A Raman scattering study of pressure-induced phase transitions in nanocrystalline Bi2MoO6

Lattice dynamics calculations and a high-pressure Raman scattering study of nanocrystalline Bi2MoO6, a member of the bismuth-layered Aurivillius family of ferroelectrics, are presented. These studies showed the onset of two reversible second-order or weakly first-order phase transitions near 2.5 and 4.5 GPa as well as some subtle structural changes at 8.2 GPa. Symmetry increases upon application of pressure and the first phase transition involves, most likely, the loss of the MoO6 tilt mode around a pseudo-tetragonal axis. The second phase transition is associated with the instability of a low wavenumber mode, which behaves as a soft mode. This soft mode most likely corresponds to the polar E-u mode of the tetragonal I4/mmm aristotype and Bi2MoO6 transforms at 4.5 GPa into the centrosymmetric orthorhombic phase. The sequence of the pressure-induced phase transitions in nanocrystalline Bi2MoO6 is similar to that observed for bulk Bi2WO6 but the critical pressures are significantly lower for the molybdenum compound. Our results also show that the critical pressure of the first phase transition is slightly lower for the nanocrystalline Bi2MoO6 (2.5 GPa) than for the microcrystalline (bulk) Bi2MoO6 (2.8 GPa).