Temperature-induced Phase Transitions in Micro-, Submicro-, and Nanocrystalline NaNbO3

Phase transitions in micro-, submicro-, and nanocrystalline NaNbO 3 were investigated by temperature-tuning Raman spectroscopy and X-ray powder diffraction method. Three powders with different average particle size showed successive phase transitions within the measured temperature range from −150 to 450 °C. The temperature characteristics of Raman active phonons in microcrystalline NaNbO 3 corresponded the one reported for bulk NaNbO 3, which transforms with increasing temperature from the ferroelectric N into the antiferroelectric P phase and finally above 373 °C ( T m3 ) into the antiferroelectric R phase. Submicrocrystalline NaNbO 3, which takes the noncentrosymmetric orthorhombic Pmc 2 1 structure at room temperature, transformed into a pseudocubic structure at 333 °C ( T s3 ). Nanocrystalline NaNbO 3 showed a diffused phase transition from an orthorhombic Pmma structure to a high-temperature phase at around 180 °C ( T n 2 ). For micro- and submicrocrystalline NaNbO 3, hysteretic phase transition behavior was found for the temperature characteristics of specific phonons. On the other hand, the characteristics obtained for nanocrystalline NaNbO 3 were much more diffused and did not show any hysteretic effect. Crystal structure refinements of the X-ray powder diffraction patterns using the Rietveld method demonstrated a hysteretic deformation of the a − b plane for microcrystalline NaNbO 3 around T m3 and of the b − c plane for submicrocrystalline NaNbO 3 around T s3 . The temperature dependence of the primitive perovskite volumes showed a very small hysteresis for microcrystalline NaNbO 3 but a clear one for submicrocrystalline NaNbO 3 . Lattice distortion of the submicrocrystalline Pmc 2 1 structure from a cubic perovskite lattice induced a particularly large contraction of parameter c around T s3 with increasing temperature, which resulted in a decrease of the primitive cell volume. This transition showed a first-order type character, which may relate to a ferroelectric−antiferroelectric transition. Rearrangement of the NbO 6 octahedra induces a transition from an orthorhombic into a pseudocubic structure.