THE ROLE OF METAL-METAL BONDS IN THE FORMATION OF REFRACTORY TRANSITION-METAL PHOSPHIDES AND SULFIDES

摘要

There are three stability requirements that must be met by solid compounds in order that they play an important role in high-temperature systems at equilibrium: 1. they must be relatively stable with respect to loss to the vapor, i.e. the partial pressures of the vapor species must be sufficiently low that the loss of species to the vapor is negligible, 2. they must be stable with respect to melting and 3. they must be stable with respect to disproportionation. Under inert gas or vacuum conditions the first condition means in particular that the partial pressure of the nonmetal must be sufficiently low that the material does not decompose appreciably through loss of nonmetal to the vapor phase. While binary compounds, M_xA_y, exist with y/x ranging to two and greater in compounds such as the di- and tri-sulfides and the po1yphosphides, those sulfides and phosphides of the early transition-metals that are important in high- temperature (T> 1500K) systems in vacuo are more metal-rich and contain metal-metal bonds that contribute significantly to the cohesive energy of the solids. Although such bonding is not essential to the formation of a material that does not lose nonmetal to the vapor phase at a rapid rate, as can be seen by the fact that the alkaline earth sulfides exhibit, relative to many other sulfides, very low sulfur pressures, it is clear that the ability of the early transition metals to form metal-metal bonds increases the range of formation of compounds "stable" at high temperatures toward the metal-rich region.