A SEMIEMPIRICAL CORRELATION OF EQUILIBRIUM PROPERTIES FOR NONSTOICHIOMETRIC TRANSITION-METAL PLUS SULFUR SYSTEMS

Transition metal sulfides have received much interest in their thermod ynamic properties essential for understanding the nature of phase tran sformation. The order-disorder transformation of the vanadium + sulfur system has been studied by several investigators. However, its experi mentally observed equilibrium properties cannot be adequately correlat ed. In this paper, a statistical thermodynamic approach to the equilib rium properties of these nonstoichiometric compounds was attempted by considering three kinds of interaction energies among various vacancy- vacancy and metal-vacancy pairs. Both composition- and temperature-dep endences of the short-range interlayer interaction energy for metal-va cancy pairs were taken into account to fit the experimental data. By a pplying the proposed semi-empirical model, equilibrium sulfur vapor pr essures of the vanadium-sulfur system at seven temperatures, those of the niobium-sulfur system at four temperatures and those of the tantal um-sulfur system at three temperatures were calculated for a wide rang e of nonstoichiometric compositions. Results were compared to correspo nding experimental data with good agreements.