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 transformat ions, one at around 2 GPa which corresponds to the previously reported tetr agonal 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 ph ase. In the case of polycrystalline samples, the disorder may arise not onl y from off-center positions of Ti atoms as was proposed earlier for single crystals but also due to grain boundaries and intergrain stresses. Raman sp ectra of other known phases of BaTiO3 are also compared for possible contri butions to the breakdown of Raman selection rules at high pressure. [S0163- 1829(98)07645-0].