Influence of MnO₂ doping on the dielectric and piezoelectric properties and the domain structure in (K_0.5Na_0.5)NbO ₃ single crystals

MnO₂-doped (K_0.5Na_0.5)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 ε_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 T_{O - T}, and Curie temperature T _C, being on the order of 193° and 416°C, respectively. The domain size of [001]-oriented KNN-Mn crystal was found to be on the order of 5-13 μm at room temperature, smaller than that of the domain size observed in the pure KNN crystal (20-30 μm). 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°C, due to the phase transformation.