Influence of MnO2 Doping on the Dielectric and Piezoelectric Properties and the Domain Structure in (K0.5Na0.5)NbO3 Single Crystals

MnO2-doped (K0.5Na0.5)NbO3 (KNN) single crystals were grown by high-temperature solution method using K2CO3-Na2CO3 eutectic composition as flux. The effect of the manganese dopant on the dielectric, piezoelectric properties and the domain structure was investigated. The MnO2-doped KNN crystals were found to exhibit higher piezoelectric coefficient d(33) and dielectric permittivity epsilon(r) when compared with pure KNN crystal, being on the order of 270 pC/N and 730, respectively, for manganese-doped level at 0.5 mol%, with slightly reduced orthorhombic to tetragonal phase transition TO-T, and Curie temperature T-C, being on the order of 193 degrees and 416 degrees C, respectively. The domain size of [001]-oriented KNN-Mn crystal was found to be on the order of 5-13 mu m at room temperature, smaller than that of the domain size observed in the pure KNN crystal (20-30 mu m). The thermal depoling experiments were performed on the [001]-poled KNN-Mn crystals, where the partial depolarization was found to occur at temperatures above 200 degrees C, due to the phase transformation.