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. [S0163-1829(98)07645-0].