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The solubility of trigonal Se in Na2S solutions and the hydrothermal growth of Se
Journal of Crystal Growth 8 (1971) 191—196 © North-HollandPublishing Co.
THE SOLUBILITY OF TRIGONAL Se IN Na2S SOLUTIONS AND THE HYDROTHERMAL GROWTH OF Se
E. D. KOLB and R. A. LAUDISE Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, U.S.A.
Received 25 June 1970; revised manuscript received 21 August 1970
The solubility of trigonal Se was measured between 25° and 80 °Cin Na2S solutions from 0.25 to 1.75 M (molar). It was found that the solubility obeyed the van ‘t Hoff relation in 1.25 M Na2S with a heat of solution of 3.88 kcal/mol while the ratio Se/S at 40° and 50 °Csuggests that the dissolving reaction is: 2. 52 +2 Se ~± Se2S The data suggest that hydrolysis of S2 - at low S2
-
concentra-
1. Introduction Trigonal (hexagonal) selenium is of interest as a nonlinear optical, piezoelectric, photoconductive and semiconductor material’ 3)• It is especially difficult to grow as single crystals because the chain and ring structure of Se melts results in its having high viscosity and tending to form glasses rather than crystals during cooling, Indeed, even when crystals are formed their morphology (spherulites, etc.) often is reminiscent of that observed for organic polymers suggesting that chain folding errors and many of the other imperfections associated with polymer crystals may occur readily in Se. Keezer4) has dissolved halogens or thallium in Se melts so as to lower the viscosity and permit the growth of crystals from the melt. These additives tend to break Se—Se bonds in chains and rings presumably by forming Se—Tl and Se—X bonds. Harrison~)has grown Se from the melt at high pressure where the melting point is elevated and viscosity is lower. Viscosity is reduced at high temperatures because the average Se chain length is shortened. Se has also been grown by the melt differential technique6) and by vapor deposition7’8) and monoclinic9’’°)Se has been crystallized from organic solvents such as carbon disulfide. Recently E. D. Kolb’1) observed that Se had appreciable solubility in sulfide solutions, grew crystals 191
tions and high temperatures is important in reducing solubility. When the hydrolysis was repressed by the addition of (OH)-, the solubility was markedly increased. The best conditions for hydrothermal growth in a temperature differential (AT) were found to be: crystallization temperature, 175°;AT —25°,95% fill and M Na2S. Melt growncycling seeds were shown to produce poor0.375 growth but temperature could be used to build up a few spontaneous nuclei into crystals of nearly 1 cm in length.
from such solutions by slow cooling at atmospheric pressure from 65 to 30 °C,and made preliminary so~ lubility measurements. Spectrochemical analysis’ 1) showed that the only significant impurities present were S and Si. It would be expected that hydrothermal crystals prepared in the absence of glass containers would be Si free, and would contain appreciably less S because the ~2 mineralizer concentrations required are lower. We decided to extend previous solubility measurements of Se in S2 solutions so as to glean information concerning the species present. If S2 concentration-ternperature regions where chain length in solution were short could be discovered, these regions would be particularly useful for the growth of non-chain folded Se crystals. Indeed studies of Se growth as a polymer from the melt compared to Se growth from small species in S2 solutions may be of interest in shedding light on mechanisms of polymer growth in general. Such studies would be expedited if the temperature region of study were extended, and indeed higher temperature growth might be expected to produce better crystals, so we decided to investigate hydrothermal Se crystallization from S2 solutions. The use of complexers12) (mineralizers) in hydrothermal work is common but with the exception of Se—S2 growth, CuC1—Cl (gel growth)13’14) and HgS—S2 (refs. 15, 16, 22) growth, ambient use of —
—
-
192
E. D. KOLB AND R. A. LAUDISE
complexers is rare. Growth of the same material from ambient to hydrothermal temperatures from the same complexer we felt would also give further understanding of the role of mineralizers in hydrothermal growth and be important in aiding in their extension to low temperature growth. Scholz’ 7) has reported that temperature cycling can be used to reduce nucleation in vapor growth and recent results indicate a similar effect in flux growth’ 8), High quality Se could best be grown on seeds of high perfection, thus making melt grown seeds unattractive, The problem of building up spontaneously nucleated crystals to a point where they may be mounted and used as seeds is severe in hydrothermal work. Consequently, we decided to investigate temperature cycling as part of our study of hydrothermal Se growth in hopes that it could be used to enlarge high quality spontaneous nuclei to the point where they could be used as seeds in subsequent growth.
