THERMODYNAMIC STUDIES OF THE IN-SE SYSTEM

Thermochemical and X-ray studies of the In-Se system have been perform ed. The temperatures of the invariant equilibria: Liq-Liql-In4Se3; Liq -In4Se3-InSe; Liq-InSe-In6Se7 and Liq-In6Se7-In2Se3, Liq-In2Se3, In2Se 3-Liq-Liq2, and In2Se3-Liq-Se have been determined to be, respectively : 794 K; 831 K; 891 K; 938 K, 1041 K, and 491.6 K, with congruent melt ing of In2Se3 1158.4 K. The experimental studies of the enthalpies of fusion for the four compounds give the following values: Delta H-m,H-I n4Se3= 7.7 kJ mol(-1) Delta H-m.InSe = 9.6, Delta H-m,H-In6Se7 = 9.14 kJ mol(-1), Delta H-f,H-In2Se3 = 0.103 kJ mol(-1) (all per mole of ato ms). A non-negligible homogeneity region at about 929 K has been revea led for In6Se7. The temperatures and enthalpies of the polymorphic tra nsitions of In2Se3 (according to the nomenclature used until now) were determined: 471 K (alpha-->beta), 913 K (beta-->gamma) and 1021 K (ga mma-->delta), respectively, with Delta H-tr(alpha-->beta) = 790 J mol( -1); Delta H-tr(beta-->gamma) = 650 J mol(-1); and Delta H-tr(gamma--> delta) = 190 J mol(-1). New schemes describing the polytypic transitio ns of the In2Se3 have been proposed. It is shown that a hexagonal modi fication a = 0.717 nm, c = 1.941 nm (most known as gamma(H)-In2Se3) is stable at room temperature while at temperatures higher than approxim ate to 823 K it transforms to another hexagonal modification a =0.4025 nn, c = 1.9235 nm (predominantly known as alpha(H)-In2Se3). This one can be quenched to room temperature and both of them produce various o ther metastable modifications on heating or on cooling. The thermodyna mic properties of the four line compounds (In4Se3, InSe, In6Se7 and In 2Se3) were revised using literature sources and our experimental data. It is possible that the InSe (and not In2Se3) is the most stable comp ound in the system indium-selenium. (C) 1998 Elsevier Science S.A.