Condensed-Phase Transitions during Effusion of Gallium Selenide

Abstract

An effusion study of gallium sesquiselenide, Ga2Se3, was conducted by simultaneous Knudsen effusion and torsion (Volmer) effusion in the range 1105−1289 K for the purpose of testing a theory of equilibrium condensed-phase transitions1 in the presence of the vapor and of improving, on that basis, understanding of this important substance at high temperatures. Below 1225 ± 10 K a phase nominally Ga2Se·Se2(s) effused congruently, principally as Ga2Se(g) and Se2(g), and above that temperature a phase nominally (Ga2Se)1+x·(Se2)1-x(l), x = 0.10, effused congruently as the same species; at 1225 ± 10 K the vapor pressure was 14 ± 1 Pa. Detailed observations of chemical events during the transition were made. A quadruple point involving GaSe(s) was found at 1155 ± 5 K and 5 ± 1 Pa. On plots of log P vs 1/T data fell in an envelope bounded above by curved three-phase lines in projection and below by straight two-phase lines, in conformity with theory. The ΔH°(298 K) of congruent vaporization of 1 mol of Ga2Se3(s) to give Ga2Se(g) and Se2(g) was 649 ± 3 kJ.