THE PHASE DIAGRAM OF THE Ga–S SYSTEM IN THE CONCENTRATION RANGE FROM 48.0 TO 60.7 MOL.% S
Abstract
Almost all binary systems composed of non-radioactive s- and sp- elements have been studied in great detail. With the exception of small details (features of non-stoichiometry of intermediate phases, liquidus line details, etc.), for almost all such systems, the corresponding T-xphase diagrams are consistently described in the literature as well as most of the P-T diagrams. In light of the foregoing, the phase diagram of the Ga – S system causes an obvious bewilderment, according to which the available literature is consistent only with the presence of GaS Ga2S3 phases congruently melting above 950 °C and eutectic between them. Thus, in widely quoted works [1, p. 106] and [2, p. 138], the intermediate solid phases mentioned are considered to be single for the given system. A completely different representation of the phase diagram is given in [3], where incongruently melting Ga2S and Ga4S5 are indicated as intermediate phases along with GaS and Ga2S3. Although several decades have passed since the publication of [3], a modern diagram of the data is given in this modern reference database.
At the same time, the questionability of some of [3] the results is manifested in the absence of any structural evidence for the Ga2S and Ga4S5 phases. The purpose of this paper is to investigate the phase relationships in the Ga – S system for the possibility of the solid phase formations with intermediate stoichiometries between GaS and Ga2S3 (for example, Ga4S5) at temperatures up to 1150 °C.
As the main method of studying the T-x phase diagram, we chose differential thermal analysis (DTA). Taking into account the propensity of the phases of the A(III)B(VI) systems to the formation of metastable states, we used equipment [4] that allows to maintain low heating rates (<1 K / min) with high linearity of temperature changes in time and high frequency of transmission of temperature Data to the computer (1 poll 1 thermocouple per 1 s). The DTA data were compared with the results of the new method of chromatothermographic analysis (CTA) [5], which allows to carry out research in the static mode, as well as with the data of high-temperature X-ray phase analysis (HT XRD).
By the DTA method, 40 samples of 24 different compositions were examined, which made it possible to obtain a fragment of the T-x diagram of the Ga – S system at a concentration interval of 48,0 – 60.7 mol % S at temperatures from room temperature to 1150 °C. The results of the experiments are presented in Fig. 1.
According to the set of available contours and the number of existing and coexisting phases, the constructed T-x diagram is in contradiction with the data [1, 2] and has a remote similarity with [3] and it is most correlated with the results of little-known works by Pardo [6, 7].
It is found that, in contrast to the low-temperature part of the diagram, where only Ga2S3 and GaS phases are present, the high-temperature part (870 – 1110 °C) of this T-x diagram is complicated. It is substantiated that in a narrow range of compositions (59.0 – 60.7 mol % S) there are three separate phases (σ, Ga2S3', Ga2S3) of different stoichiometry. These solid phases are connected with each other, as well as with gallium monosulfide, by peritectoid and eutectoid transformations.
It is shown that the eutectic equilibrium, which was previously associated with the reaction
GaS + Ga2S3 ⇄ L , (1)
in fact corresponds to the eutectoid equilibrium
GaS + Ga2S3 ⇄ σ, (2)
which is realized at t » 880°С, where s is a solid phase with a sulfur content of about 59.0 mol %, and Ga2S3 is a low-temperature modification of Ga2S3. Actually, the eutectic equilibrium corresponds to the reaction
GaS + L ⇄ σ (3)
and has a temperature of 909,5 ± 2.2 °С.
In order to define two closely spaced contours (909.5 and 921.6 °C), special techniques have been developed, which are discussed in this paper. These methods made it possible to establish that the s-phase decays at a temperature of about 922 °C according to the peritectic reaction
σ ⇄ L + Ga2S3', (4)
where Ga2S3' is a high-temperature modification of gallium sesquisulfide. Thus, the detected s-phase with a sulfur content of about 59.0 mol % exists in a very narrow temperature range (877 – 922 °C).
The Ga2S3 phase formed by the last of these reactions is obtained from the low-temperature modification of Ga2S3 and the σ-phase by eutectoid transformation at ~ 911 °С:
σ + Ga2S3 ⇄ Ga2S3'. (5)
The high-temperature modification of Ga2S3' is enriched with gallium compared with the low-temperature one, both phases coexisting in the temperature range of ~ 911 – 1006 °C. When the final temperature is reached, the low-temperature form of Ga2S3 melts peritectically:
Ga2S3 ⇄ Ga2S3' + L. (6)
The high-temperature phase of Ga2S3' exists in the temperature range 911 – 1109 °C and when the upper value of the interval is reached, it melts congruently. The gallium monosulfide GaS (~ 969 °C) also melts congruently.
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References
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