Our cycling experiments were conducted using a Leeds and Northrup “Trend Trak” Curve Follower which supplied a programmable suppression bias to the control thermocouple of the autoclave top heater controller. The latter was a Leeds and Northrup 6263 Electromax Controller of the duration adjusting type. The programmable feature was obtained by drawing a -i-” black ink curve or cam on a translucent disc which was rotated through 360°once in 24 hr. A photocell detector scanned the inner edge of the program curve or cam though an appropriate bridge circuit. A millivolt output was supplied proportional to radius points on the disc graduated from 0—100 for a total span equal to 0—5 mV. The cam was drawn to provide a 0—50 span (0—2.5 mY) in equal 3 hr segments. The millivolt output was fed in series opposition with the control thermocouple as the suppression bias which resulted in the
TMIN NUTRlENT~20O°C
T
2. Experimental
TMAXSEED~2IO°C
I HR.
2. 1. SOLUBILITY MEASUREMENTS Solubility measurements were made by determining the loss in weight of crystalline trigonal Se equilibrated for times greater than the equilibrium time in closed stirred flasks. Temperature was controlled to better than ±0.3°C.A more complete description of the apparatus and technique has been given by Kolb’ 1), For the study trigonal Se was crystallized from amorphous Se dissolved in 1.25 M (molar) Na2S by evaporation. 2.2.
-10 C
~,
HF TMAX NUTRIENT 202~C I
SEED
l85~C
MIN.
~T
+
I7~C
HYDROTHERMAL GROWTH
Hydrothermal crystallization was conducted in short (1” diameter x 3” length) and long (1*” x 7” 9).diameter Furnaceconlength) Pt lined Morey type autoclaves’ figuration and temperature measurement and control were as has been described previously and temperatures were measured on the skin of the autoclave by strapped external thermocouples’9). Seeds were either tightly fastened to the seal wafer or suspended on noble metal wires in the growth zone. The growth and dissolving zones were separated by a 5 % open baffle. All starting materials were reagent purity or better except trigonal Se nutrient for the growth study which was crystallized from amorphous Se dissolved in 1.25 M Na 2S by evaporation.
Fig. 1.
Typical temperature variations used in Se growth by cycling.
autoclave top temperature cycling. Fig. 1 shows a typical time trace for our cycling experiments. 3. Results and discussion ~ 1. SOLUBILITY Fig. 2”) shows the dependence of the solubility of trigonal Se upon temperature in 0.75 M (molar) and 1) 1.25 M Na2S. Preliminary solubility measurements’
Se
THE SOLUBILITY OF TRIGONAL
IN
Na2S
193
SOLUTIONS
I.25M Na2S 20
— —
.
-0’ —
1.26
— —
l.24
-
0
18
16
,
I.25M NO2S AMORPHOUS Se
_-
0
0
1.22
~NO2S
1.20 1.18 1.16
4 I-.
TRIGONAL Se I-
1.12
/
~l0 -J D 08. -i C’,
1.14
0
075M NO2S
1.10 0o1.06
~l.04 .02.
o
6 1.00 4.
.98
.96 2
~4.
8
.92 0
I
I
20
30
I
40
50
TEMPERATURE
I
I
I
60
70
80
.9C
(C)
I
28
2.9
3.0
x
I
3.1
I
3.2
3.3
34
~
FIG.2
Fig. 2.
Se solubility vs temperature in 1.25 and 0.75 M Na2S.
indicated that the measured solubilities of amorphous Se were considerably greater. The departure from linearity 2—.at higher temperatures is due to the hydrolysis of s S2+H 2O~±HS+OH (1)
Fig. 3.
Log Se solubility vs reciprocal absolute temperature in 1.25 M Na2S.
30
.
0 40 050
F~25
-j
~20
HS+H2O~±H2S+0H.
