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First crystallization of arsenic trisulfide from bulk glass: The synthesis of orpiment
Volume
4, number
5.6.7
MATERIALS
July 1986
LETTERS
FIRST CRYSTALLIZATION OF ARSENIC TRISULFIDE THE SYNTHESIS OF ORPIMENT C.Y. YANG, Ikpartmenr
cjj Phvsrcs. Norrh
Received 28 April
We report absorption As,S,.
M.A. PAESLER Carolina
FROM BULK GLASS:
and D.E. SAYERS Stare Unwersrtv
UI Rulergh,
Raletgh. NC 27695-8102.
C’SA
1986
the first laboratory
synthesis of crystalline
fine structure techniques. we first determined
Based on these results we identified
our sample was confirmed
with X-ray
arsenic trisulfide.
As,S,.
from bulk glass. Using extended
X-ray
a thermal cycle that results in a strong tendency to ordermg for glassy
a recipe for. and produced
and Raman experiments.
samples of. crystalline
As,&.
The crystalline
form of
We suggest how our technique might he used for synthesis of
other elusive crystals
The literature abounds [ 1] with studies of the chalcogenide alloy, arsenic trisulfide or As2S3. In its glassy form it is a textbook case for investigations of: glass forming, chemical versus random ordering; twoversus three-fold dimensionality, short- versus longrange order; photostructural changes; and probes of local order and defects. In addition, research is driven by technological interest in the material as an optical memory storage medium. Studies of the crystalline form of As2S3, called orpiment, have been limited to naturally occurring material because of the tendency of As,S, to decompose into realgar (As4S4) and free sulfur via the reaction 2As2S3 + As,S,
+ S, ,
when attempting to crystallize t’he bulk glass. Furthermore. the crystallization kinetics of As,S,, which is an excellent glass-former, are so slow that cooling of the liquid at normal cooling rates results in a glass. That is, it has not been possible to grow crystals from the melt. In addition it has not been possible to produce crystals of As2S3 by vapor transport [2], though a suggestion that very small crystals can be produced has appeared in the lite,rature [3]. Small needle shaped crystals of As,S, have been prepared [4] under pressure from solution in CS, and also hydrothermally, but we know of no successful 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
synthesis of bulk orpiment. In particular, we know of no claims in the literature of the direcr c~stallization of glassy As$,. Using extended X-ray absorption fine structure (EXAFS) we have determined a technique for forming bulk crystals of As,S3. With properly chosen parameters for melting, quenching into ice water and subsequent long slow annealing, we have synthesized orpiment for the first time in the laboratory. These results suggest the possibility of forming other crystalline materials that have eluded laboratory synthesis despite chemical arguments that they could or even should - exist. Perhaps more important is the identification of a new technique for determining with EXAFS a thermal cycle that drives a material into an ordered state while avoiding a lower free energy phase separated state. Samples of 99.999% pure amorphous As2S3 were placed in quartz ampoules, repeatedly evacuated to lO--6 Torr and back-filled with argon, and finally sealed. A family of identically prepared sealed ampoules were heated in a rocking oven to a fictive temperature of 650°C for 24 h and ice-quenched. Each of these ampoules was then annealed to a temperature ranging from 200 to 400°C (call this temperature Ta) for 12 h and ice-quenched before being returned to room temperature. These glass samples of AS,S, as well as samples of realgar and orpiment were 233
Volume 4, number 5,6,7
MATERIALS LETTERS
separately ground to fine powders and placed on Scotch Magic Tape for EXAFS measurements. From the EXAFS experiments, the relative disorder (the temperature-independent component of the Debye-Waller factor, Ao2) was measured using both the ratio [5] and the curve fitting [6] techniques. The results of both methods were in good agreement. In order to avoid the thickness effects (an important source of error in the measurement of the DebyeWaller term), various thicknesses of uniform sample were measured. All of the samples used in this work had an edge step of A(Px) < 1 .l so that the EXAFS does not change for different thicknesses. Measurements were performed at the National Synchrotron Light Source on the X-l IA beam line’with an Si(ll1) monochromator. Crystalline As was placed before a third monochromator to check photon energy shift. A plot of the relative disorder versus the quenching temperature Ta is shown in fig. 1. As a function of TQ, it is clear that Au2 exhibits a minimum at 300°C occurring between the glass transition temperature 7” and the melting temperature T’ (205 + 5 and 3 10 + YC, respectively [7] ). The decrease in Au2 near To = 300°C indicates a strong tendency towards ordering at this temperature. Thkrefore, we chose a sample which had undergone the 65O’C fictive temperature anneal and icequenching cycle, and annealed this sample for 12 days at 300°C. We removed the sample from the oven and allowed it to cool in air to room temperature.
