  Nb-substituting Fe impurity in KNbO3 is studied in first-principles supercell
calculations by the linear muffin-tin orbital method. Possible ways to account
for the impurity charge compensation are discussed. Calculations are done in
the local density approximation (LDA) and, for better description of Coulomb
correlation effects within the localized impurity states, also in the LDA+U
scheme. The achievements and problems encountered in both approaches are
analyzed. It is found that the impurity possess either a low-spin configuration
(with 0 or 1 compensating electron), or a high-spin configuration (with 2 or 3
compensating electrons), the latter two apparently corresponding to practically
relevant rechargeable impurity states.

Materials Science