First-principles calculations for Fe impurities in KNbO3

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.