? s
GO
a,
l-
The sample had converted to a yellowish solid with a very small fraction (estimated to be much less than 1% by volume) of red flakes (presumed to be amorphous As2S3) remaining at a few places on the walls of the ampoule. After removal of the few flakes of red material under a microscope, a portion of the remaining sample was ground into a powder to measure the Xray diffraction spectrum in a Debye-Scherrer camera. Sharp lines were observed, and comparison to a spectrum of naturally occurring orpiment, revealed the photographs to be identical. Indeed, there were no missing lines and no additional lines in the picture of the crystallized sample when compared to that of the naturally occurring crystalline specimen. To further confirm that we had indeed synthesized orpiment, we took a Raman spectrum of our crystallized sample and compared it to the known spectrum of naturally occurring orpiment [8]. The results are shown in fig. 2 where we plot on the top the spectrum of our synthesized sample and on the bottom the spectrum of naturally occurring orpiment. The spectrum of our sample has the sharp lines indicative of a crystalline material, and a one-to-one correspondence
Ts L
200
400 Quenching
Temperature,
600 T,P’C)
Fig. 1. Value of the relative disorder Aa2 (the temperatureindependent component of the Debye-Waller factor) versus quenching temperature, Te .for samples of pur_eamorphous As2Ss. All samples were imtlally ice-quenched from a fictive temperature of 650°C. The minimum near Ta = 300 “C indicates a strong tendency to ordering at this temperature. 234
July 1986
Fig. 2. Raman spectrum of naturally occurring (curve B) and synthesized (curve A) crystalline As2Ss. The existence and the sharpness of the features in the top spectrum laboratory confirm piment.
the identification of the synthesized Data of curve B are from ref. [ 81.
material
as or-
Volume 4, number
5,6,7
MATERIALS
between its features and those identified as crystalline Raman-active modes in the naturally occurring sample. From the X-ray and Raman results we concluded that the sample annealed at 3OO’C for 12 days (after a 65O’C fictive temperature anneal and ice-quench) was laboratory synthesized crystalline As,S,. To determine the range of temperatures of Ta for which crystallization will occur, we have annealed samples at 250 and 35O’C for two months (at this writing) and have found no evidence of crystallization. Apparently the acceptable range of temperatures for which crystallization will occur is quite narrow. In other words, the potential barrier between the glassy and crystalline state has a deep and narrow minimum near a value of Ta = 3OO’C. In addition to providing a recipe for preparation of crystalline As,S,, we feel our results have opened the door for the possible synthesis of other crystals. We are currently using our EXAFS technique to determine if Ge,S, might also be synthesized in crystalline form by determining a tendency to ordering manifested as a decrease in the temperature-independent component of the Debye-Waller factor for some specific recipe. There are no known crystalline forms of this material despite the facts that in solution the molecular ion Ge4S6 is known to exist, and a eutectic occurs in the Ge-S phase diagram at this composition [9]. In addition an attempt is being made to synthesize the selenide counterpart of realgar, that is, As4Se4. It is a matter of contention whether or not this material has been synthesized, although this crystalline sister of As4S4 certainly can be expected to be stable.
LETTERS
July 1986
We thank G. Lucovsky for helpful discussions and J.M. Lee for performing the Raman experiments. We also thank Steve Heald for help on the beam line. Research was supported by NSF grant DMR-8407265. We also gratefully acknowledge the support of the Department of Energy under Contract DE-AS05-80ER10742 for the development of the beamline X-l 1 and for partial support of this research.
References [II R. Zallen, The physics of amorphous
solids (WileyInterscience, New York, 1983) and references therein. [21 B.A. Smith, N. Cowlan and A.M. Shamah, Phil. Mag. B39 (1976) 111. [31 C. Bowlt and B.N. Ghosh, Brit. J. Appl. Phys. 16 (1965) 1762. [41 S.A. Dembovskii, Yu.A. Polyakov and A.A. Vaipolin, Izv. Akad. Nauk SSSR Neorg. Mat. 4 (1986) 767. 151 E.A. Stern, D.E. Sayers and F.W. Lytle, Phys. Rev. Bll (1975) 4836. [61 A.D. Cox, in: EXAFS for inorganic systems, eds. C.D. Garner and S.S. Hasnain (Daresbury Laboratory, Daresbury, 1981). (71 M.B. Myers and E.J. Felty, Mat. Res. Bull. 2 (1967) 535. [81 R. Zallen, M.L. Slade and A.T. Ward, Phys. Rev. B12 (1971) 4257. 191 A. Feltz, K. Zickmtiller and G. Pfaff, in: Proceedings of the 7th International Conference on Amorphous and Liquid Semiconductors, eds. E.E. Owen and W.E. Spear (Stevenson, Dundee, 1977) p. 125.
