High-pressure Raman study of a relaxor ferroelectric:: The Na0.5Bi0.5TiO3 perovskite -: art. no. 174106

We report high-pressure Raman measurements (up to 19 GPa) on the perovskite-type relaxer ferroelectric sodium-bismuth-titanate. Na0.5Bi0.5TiO3 (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 NET. Raman spectroscopy is shown to be an effective technique to investigate the pressure-dependent behavior in relaxor Ferroelectrics.