Non-Stoichiometry and Bonding in Refractory Monocarbides

Abstract

The refractory monocarbides with the sodium-chloride type structure have a wide homogeneity range. Equations have been derived from statistical considerations to give the activity of each component as a function of composition and temperature based on the interaction energies between the various components. From the activity measurements as a function of composition in various binary carbides and nitrides of the Groups IV and V transition elements and in the ternary zirconium-niobium-carbon system, the various interaction energies were calculated. The metal to metal interaction energy is the same in the carbides as it is in the pure metals and metallic solutions; therefore, the metal to metal bonding is the same in pure metals and in the carbides. The high melting point and other refractory properties of the carbide arise from the metal-carbon bond. The metal-carbon bond is stronger in Group IV carbides than in Group V carbides. The metal-carbon bond energy decreases as the atomic weight of the metal increases. The experimental results also indicate that the metal-carbon bond is such that the carbon atom is ionized--the ionization increasing with the bond strength.