235
4, number
5.6.7
MATERIALS
July 1986
LETTERS
FIRST CRYSTALLIZATION OF ARSENIC TRISULFIDE THE SYNTHESIS OF ORPIMENT C.Y. YANG, Ikpartmenr
cjj Phvsrcs. Norrh
Received 28 April
We report absorption As,S,.
M.A. PAESLER Carolina
FROM BULK GLASS:
and D.E. SAYERS Stare Unwersrtv
UI Rulergh,
Raletgh. NC 27695-8102.
C’SA
1986
the first laboratory
synthesis of crystalline
fine structure techniques. we first determined
Based on these results we identified
our sample was confirmed
with X-ray
arsenic trisulfide.
As,S,.
from bulk glass. Using extended
X-ray
a thermal cycle that results in a strong tendency to ordermg for glassy
a recipe for. and produced
and Raman experiments.
samples of. crystalline
As,&.
The crystalline
form of
We suggest how our technique might he used for synthesis of
other elusive crystals
The literature abounds [ 1] with studies of the chalcogenide alloy, arsenic trisulfide or As2S3. In its glassy form it is a textbook case for investigations of: glass forming, chemical versus random ordering; twoversus three-fold dimensionality, short- versus longrange order; photostructural changes; and probes of local order and defects. In addition, research is driven by technological interest in the material as an optical memory storage medium. Studies of the crystalline form of As2S3, called orpiment, have been limited to naturally occurring material because of the tendency of As,S, to decompose into realgar (As4S4) and free sulfur via the reaction 2As2S3 + As,S,
+ S, ,
when attempting to crystallize t’he bulk glass. Furthermore. the crystallization kinetics of As,S,, which is an excellent glass-former, are so slow that cooling of the liquid at normal cooling rates results in a glass. That is, it has not been possible to grow crystals from the melt. In addition it has not been possible to produce crystals of As2S3 by vapor transport [2], though a suggestion that very small crystals can be produced has appeared in the lite,rature [3]. Small needle shaped crystals of As,S, have been prepared [4] under pressure from solution in CS, and also hydrothermally, but we know of no successful 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
synthesis of bulk orpiment. In particular, we know of no claims in the literature of the direcr c~stallization of glassy As$,. Using extended X-ray absorption fine structure (EXAFS) we have determined a technique for forming bulk crystals of As,S3. With properly chosen parameters for melting, quenching into ice water and subsequent long slow annealing, we have synthesized orpiment for the first time in the laboratory. These results suggest the possibility of forming other crystalline materials that have eluded laboratory synthesis despite chemical arguments that they could or even should - exist. Perhaps more important is the identification of a new technique for determining with EXAFS a thermal cycle that drives a material into an ordered state while avoiding a lower free energy phase separated state. Samples of 99.999% pure amorphous As2S3 were placed in quartz ampoules, repeatedly evacuated to lO--6 Torr and back-filled with argon, and finally sealed. A family of identically prepared sealed ampoules were heated in a rocking oven to a fictive temperature of 650°C for 24 h and ice-quenched. Each of these ampoules was then annealed to a temperature ranging from 200 to 400°C (call this temperature Ta) for 12 h and ice-quenched before being returned to room temperature. These glass samples of AS,S, as well as samples of realgar and orpiment were 233
Volume 4, number 5,6,7
MATERIALS LETTERS
separately ground to fine powders and placed on Scotch Magic Tape for EXAFS measurements. From the EXAFS experiments, the relative disorder (the temperature-independent component of the Debye-Waller factor, Ao2) was measured using both the ratio [5] and the curve fitting [6] techniques. The results of both methods were in good agreement. In order to avoid the thickness effects (an important source of error in the measurement of the DebyeWaller term), various thicknesses of uniform sample were measured. All of the samples used in this work had an edge step of A(Px) < 1 .l so that the EXAFS does not change for different thicknesses. Measurements were performed at the National Synchrotron Light Source on the X-l IA beam line’with an Si(ll1) monochromator. Crystalline As was placed before a third monochromator to check photon energy shift. A plot of the relative disorder versus the quenching temperature Ta is shown in fig. 1. As a function of TQ, it is clear that Au2 exhibits a minimum at 300°C occurring between the glass transition temperature 7” and the melting temperature T’ (205 + 5 and 3 10 + YC, respectively [7] ). The decrease in Au2 near To = 300°C indicates a strong tendency towards ordering at this temperature. Thkrefore, we chose a sample which had undergone the 65O’C fictive temperature anneal and icequenching cycle, and annealed this sample for 12 days at 300°C. We removed the sample from the oven and allowed it to cool in air to room temperature.