(2)
~ ~iis
The hydrolysis reactions proceed further to the right 2
as temperature is increased and the decrease in S concentration due tosolutions hydrolysisaccounting is relativelyfor lessthe in high 2 concentration fact S that the linear region extends to a higher temperature in 1.25 M than in 0.75 M Na 2S. When solubility measurements were made in 10 M NaOH solutions where the Na2S concentrations were 0.75 M, the solubility increased by more than a factor of two, as would be expected, since (OH)— represses eqs. (I) and (2). Fig. 3 shows that the van ‘t Hoff relation is obeyed in 1.25heats M Na2S. Insufficient dataS2is available to calculate of solution at other concentrations, —
-
~
~
0 5 -
0 Fig. 4.
0.25
0.5
0.75
.0
1.25
1.5
.75
2.0
Na2 s MOLARITY Se solubility as a function of Na2S concentration.
The slope of fig. 3 gives a heat of solution of 3.88 kcal/mol in 1.25 M Na2S. 2 Fig. 4 showsAsthecandependence of fig. solubility on Ss/c concentration. be seen from 4 the ratio
194
E. D. KOLB AND R. A. LAUDISE
where s is Se solubility and c is concentration of Na2S
~
is aconstant.Agen:ralized dissolving reaction for Se 2 ~(SenSm)2m. nSe+mS The equilibrium constant for eq. (3) will be
(3)
where[]indicat:s:ctivities. ln terms of s and c, eq. (4) becomes
(4)
s/n =
~—.
___________
~
~
_____ _______
(5)
[c_(m/n)s]m
_____ _______
For lack of activity constant data we are forced to assume that concentrations approximate activities. From the large solubilities measured it is obvious that K 3~l.Sothat c = (m/n)s. (6) Fig. 4 shows that s/c is constant. By the use of appropriate density data solutions Its/cwas in found terms 2 for canNa2S be calculated. of vary molesbetween Se/moles S and 2.21 over the S2 concentrato 1.99 tion range of fig. 4. This indicates that the most probable value for n is 2 and for ,n is 1. Thus the dissol~ing reaction is 2Se-~-S2 ~ (Se 2 (7) 2S) at least between 40 and 50 °Cand up to 1.75 M Na 2S. This is in agreement with the findings of Greiver and 20) who studied the reaction up to 1.46 M Na Zaitseva 2S (presumably) at room temperature. 21) has studied the nature of species present in Ward polysulfide solutions by Raman spectroscopy and postulates (S~2) as most important. By analogy, Se~2is suggested as the predominant species in Se/S2 solutions. The present work indicates that a more reasonable species is (Se 2). The solubility of2S Se in 0.94 M Na 2S as a function of NaC1 concentration was measured, and it was found that Cl has no effect on the solubility of Se. Thus Cl does not appear to act as a chain terminator in aqueous media as it does in the melt.
___
-
-
~
-
3.2.
HYDROTHERMAL CRYSTAL GROWTH
Table 1 lists typical successful conditions for Se growth with a constant temperature differential (AT). Experiments in series (1), (2) and (3) all produced thin
Fig. 5.
6X (a) Needle habit Se crystals. (b) Equiaxial Se crystals. (c) Large Se needles grown by temperature cycling.
THE SOLUBILITY OF TRIGONAL
TABLE
Se
IN
Na2S
195
SOLUTIONS
I
Constant differential growth of Se AT
Growth temperature
*
(1) 145_160* (2) 175
15_40* 25
(3) 160_I75*
15_40*
%
fill
85 85
Solvent
Results
0.025—0.25 M Na2S* 0.025 M Na2S*
small hollow needles medium sized hollow needles
85_90* 0.025 M—0.25 M Na25* 0.5 M—5.0 M NaCl* needles 0—10.0 M LiCl* (4) 175 25 95 0.375 M Na2S blocky crystals When a range of conditions is given this indicates that a number of experiments covering this range were carried out.
