THE CUBIC HIGH-TEMPERTURE MODIFICATION OF GALLIUM SULPHIDE (xS = 59 mol %) AND THE T, x-DIAGRAM OF THE Ga – S SYSTEM

Abstract

Recently, in our previous works we have argued that in contrast to traditional views on the Ga–S system, the phase diagram of this system is more sophisticated in the regions of pre-melting temperatures and concentrations between 50 and 60 mol % of sulphur. When using two independent methods of thermal analysis (DTA and ChTA), it was argued that in a narrow temperature range (877–922°C) there is a new phase (σ-phase) with a sulphur content of ~ 59.0 mol %. Objective. The goal of this work was to provide direct evidence of the existence of the high-temperature phase in the Ga–S system (σ-phase). Another task was to construct the the Ga–S phase diagram in the concentration range from 30.0 to 60.7 mol % S. Methods and methodology. A high speed quenching procedure and further structural analysis with XRD (Empyrean B. V.) and transmission electron microscopy (TEM: FEI Teсnai G2 F20 S TWlN) were used to solve the tasks. The high-temperature XRD (Thermo ARL X’TRA ) was also applied. The T-x diagram of the Ga–S system was verifi ed with the help of the thermal analysis techniques: DTA and ChTA. Results. It was found that the σ-phase has a defective zinc-blende structure (a = 5.210 Å according to XRD and 5.217 Å according to TEM). Out of the range of stability, this phase easily decomposes to the heterogeneous composite of monoclinic Ga2S3 (Cc, a = 11.14 Å, b = 6.41 Å, c = 7.04 Å, β = 121.22°) and layered hexagonal β-GaS (P63/mmc, a = 3.59 (1) Å, c = 15.47 Å). The obtained results were verifi ed with the use of the high-temperature XRD and chromathermographic analysis. Unfortunately, substantial vapour pressure over the GaS–Ga2S3 alloys at high temperatures does not make it possible to obtain the XRD-data for above 800 °C. At 800 °C and below only GaS and Ga2S3 phases were detected in the diffraction patterns. There is a small change in the calculated lattice parameters of monoclinic Ga2S3 with a change in temperature. The lattice constant a for hexagonal GaS also shows insignifi cant changes with the change of temperature: from 3.580(9) at 25 °C to 3.603(9) Å at 800 °С. On the contrary, the lattice constant c for GaS increases more with temperature: from 15.50(1) at 25 °С to 15.62(1) Е at 800 °C. The detected weak temperature changes in the lattice constants for Ga2S3 and GaS are characteristic of such rigid structures as sulphides of light elements. A notable increase in the lattice parameter c for GaS with increasing temperature that is seen to be an explicit exception to that rule is well explained by the layered structure of gallium monosulphide. For that structure the increase in the c parameter with temperature is connected with the growth of van der Waals gaps (i. e. with the growth of the distance between the S–Ga–Ga–S packets). Conclusion. Therefore, we proved that the σ-phase exists as thermodynamically stable phase in a narrow temperature and concentration range. However, it can be quickly quenched and can be characterized by structural methods as a cubic zinc-blende-type phase. Considering the fact that the σ-phase in the Ga–S system and the In3–xS4-phase are similar in composition (molar sulphur content), differences in these structures are also discussed.

Keywords

• gallium sulfi des
• phase diagram
• gallium sesquisulfi de
• zinc-blende structure structures.