Electronic Structure and the Chemical Bond in Non-stoichiometric Refractory Compounds of Transition Metals in Sub-groups IVa and Va

Studies of the electronic structure and of the chemical bond in non-stoichiometric binary compounds based on the transition metals of sub-groups IVa and Va are systematically reviewed. The main phenomenological models are examined, and the results of calculations by band and by cluster models of the distribution of electron density are compared; changes in the chemical bond caused by the presence of point vacancies in the metalloid and in the metal sub-lattice are discussed. Calculated results are compared with the results of studies of non-stoichiometric compounds by X-ray emission and photoelectron spectroscopy methods, and with various physicochemical properties. The relative merits of the two approaches (using band and cluster models respectively) in studies of electronic processes associated with the formation of point vacancies in crystals are discussed. 159 references.