Phase Transitions and Properties of Doped KNbO3 Crystals

Single crystals of KNbO3 doped with ions of various valency are grown and studied. The electrical conductivity, thermoelectric power, and optical absorption of these crystals are found to be characteristic of oxide semiconductors of controlled-valency type. They may be explained by two different processes: excitation of electrons from donor centres to the extended states in conduction band and hopping of the localized charge carriers (small-radius polarons). Heterovalent substitutions affect also the dielectric properties of KNbO3. The permittivity data in paracentric region fit well to the Curie-Weiss law but the constants C and T0 can be changed to a great extent. The experimental data are analyzed in terms of thermodynamics taking into account the influence of electronic defects on the crystal polarization.