Raman scattering study of the high temperature phase transitions of NaTaO3

The phase transition sequence of sodium tantalate (NaTaO3) ceramic samples has been investigated by Raman spectroscopy, from room temperature up to 640 °C. At room temperature, NaTaO3 shows an orthorhombically distorted perovskite structure belonging to the Pbnm space group. Raman and infrared vibrational spectra at 20 °C are in relative good agreement with group theory predictions for this group. At high temperatures, NaTaO3 shows three phase transitions, in the sequence Pbnm (∼450 °C) Cmcm (∼560 °C) P4/mbm (∼620 °C) Pm3¯m. The Raman spectra for the two intermediate high temperature phases showed a decreasing of the number of the active modes, accompanied by band broadening, and were well compatible with structural symmetry increasing at higher temperatures.