A SEMI-EMPIRICAL CORRLATION OF EQUILIBRIUM PROPERTIES FOR NONSTOICHIOMETRIC TRANSITION METAL + SULFUR SYSTEMS

Transition metal sulfides have received much interest in their thermodynamic properties essential for understanding the nature of phase transformation. The order-disorder transformation of the vanadium + sulfur system has been studied by several investigators. However, its experimentally observed equilibrium properties cannot be adequately correlated.

In this paper, a statistical thermodynamic approach to the equilibrium properties of these nonstoichio-metric compounds was attempted by considering three kinds of interaction energies among various vacancy-vacancy and metal-vacancy pairs. Both composition- and temperature-dependences of the short-range interlayer interaction energy for metal-vacancy pairs were taken into account to fit the experimental data.

By applying the proposed semi-empirical model, equilibrium sulfur vapor pressures of the vanadium-sulfur system at seven temperatures, those of the niobium-sulfur system at four temperatures and those of the tantalum-sulfur system at three temperatures were calculated for a wide range of nonstoichiometric compositions. Results were compared to corresponding experimental data with good agreements.