CHARGE-TRANSFER IN MULTICOMPONENT OXIDES

The transfer of charge between different ions in an oxide plays an ess ential role in the stability of these compounds. Since small variation s in charge can introduce large changes in the total energy, a correct description of this phenomenon is critical. In this work, we show tha t the ionic charge in oxides can strongly depend on its atomic environ ment. A model to assign point charges to atoms as a function of their atomic environment has recently been proposed for binary alloys [C. Wo lverton, A. Zunger, S. Froyen, and S.-H. Wei, Phys. Rev. B 54, 7843 (1 996)] and proven to be very successful in screened solids such as semi conductors and metals. Here, we extend this formalism to multicomponen t oxides and we assess its applicability. The simple point-charge mode l predicts a linear relation between the charge on an atom and the num ber of unlike neighbors, and between the net value of the charge and t he Coulomb field at a given site. The applicability of this approach i s tested in a large-supercell self-consistent tight-binding calculatio n for a random Zr-Ca-O alloy. The observed fluctuations of the ionic c harge about the average linear behavior (as a function of the number o f unlike neighbors) was larger than 0.25 electrons even when many shel ls of atomic neighbors were considered in the fit. This variation is s ignificant since it can introduce large errors in the electrostatic en ergy. On the other hand, for small absolute values of the charge, the ionic charge varied linearly with the Coulomb held, in agreement with previous findings. However, for large Coulomb fields, this function sa turates at the formal chemical charge. [S0163-1829(98)04207-6].