate with excessive MoO3 have a typical blueish colour. They were spectroscopically analysed to have molybdenum impurities concentrations of lower oxidation degrees 5and Mo~3)whose in the crystals (Mo~ increase with increasing content of the molybdenum anhydride in the solution melt. The occurrence of these impurities in crystals of YA13 [B03]4 was earlier discussed in ref. [6].
References [1] NJ. Leonyuk, A.V. Pashkova and T.D. Semenova, Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 11(1975)181. [2] V.I.Spitsyn, I.M. Kuleshov, Zh. Obshch. Khim. 21 [31 R.L. Seifert,in: High-Temperature Technology, Ed. J.E. Campbell (Wiley, New York, 1957) p. 421. [41J.M. Coe and D. Elwell, J. CrystalGrowth 33 (1976) 155. [51 N.!. Leonyuk and A.V. Pashkova, in: Problemy Kristallologli (MGU Publishers, Moscow, 1976) p. 305 (in Rus[6] N.LLeonyuk, N.!. Eremin and A.V. Pashkova, Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 7(1976)1249.