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.