Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free (K_0.5 Na_0.5) NbO₃ single crystal

(K_0.5Na_0.5)NbO₃ (KNN) single crystals were grown using a high temperature flux method. The dielectric permittivity was measured as a function of temperature for [001]-oriented KNN single crystals. The ferroelectric phase transition temperatures, including the rhombohedral-orthorhombic T_{R - O}, orthorhombic-tetragonal T_{O - T} and tetragonal-cubic T_C were found to be located at -149  ^{{ring operator}}C, 205 ^{{ring operator}}C and 393 ^{{ring operator}}C, respectively. The domain structure evolution with an increasing temperature in [001]-oriented KNN single crystal was observed using polarized light microscopy (PLM), where three distinguished changes of the domain structures were found to occur at -150  ^{{ring operator}}C, 213 ^{{ring operator}}C and 400 ^{{ring operator}}C, corresponding to the three phase transition temperatures.