Lattice dynamics and pressure-induced phase transitions in Bi2W2O9: High-pressure Raman study

Lattice dynamics calculations and high-pressure Raman scattering study of Bi2W2O9, which is an m=2 member of the cation-deficient bismuth layered Aurivillius family of compounds, are presented. These studies showed the onset of two reversible second-order phase transitions near 2.8 and 4.8 GPa. The pressure dependence of Raman bands provides strong evidence that the first phase transition involves the loss of the WO6 tilt mode around pseudotetragonal axis. The second transition is likely related to some shift of the W atoms and/or tilting of the WO6 octahedra plus shift of the Bi atoms or the whole Bi2O2 layers. In contrast to the m=1 cation-deficient bismuth layered compounds, Bi2WO6 and Bi2MoO6, no soft-mode behavior was observed for Bi2W2O9 and the transition at 4.8 GPa leads to symmetry lowering.