? s
GO
a,
l-
The sample had converted to a yellowish solid with a very small fraction (estimated to be much less than 1% by volume) of red flakes (presumed to be amorphous As2S3) remaining at a few places on the walls of the ampoule. After removal of the few flakes of red material under a microscope, a portion of the remaining sample was ground into a powder to measure the Xray diffraction spectrum in a Debye-Scherrer camera. Sharp lines were observed, and comparison to a spectrum of naturally occurring orpiment, revealed the photographs to be identical. Indeed, there were no missing lines and no additional lines in the picture of the crystallized sample when compared to that of the naturally occurring crystalline specimen. To further confirm that we had indeed synthesized orpiment, we took a Raman spectrum of our crystallized sample and compared it to the known spectrum of naturally occurring orpiment [8]. The results are shown in fig. 2 where we plot on the top the spectrum of our synthesized sample and on the bottom the spectrum of naturally occurring orpiment. The spectrum of our sample has the sharp lines indicative of a crystalline material, and a one-to-one correspondence
Ts L
200
400 Quenching
Temperature,
600 T,P’C)
Fig. 1. Value of the relative disorder Aa2 (the temperatureindependent component of the Debye-Waller factor) versus quenching temperature, Te .for samples of pur_eamorphous As2Ss. All samples were imtlally ice-quenched from a fictive temperature of 650°C. The minimum near Ta = 300 “C indicates a strong tendency to ordering at this temperature. 234
July 1986
Fig. 2. Raman spectrum of naturally occurring (curve B) and synthesized (curve A) crystalline As2Ss. The existence and the sharpness of the features in the top spectrum laboratory confirm piment.
the identification of the synthesized Data of curve B are from ref. [ 81.
material
as or-
Volume 4, number
5,6,7
MATERIALS
between its features and those identified as crystalline Raman-active modes in the naturally occurring sample. From the X-ray and Raman results we concluded that the sample annealed at 3OO’C for 12 days (after a 65O’C fictive temperature anneal and ice-quench) was laboratory synthesized crystalline As,S,. To determine the range of temperatures of Ta for which crystallization will occur, we have annealed samples at 250 and 35O’C for two months (at this writing) and have found no evidence of crystallization. Apparently the acceptable range of temperatures for which crystallization will occur is quite narrow. In other words, the potential barrier between the glassy and crystalline state has a deep and narrow minimum near a value of Ta = 3OO’C. In addition to providing a recipe for preparation of crystalline As,S,, we feel our results have opened the door for the possible synthesis of other crystals. We are currently using our EXAFS technique to determine if Ge,S, might also be synthesized in crystalline form by determining a tendency to ordering manifested as a decrease in the temperature-independent component of the Debye-Waller factor for some specific recipe. There are no known crystalline forms of this material despite the facts that in solution the molecular ion Ge4S6 is known to exist, and a eutectic occurs in the Ge-S phase diagram at this composition [9]. In addition an attempt is being made to synthesize the selenide counterpart of realgar, that is, As4Se4. It is a matter of contention whether or not this material has been synthesized, although this crystalline sister of As4S4 certainly can be expected to be stable.
LETTERS
July 1986
We thank G. Lucovsky for helpful discussions and J.M. Lee for performing the Raman experiments. We also thank Steve Heald for help on the beam line. Research was supported by NSF grant DMR-8407265. We also gratefully acknowledge the support of the Department of Energy under Contract DE-AS05-80ER10742 for the development of the beamline X-l 1 and for partial support of this research.
References [II R. Zallen, The physics of amorphous
solids (WileyInterscience, New York, 1983) and references therein. [21 B.A. Smith, N. Cowlan and A.M. Shamah, Phil. Mag. B39 (1976) 111. [31 C. Bowlt and B.N. Ghosh, Brit. J. Appl. Phys. 16 (1965) 1762. [41 S.A. Dembovskii, Yu.A. Polyakov and A.A. Vaipolin, Izv. Akad. Nauk SSSR Neorg. Mat. 4 (1986) 767. 151 E.A. Stern, D.E. Sayers and F.W. Lytle, Phys. Rev. Bll (1975) 4836. [61 A.D. Cox, in: EXAFS for inorganic systems, eds. C.D. Garner and S.S. Hasnain (Daresbury Laboratory, Daresbury, 1981). (71 M.B. Myers and E.J. Felty, Mat. Res. Bull. 2 (1967) 535. [81 R. Zallen, M.L. Slade and A.T. Ward, Phys. Rev. B12 (1971) 4257. 191 A. Feltz, K. Zickmtiller and G. Pfaff, in: Proceedings of the 7th International Conference on Amorphous and Liquid Semiconductors, eds. E.E. Owen and W.E. Spear (Stevenson, Dundee, 1977) p. 125.
235