Noncentrosymmetric phase of submicron NaNbO₃ crystallites

The temperature and pressure characteristics of a noncentrosymmetric crystal modification of NaNbO₃ were studied by Raman spectroscopy. A transition towards the bulk-like structure of NaNbO₃ occurs in the temperature range from 280 to 360 °C. High-pressure Raman spectroscopy revealed successive pressure-induced phase transitions at around 2, 6.5 and 10 GPa. Raman scattering profiles recorded above 7 GPa correspond to those reported for the bulk. The temperature-induced spectral changes were completely reversible between −150 and 450 °C. Those induced by pressure were almost reversible from ambient pressure up to 15.9 GPa. Piezoresponse force microscopy demonstrated the occurrence of piezoelectric activity for submicron NaNbO₃ crystals with particle size ranging from 200 to 400 nm. The noncentrosymmetric crystallographic structure plays a critical role for the enhancement of piezoelectricity.