Evidence for existence of Functional Monoclinic Phase in Sodium Niobate based Solid Solution by Powder Neutron Diffraction

We have carried out systematic temperature-dependent neutron diffraction measurements in conjunction with dielectric spectroscopy from 6 to 300 K for sodium niobate based compounds (1-x) NaNbO3 -xBaTiO3 (NNBTx). The dielectric constant is measured both as a function of temperature and frequency. It shows an anomaly at different temperatures in cooling and heating cycles and exhibits a large thermal hysteresis of 150 K for the composition x=0.03. The dielectric constant is found to be dispersive in nature and suggests a relaxor ferroelectric behavior. In order to explore structural changes as a function of temperature, we analyzed the powder neutron diffraction data for the composition x=0.03 and 0.05, respectively. Drastic changes are observed in the powder profiles near 30.6{\deg}, 32.1{\deg} and 34.6{\deg} in the diffraction pattern below 200 K during cooling and above 190 K in heating cycles, respectively.The disappearance of superlattice reflection and splitting in main perovskite peaks provide a signature for structural phase transition. We observed stabilization of a monoclinic phase (Cc) at low temperature. This monoclinic phase is believed to provide a flexible polarization rotation and considered to be directly linked to the high performance piezoelectricity in materials. The thermal hysteresis for composition x=0.03 is larger than x=0.05. This suggests that addition of BaTiO3 in NaNbO3 suppresses the thermal hysteresis. It is also observed that the structural phase transition temperature decreases on increasing dopant concentration.