Effects of atomic and vacancy ordering in nonstoichiometric carbides

An analysis of the electronic structure of highly nonstoichiometric transition-metal carbides in both ordered and disordered states is given, which explains the influence of atomic and vacancy order on the physical properties of the carbides. It is shown that the dependence of carbide properties on the arrangement of carbon atoms on the nonmetal sublattice is related to both the crystal symmetry space group and the symmetry of the nearest-neighbour environment of the carbid atoms. By considering order-disorder phase transition data, experimental information on atomic and vacancy order in carbides is obtained in order to explain the nature of superstructures at high vacancy concentrations. 161 refs.; 21 figs.; 6 tabs