TABLE
2
Temperature cycling experiments Run number 1770 1798
~
~~d*
210°C 210
AT (atTm~x~~d) —10 —10
AT
Tmin ~d
(at 175 185
Nucleation
Tmin ~~d)
+10 +17
—
none few well formed needles on baffle
Tma~s~d is the maximum temperature at the top of the vessel in the region where seeds are ordinarily placed. In the cycling runs no seeds were present; AT (at Tma. ~d) is the difference in temperature between dissolving and seed regions of the vessel when the seed region is at maximum temperature. Tmi,, ~ is the minimum temperature at the top of the vessel. *
hollow needles on the seal wafer of the sort shown in fig. 5(a). Experiments in series (4) produced more nearly equi-axial crystals on the seal wafer of the sort shown in fig. 5(b). The habit of fig. 5(b) is more useful for physical studies and would be more likely to lead to sound growth on seeds. The poor results of the experiments of series (1), (2) and (3) show respectively: (I) low temperature, regardless of 52_ concentration produces a poor morphology (2) low solubilities present at low ~2_ concentrations produce a poor morphology (3) C1 is ineffective in modifying morphology as might be expected from its lack of effect on solubility. From the foregoing we conclude that the best hydrothermal conditions for growth are: Crystallization temperature, 175°; % fill, 95; AT, 25; and Solvent, 0.375 M Na2S Seeded growth experiments on (I l~0) and (0001) seeds were conducted and considerable epitaxy occurred. However, due to the relatively poor quality of the melt grown seeds, perfection was poor. It was decided to increase the size of the spontaneously nucleated
seeds by the use of Scholz temperature cycling technique’ 7), The conditions of the cycling runs are given in table 2 and correspond to the traces of fig. 1. All fills were 90 % and the solvent was 0.025 and 0.375 M Na2S. All runs were 12 days. As might be expected, when AT at Tmax = AT at T’,,,~at the seed no growth occurs. When AT at the seed oscillates between —10 and + 17 °Cand the growth and nutrient zones are cycled nucleation occurs at the baffle. Scholz’ 7) has shown that cycling both ends of a system will cause nucleation at a region near the middle. Growth zone temperature was cycled 25°and nutrient 2°as measured by external thermocouples. It is probable that the true internal temperature ofthe nutrient cycled by considerably more than 2°.Some of the needles formed under these conditions had <0001> dimensions (fig. Sc) approaching 1 cm; however, close examination showed them to be hollow. It is surprising that under base concentration conditions where constant temperature differential growth was equiaxial, temperature cycling growth was needle like. Acknowledgments We would like to thank A. J. Caporaso for conduct-
196
F. D. KOLB AND R. A. LAUDISE
ing the hydrothermal growth experiments and D. J. Nitti for assistance in the identification of phases by X-ray diffraction,
10)
11) 12)
References 1) J. P. Kaminow, Quantum Electr., 2QE4-23 (1968). 2) M. C. Teich and T. Kaplan, IEEE J. Quant. Etectr. 2 (1966) 702. 3) J. E. Adams andW. Haas, in: Physics of Selenium and Telluriurn, Ed. W. Charles Cooper (Pergamon Press, 1970) p. 293 if. 4) R. C. Keezer, C. Wood and J. W. Moody, in: Proc. mt. Conf Cryst. Growth, Ed. H. S. Peiser (Pergamon Press, New York, 1967) p. 119 if. 5) D. E. Harrison, J. AppI. Phys. 36 (1965) 1680. 6) T. Stubb, in: Recent Advances in Selenium Physics, Ed.: European Selenium and Tellurium Committee (ESTC) (Pergamon Press, 1965) p. 53 if. 7) F. Eckart, in: Recent Advances in Selenium Physics, Ed.: European Selenium and Tellurium Committee (ESTC) (Pergamon Press, 1965) p. 85 if. 8) C. H. Griffiths and H. Sang, in: Physics of Selenium and Tellurium, Ed. W. Charles Cooper (Pergamon Press, 1967) p. 135 if. 9) G. B. Abdullayev, Y. G. Asadov and K. P. Mamedov, in:
13) 14) 15) 16) 17)
18) 19)
20) 21) 22)
Physics of Selenium and Tellurium, Ed. W. Charles Cooper (Pergamon Press, 1967) p. 179 if. J. lyima, J. Taynai and M. A. Nicolet, in: Physics ofSelenium and Tellurium, Ed. W. Charles Cooper (Pergamon Press, 1967) p. 199 if. E. D. Kolb, in: Physics of Selenium and Tellurium, Ed. W. Charles Cooper (Pergamon Press, 1967) p. 155 if. R. A. Laudise, in: Proc. mt. Coal Cryst. Growth, Ed. H. S. Peiser (Pergamon Press, New York, 1966), p. 3 if. H. K. Henisch, J. Dennis and J. Hanoka, J. Phys. Chem. Solids 26 (1965) 493. A. F. Armington and J. J. O’Connor, Mat. Res. Bull. 2 (10) (1967) 907. S. D. Scott and H. L. Barnes, Mat. Res. Bull. 4 (1969) (1969) 897. A. F. Armington and J. J. O’Connor, J. Crystal Growth 6 (1970) 278. H. Scholz and R. Kiuckow, in: Proc. mt. Calif Cryst. Growth, Ed. H. S. Peiser (Pergamon Press, New York, 1966) p. 475 if. W. Hintzmann and G. Muller-Vogt, J. Crystal Growth 5 (1969) 274. R. A. Laudise and J. W. Nielsen, in: Solid State Physics, Eds. F. Seitz and D. Turnbull, Vol. 12 (Academic Press, New York, 1961) p. 149 if. T. N. Greiver and I. G. Zaitseva, Zh. Prikl. Khim. 40 (1967) 1683. A. T. Ward, Mat. Res. Bull. 4 (1969) 581. Y. Toudic and R. Aumont, Compt. Rend. 269 (1969) 74.
