ELECTRIC-FIELD OF AN ISOLATED CHARGED IMPURITY IN AN IONIC-CRYSTAL

The character of the behavior of the electric field created by a charg ed impurity in an ionic crystal is studied on the basis of both cluste r and analytical approaches. In the cluster approach about 30 000 ions surrounding the impurity are taken into account. These ions are descr ibed in a model of polarizable sites. A direct calculation shows that the asymptote of the electric field of a charged impurity at lattice p oints can differ strongly from the one given by the Coulomb equation w ritten for a homogeneous polarizable medium. The behavior of the elect ric field at intermediate distances, where the asymptotic behavior can not yet be used, is studied in detail. It is found that the electric f ield is increased significantly in comparison to the Coulomb field in the region near the defect. The size of this (strongly polarized) regi on increases as the dielectric constant increases. These data are in q ualitative agreement with the results obtained by Vikhnin et al. and a ccount for the results of recent experiments designed to investigate p olarization in reduced virtual ferroelectrics. (C) 1998 American Insti tute of Physics.