High-pressure Raman study of a relaxor ferroelectric: The Na_0.5Bi_0.5TiO₃ perovskite

We report high-pressure Raman measurements (up to 19 GPa) on the perovskite-type relaxor ferroelectric sodium-bismuth-titanate, Na_0.5Bi_0.5TiO₃ (NBT). Distinct changes in the Raman spectra have been analyzed in the light of a rhombohedral-to-orthorhombic (R3c-to-Pnma) phase transition. Results show that this transition, involving a change in the tilt system and the cation displacement, does not occur in a single step, but goes through an intermediate phase (2.7 to 5 GPa). The frequency evolution of characteristic bands in the Raman spectra allows us to propose a scenario where in the early stage of the transition a change in the A-cation displacement ([111]_p→[010]_p) takes place, while at least one other change, i.e., B-site cation displacement ([111]_p→[000]) or the tilt change (a^-a^-a^-→a^-b⁺a^-), appears to happen only at higher pressures. A pressure-induced breakdown of the Raman intensity, preceding the phase transition, has been observed for the bands at 135 and 275 cm^-1. It is suggested that a change in the polar character of nanosized Bi³⁺TiO₃ and Na¹⁺TiO₃ clusters is at the origin of this observation, being, in fact, the signature of a pressure-induced relaxor-to-antiferroelectric crossover in NBT. Raman spectroscopy is shown to be an effective technique to investigate the pressure-dependent behavior in relaxor ferroelectrics.