THE SOLUBILITY OF TRIGONAL Se IN Na2S SOLUTIONS AND THE HYDROTHERMAL GROWTH OF Se
E. D. KOLB and R. A. LAUDISE Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, U.S.A.
Received 25 June 1970; revised manuscript received 21 August 1970
The solubility of trigonal Se was measured between 25° and 80 °Cin Na2S solutions from 0.25 to 1.75 M (molar). It was found that the solubility obeyed the van ‘t Hoff relation in 1.25 M Na2S with a heat of solution of 3.88 kcal/mol while the ratio Se/S at 40° and 50 °Csuggests that the dissolving reaction is: 2. 52 +2 Se ~± Se2S The data suggest that hydrolysis of S2 - at low S2
-
concentra-
1. Introduction Trigonal (hexagonal) selenium is of interest as a nonlinear optical, piezoelectric, photoconductive and semiconductor material’ 3)• It is especially difficult to grow as single crystals because the chain and ring structure of Se melts results in its having high viscosity and tending to form glasses rather than crystals during cooling, Indeed, even when crystals are formed their morphology (spherulites, etc.) often is reminiscent of that observed for organic polymers suggesting that chain folding errors and many of the other imperfections associated with polymer crystals may occur readily in Se. Keezer4) has dissolved halogens or thallium in Se melts so as to lower the viscosity and permit the growth of crystals from the melt. These additives tend to break Se—Se bonds in chains and rings presumably by forming Se—Tl and Se—X bonds. Harrison~)has grown Se from the melt at high pressure where the melting point is elevated and viscosity is lower. Viscosity is reduced at high temperatures because the average Se chain length is shortened. Se has also been grown by the melt differential technique6) and by vapor deposition7’8) and monoclinic9’’°)Se has been crystallized from organic solvents such as carbon disulfide. Recently E. D. Kolb’1) observed that Se had appreciable solubility in sulfide solutions, grew crystals 191
tions and high temperatures is important in reducing solubility. When the hydrolysis was repressed by the addition of (OH)-, the solubility was markedly increased. The best conditions for hydrothermal growth in a temperature differential (AT) were found to be: crystallization temperature, 175°;AT —25°,95% fill and M Na2S. Melt growncycling seeds were shown to produce poor0.375 growth but temperature could be used to build up a few spontaneous nuclei into crystals of nearly 1 cm in length.
