Dielectric anomalies of (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 single crystals with the tungsten bronze structure

Tetragonal tungsten bronze (K0.5Na0.5)(0.2)(Sr0.75Ba0.25)(0.9)Nb2O6 single crystals have been investigated by dielectric spectroscopy. Ferroelectric phase transition at about 420 K shows a thermal hysteresis with some diffuseness, implying a first-order nature of the phase transition. Below the transition temperature where domain wall contribution is expected to be dominant in the dielectric susceptibility, logarithmic dielectric dispersion in addition to the polydispersive freezing loss peak has been observed. The former seems to exist in a wide temperature range and may be understood within the framework of relaxations of elastic interfaces in disordered media. Upon further decreasing temperature, very active transverse dynamics were revealed along the directions perpendicular to the polar c axis by dielectric and pyroelectric measurements. The dielectric relaxation rate followed the Arrhenius law, and the distribution of the relevant activation energy has been obtained from the scaling of the temperature- and frequency-dependent dielectric losses.