  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.
