We report high-pressure Raman measurements (up to 19 GPa) on the perovskite-type relaxor ferroelectric sodium-bismuth-titanate, ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$ (NBT). Distinct changes in the Raman spectra have been analyzed in the light of a rhombohedral-to-orthorhombic $(R3c-\mathrm{t}\mathrm{o}-\mathit{Pnma})$ 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 $\left(\right[111{]}_p\to \left[010{]}_p\right)$ takes place, while at least one other change, i.e., B-site cation displacement $\left(\right[111{]}_p\to \left[000\right])$ or the tilt change ${(a}^{-}{a}^{-}{a}^{-}\to a^{-}{b}^{+}{a}^{-}),$ 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 ${\mathrm{cm}}^{-1}.$ It is suggested that a change in the polar character of nanosized ${\mathrm{Bi}}^{3+}{\mathrm{TiO}}_3$ and ${\mathrm{Na}}^{1+}{\mathrm{TiO}}_3$ 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.

• Received 29 August 2000

DOI:https://doi.org/10.1103/PhysRevB.63.174106