Semi-empirical calculations of hole polarons in MgO and KNbO₃ crystals

The semi-empirical quantum chemical INDO method has been used for cluster and large unit cell calculations of hole polarons bound to a cation vacancy in highly ionic MgO and partly covalent perovskite KNbO₃. In both cases a hole is well localized on an oxygen atom displaced towards the vacancy. The calculated optical and thermal ionization energies for V^- and V⁰ centers are in excellent agreement with experimental data for MgO. In KNbO₃ we predict the existence of one-site and two-site (molecular) polarons with close absorption energies (≈1 eV). The relevant experimental data are discussed.