The T-x Diagram of the Ga – Se System in the Composition Range from 48.0 to 61.5 mol % Se According to Thermal Analysis Data

Abstract

Purpose. In this study we discuss the Ga – Se and Ga – S systems, which have very similar shaped
phase diagrams, as well as the structures and properties of their binary phases. With this consideration
in mind, we anticipated discovering the selenium analogue of the sulphide cubic hightemperature
phase Ga₂+dS₃. This recently detected phase (the so-called s-phase) exists in the Ga
– S system in a narrow range of temperatures (~878 – 922 °C) and compositions (approximately
59 mol % S). Additionally, we found it necessary to check the other phase relationships in the Ga
– Se system. The goal of this work was to conduct a thermographic study (DTA) of the Ga – Se
T-x diagram in the concentration range from 48.0 to 61.5 mol % Se and temperature range from
500 to 1100 °С and to fi nd out whether there is a selenium analogue of the sulphide s-phase.*
Methods and methodology. 40 alloys of the Ga – Se system with various compositions were
investigated. All the alloys were prepared by the fusion of weights of the metallic gallium and
selenium in vacuumed and sealed quartz ampoules for 3 hours at 1100 °С. The ingots, weighing
from 1 to 4 kg, were quantitatively transferred to Stepanov’s vessels, then vacuumed and sealed.
Thermographs were obtained for the temperature range from 0.9 to 4.2 К/min with a temperature
variation rate from 500 °C to 1100 °C.
Results. Based on the results of the differential thermal analysis, the T-x diagram of the Ga - Se
system was studied in the concentration range from 48.0 to 61.5 mol % Se. The congruent melting temperatures of the g-GaSe and Ga₂Se₃ phases (944 ±2 and 1010 ±2 °С correspondingly), the
temperature of the monotectic reaction L₂ = L₂ + e-GaSe (916 ± 2 °С), as well as the coordinates
of the eutectic point between g-GaSe and Ga₂Se₃ (888 ± 2 °С, composition ~55.5 mol % Se), correlate
well with the reported data. The novelty of the results is that we obtained evidence of the
retrograde solidus of the g-GaSe phase on the part of selenium (with a homogeneity region of
several tenths of mol % at temperatures above the eutectic) and evidence of the independent
existence of the compositionally similar e-GaSe and g-GaSe phases. In this case, the richer gallium-
enriched phase e-GaSe peritectically melts with the formation of a liquid (L2) and solid
g-GaSe (921 ±2 °С). However, at this stage no evidence in favour of the existence of the hightemperature
modifi cation type such as Ga₂+dSe₃ was obtained

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