from such solutions by slow cooling at atmospheric pressure from 65 to 30 °C,and made preliminary so~ lubility measurements. Spectrochemical analysis’ 1) showed that the only significant impurities present were S and Si. It would be expected that hydrothermal crystals prepared in the absence of glass containers would be Si free, and would contain appreciably less S because the ~2 mineralizer concentrations required are lower. We decided to extend previous solubility measurements of Se in S2 solutions so as to glean information concerning the species present. If S2 concentration-ternperature regions where chain length in solution were short could be discovered, these regions would be particularly useful for the growth of non-chain folded Se crystals. Indeed studies of Se growth as a polymer from the melt compared to Se growth from small species in S2 solutions may be of interest in shedding light on mechanisms of polymer growth in general. Such studies would be expedited if the temperature region of study were extended, and indeed higher temperature growth might be expected to produce better crystals, so we decided to investigate hydrothermal Se crystallization from S2 solutions. The use of complexers12) (mineralizers) in hydrothermal work is common but with the exception of Se—S2 growth, CuC1—Cl (gel growth)13’14) and HgS—S2 (refs. 15, 16, 22) growth, ambient use of —
—
-
192
E. D. KOLB AND R. A. LAUDISE
complexers is rare. Growth of the same material from ambient to hydrothermal temperatures from the same complexer we felt would also give further understanding of the role of mineralizers in hydrothermal growth and be important in aiding in their extension to low temperature growth. Scholz’ 7) has reported that temperature cycling can be used to reduce nucleation in vapor growth and recent results indicate a similar effect in flux growth’ 8), High quality Se could best be grown on seeds of high perfection, thus making melt grown seeds unattractive, The problem of building up spontaneously nucleated crystals to a point where they may be mounted and used as seeds is severe in hydrothermal work. Consequently, we decided to investigate temperature cycling as part of our study of hydrothermal Se growth in hopes that it could be used to enlarge high quality spontaneous nuclei to the point where they could be used as seeds in subsequent growth.
Our cycling experiments were conducted using a Leeds and Northrup “Trend Trak” Curve Follower which supplied a programmable suppression bias to the control thermocouple of the autoclave top heater controller. The latter was a Leeds and Northrup 6263 Electromax Controller of the duration adjusting type. The programmable feature was obtained by drawing a -i-” black ink curve or cam on a translucent disc which was rotated through 360°once in 24 hr. A photocell detector scanned the inner edge of the program curve or cam though an appropriate bridge circuit. A millivolt output was supplied proportional to radius points on the disc graduated from 0—100 for a total span equal to 0—5 mV. The cam was drawn to provide a 0—50 span (0—2.5 mY) in equal 3 hr segments. The millivolt output was fed in series opposition with the control thermocouple as the suppression bias which resulted in the
TMIN NUTRlENT~20O°C
T
2. Experimental
TMAXSEED~2IO°C
I HR.
2. 1. SOLUBILITY MEASUREMENTS Solubility measurements were made by determining the loss in weight of crystalline trigonal Se equilibrated for times greater than the equilibrium time in closed stirred flasks. Temperature was controlled to better than ±0.3°C.A more complete description of the apparatus and technique has been given by Kolb’ 1), For the study trigonal Se was crystallized from amorphous Se dissolved in 1.25 M (molar) Na2S by evaporation. 2.2.
-10 C
~,
HF TMAX NUTRIENT 202~C I
SEED
l85~C
MIN.
~T
+
I7~C
HYDROTHERMAL GROWTH
Hydrothermal crystallization was conducted in short (1” diameter x 3” length) and long (1*” x 7” 9).diameter Furnaceconlength) Pt lined Morey type autoclaves’ figuration and temperature measurement and control were as has been described previously and temperatures were measured on the skin of the autoclave by strapped external thermocouples’9). Seeds were either tightly fastened to the seal wafer or suspended on noble metal wires in the growth zone. The growth and dissolving zones were separated by a 5 % open baffle. All starting materials were reagent purity or better except trigonal Se nutrient for the growth study which was crystallized from amorphous Se dissolved in 1.25 M Na 2S by evaporation.
Fig. 1.
Typical temperature variations used in Se growth by cycling.
autoclave top temperature cycling. Fig. 1 shows a typical time trace for our cycling experiments. 3. Results and discussion ~ 1. SOLUBILITY Fig. 2”) shows the dependence of the solubility of trigonal Se upon temperature in 0.75 M (molar) and 1) 1.25 M Na2S. Preliminary solubility measurements’
Se
THE SOLUBILITY OF TRIGONAL
IN
Na2S
193
SOLUTIONS
I.25M Na2S 20
— —
.
-0’ —
1.26
— —
l.24
-
0
18
16
,
I.25M NO2S AMORPHOUS Se
_-
0
0
1.22
~NO2S
1.20 1.18 1.16
4 I-.
TRIGONAL Se I-
1.12
/
~l0 -J D 08. -i C’,
1.14
0
075M NO2S
1.10 0o1.06
~l.04 .02.
o
6 1.00 4.
.98
.96 2
~4.
8
.92 0
I
I
20
30
I
40
50
TEMPERATURE
I
I
I
60
70
80
.9C
(C)
I
28
2.9
3.0
x
I
3.1
I
3.2
3.3
34
~
FIG.2
Fig. 2.
