High-pressure Raman studies of polycrystalline BaTiO₃

We report Raman-scattering studies of polycrystalline BaTiO₃ under pressure up to 8.6 GPa. Our data show evidence for two structural phase transformations, one at around 2 GPa which corresponds to the previously reported tetragonal to cubic phase transition and a new one near 5 GPa. The persistence of strong Raman lines well beyond the tetragonal to cubic phase transition suggests the presence of significant disorder in the high-pressure cubic phase. In the case of polycrystalline samples, the disorder may arise not only from off-center positions of Ti atoms as was proposed earlier for single crystals but also due to grain boundaries and intergrain stresses. Raman spectra of other known phases of BaTiO₃ are also compared for possible contributions to the breakdown of Raman selection rules at high pressure.