It was recalled that defects in ionic solids were often modelled by using the crystal-field approximation; which ignored the electronic structure of the host crystal. This model had limited applicability to diffuse states which extended into a bulk crystal. In order to study host electronic structure effects, the crystal-field model was extended so as to include a position-dependent effective mass. The electronic problem could be stated so that the effective-mass correction appeared as a state-dependent potential that was proportional to the local kinetic energy. This effective potential could be attractive or repulsive; depending upon the ratio of the effective mass to the actual mass, and upon whether the electronic wave function was oscillatory or damped. In the case of alkali halides, for which the above ratio was less than unity, it was found that the effective mass led to more deeply bound defect ground states while excited states were less deeply bound relative to the crystal-field approximation. It was concluded that this kinetic effect could be significant in the case of weakly-bound defects such as the F’ center.

D.Y.Smith, M.S.Malghani: Materials Science Forum, 1997, 239-241, 369-72