Se solubility vs temperature in 1.25 and 0.75 M Na2S.
indicated that the measured solubilities of amorphous Se were considerably greater. The departure from linearity 2—.at higher temperatures is due to the hydrolysis of s S2+H 2O~±HS+OH (1)
Fig. 3.
Log Se solubility vs reciprocal absolute temperature in 1.25 M Na2S.
30
.
0 40 050
F~25
-j
~20
HS+H2O~±H2S+0H.
(2)
~ ~iis
The hydrolysis reactions proceed further to the right 2
as temperature is increased and the decrease in S concentration due tosolutions hydrolysisaccounting is relativelyfor lessthe in high 2 concentration fact S that the linear region extends to a higher temperature in 1.25 M than in 0.75 M Na 2S. When solubility measurements were made in 10 M NaOH solutions where the Na2S concentrations were 0.75 M, the solubility increased by more than a factor of two, as would be expected, since (OH)— represses eqs. (I) and (2). Fig. 3 shows that the van ‘t Hoff relation is obeyed in 1.25heats M Na2S. Insufficient dataS2is available to calculate of solution at other concentrations, —
-
~
~
0 5 -
0 Fig. 4.
0.25
0.5
0.75
.0
1.25
1.5
.75
2.0
Na2 s MOLARITY Se solubility as a function of Na2S concentration.
The slope of fig. 3 gives a heat of solution of 3.88 kcal/mol in 1.25 M Na2S. 2 Fig. 4 showsAsthecandependence of fig. solubility on Ss/c concentration. be seen from 4 the ratio
194
E. D. KOLB AND R. A. LAUDISE
where s is Se solubility and c is concentration of Na2S
~
is aconstant.Agen:ralized dissolving reaction for Se 2 ~(SenSm)2m. nSe+mS The equilibrium constant for eq. (3) will be
(3)
where[]indicat:s:ctivities. ln terms of s and c, eq. (4) becomes
(4)
s/n =
~—.
___________
~
~
_____ _______
(5)
[c_(m/n)s]m
_____ _______
For lack of activity constant data we are forced to assume that concentrations approximate activities. From the large solubilities measured it is obvious that K 3~l.Sothat c = (m/n)s. (6) Fig. 4 shows that s/c is constant. By the use of appropriate density data solutions Its/cwas in found terms 2 for canNa2S be calculated. of vary molesbetween Se/moles S and 2.21 over the S2 concentrato 1.99 tion range of fig. 4. This indicates that the most probable value for n is 2 and for ,n is 1. Thus the dissol~ing reaction is 2Se-~-S2 ~ (Se 2 (7) 2S) at least between 40 and 50 °Cand up to 1.75 M Na 2S. This is in agreement with the findings of Greiver and 20) who studied the reaction up to 1.46 M Na Zaitseva 2S (presumably) at room temperature. 21) has studied the nature of species present in Ward polysulfide solutions by Raman spectroscopy and postulates (S~2) as most important. By analogy, Se~2is suggested as the predominant species in Se/S2 solutions. The present work indicates that a more reasonable species is (Se 2). The solubility of2S Se in 0.94 M Na 2S as a function of NaC1 concentration was measured, and it was found that Cl has no effect on the solubility of Se. Thus Cl does not appear to act as a chain terminator in aqueous media as it does in the melt.
___
-
-
~
-
3.2.
HYDROTHERMAL CRYSTAL GROWTH
Table 1 lists typical successful conditions for Se growth with a constant temperature differential (AT). Experiments in series (1), (2) and (3) all produced thin
Fig. 5.
6X (a) Needle habit Se crystals. (b) Equiaxial Se crystals. (c) Large Se needles grown by temperature cycling.
THE SOLUBILITY OF TRIGONAL
TABLE
Se
IN
Na2S
195
SOLUTIONS
I
Constant differential growth of Se AT
Growth temperature
*
(1) 145_160* (2) 175
15_40* 25
(3) 160_I75*
15_40*
%
fill
85 85
Solvent
Results
0.025—0.25 M Na2S* 0.025 M Na2S*
small hollow needles medium sized hollow needles
85_90* 0.025 M—0.25 M Na25* 0.5 M—5.0 M NaCl* needles 0—10.0 M LiCl* (4) 175 25 95 0.375 M Na2S blocky crystals When a range of conditions is given this indicates that a number of experiments covering this range were carried out.
