Nonstoichiometry in steps on AB crystals (I). General aspects, including bulk imperfections

Starting with a quasichemical treatment, simple model calculations are given for the equilibrium composition in surface steps containing A and B kinks, the amount and ratio of which is varied by changing the adjacent gas composition. In connection with deviations from step stoichiometry, the total amount of kinks increases strongly. A nearest neighbour approximation has been applied to the attachment of A or B atoms at kinks from the gas phase, and also from the bulk by formation of vacancies V A or V B . These processes can be formulated separately for the two elements, taking into account the effective formation of bulk atoms of the other kind (B B or B A ).