Raman spectroscopic measurements are carried out to investigate the structural phase transitions as a function of composition in modified sodium niobate [(1‐x) NaNbO3‐xBaTiO3:NNBTx] for x = 0.0 to 0.15 at room temperature. The characteristic antiferroelectric modes at around 93.4 and 123.6 cm−1, along with a mode at 155.5 cm−1 were found to disappear across the structural phase transition from antiferroelectric orthorhombic phase (Pbcm) to ferroelectric orthorhombic phase (Pmc21) for x > 0.02. The redistribution of intensities and positions of the Raman lines in bending (150–350 cm−1) and stretching modes (>550 cm−1) on increasing the concentration x > 0.05, also confirms the occurrence of another phase transition from ferroelectric orthorhombic phase (Pmc21) to another ferroelectric orthorhombic phase (Amm2) phase across x~0.10. The phase transitions as observed from Raman measurements are consistent with previous X‐ray diffraction study.
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 2θ ∼ 30.6°, 32.1° and 34.6° 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.
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