TABLE
2
Temperature cycling experiments Run number 1770 1798
~
~~d*
210°C 210
AT (atTm~x~~d) —10 —10
AT
Tmin ~d
(at 175 185
Nucleation
Tmin ~~d)
+10 +17
—
none few well formed needles on baffle
Tma~s~d is the maximum temperature at the top of the vessel in the region where seeds are ordinarily placed. In the cycling runs no seeds were present; AT (at Tma. ~d) is the difference in temperature between dissolving and seed regions of the vessel when the seed region is at maximum temperature. Tmi,, ~ is the minimum temperature at the top of the vessel. *
hollow needles on the seal wafer of the sort shown in fig. 5(a). Experiments in series (4) produced more nearly equi-axial crystals on the seal wafer of the sort shown in fig. 5(b). The habit of fig. 5(b) is more useful for physical studies and would be more likely to lead to sound growth on seeds. The poor results of the experiments of series (1), (2) and (3) show respectively: (I) low temperature, regardless of 52_ concentration produces a poor morphology (2) low solubilities present at low ~2_ concentrations produce a poor morphology (3) C1 is ineffective in modifying morphology as might be expected from its lack of effect on solubility. From the foregoing we conclude that the best hydrothermal conditions for growth are: Crystallization temperature, 175°; % fill, 95; AT, 25; and Solvent, 0.375 M Na2S Seeded growth experiments on (I l~0) and (0001) seeds were conducted and considerable epitaxy occurred. However, due to the relatively poor quality of the melt grown seeds, perfection was poor. It was decided to increase the size of the spontaneously nucleated
seeds by the use of Scholz temperature cycling technique’ 7), The conditions of the cycling runs are given in table 2 and correspond to the traces of fig. 1. All fills were 90 % and the solvent was 0.025 and 0.375 M Na2S. All runs were 12 days. As might be expected, when AT at Tmax = AT at T’,,,~at the seed no growth occurs. When AT at the seed oscillates between —10 and + 17 °Cand the growth and nutrient zones are cycled nucleation occurs at the baffle. Scholz’ 7) has shown that cycling both ends of a system will cause nucleation at a region near the middle. Growth zone temperature was cycled 25°and nutrient 2°as measured by external thermocouples. It is probable that the true internal temperature ofthe nutrient cycled by considerably more than 2°.Some of the needles formed under these conditions had <0001> dimensions (fig. Sc) approaching 1 cm; however, close examination showed them to be hollow. It is surprising that under base concentration conditions where constant temperature differential growth was equiaxial, temperature cycling growth was needle like. Acknowledgments We would like to thank A. J. Caporaso for conduct-
196
F. D. KOLB AND R. A. LAUDISE
ing the hydrothermal growth experiments and D. J. Nitti for assistance in the identification of phases by X-ray diffraction,
10)
11) 12)
References 1) J. P. Kaminow, Quantum Electr., 2QE4-23 (1968). 2) M. C. Teich and T. Kaplan, IEEE J. Quant. Etectr. 2 (1966) 702. 3) J. E. Adams andW. Haas, in: Physics of Selenium and Telluriurn, Ed. W. Charles Cooper (Pergamon Press, 1970) p. 293 if. 4) R. C. Keezer, C. Wood and J. W. Moody, in: Proc. mt. Conf Cryst. Growth, Ed. H. S. Peiser (Pergamon Press, New York, 1967) p. 119 if. 5) D. E. Harrison, J. AppI. Phys. 36 (1965) 1680. 6) T. Stubb, in: Recent Advances in Selenium Physics, Ed.: European Selenium and Tellurium Committee (ESTC) (Pergamon Press, 1965) p. 53 if. 7) F. Eckart, in: Recent Advances in Selenium Physics, Ed.: European Selenium and Tellurium Committee (ESTC) (Pergamon Press, 1965) p. 85 if. 8) C. H. Griffiths and H. Sang, in: Physics of Selenium and Tellurium, Ed. W. Charles Cooper (Pergamon Press, 1967) p. 135 if. 9) G. B. Abdullayev, Y. G. Asadov and K. P. Mamedov, in:
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