Phase Equilibria in the Cu-Bi-Se System and Thermodynamic Properties of Copper Selenobismuthates

The Cu-Bi-Se system was studied using DTA, X-ray powder diffraction, and electromotive force (e.m.f.) measurements in (-) Cu (solid)|Cu(4)RbCl(3)I(2) (solid)|(Cu in alloy) (solid) (+) concentration cells in the range 300-430 K. Polythermal sections and a 300-K isothermal section of the phase diagram are constructed, as well as a liquidus surface projection. The existence of ternary compounds CuBi(3)Se(5), CuBiSe(2), and Cu(3)BiSe(3) is verified, and phase fields involving them are established. Primary separation fields and the types and coordinates of invariant and monovariant equilibria are determined. E.m.f. data were used to calculate partial molar functions (Delta(G) over bar, Delta(H) over bar, Delta(S) over bar) of copper in alloys and the standard thermodynamic functions of formation and entropies of the ternary compounds and the end-member (Cu(9)BiSe(6)) of solid solutions based on the low-temperature Cu(2)Se phase.