Temperature-dependent Raman scattering study of cation-deficient Aurivillius phases: Bi2WO6 and Bi2W2O9

Temperature-dependent Raman scattering experiments were performed on Bi2WO6 and Bi2W2O9. Significant changes in the phonon properties of Bi2WO6 were observed as the temperature was increased due to decreased distortion from the B2cb structure. It was shown that instability of the 57 cm-1 mode that behaved as a soft mode under pressure is not responsible for the Pca21 to B2cb phase transition taking place in Bi2WO6 at 933 K. This result confirmed that this mode is not related to the X2+ tilt mode, which disappears upon change in symmetry from Pca21 to B2cb. Bi2W2O9 does not exhibit any structural phase transition in the 298-800 K range. However, the temperature dependence of Raman bands indicated that the Bi2W2O9 structure evolves with decreasing temperature from 800 to 298 K towards a more symmetric structure that was reported above 2.8 GPa at room temperature. This structural change is driven by displacement of the W atoms and is different from that exhibited by Bi2WO6 and other members of the Aurivillius family but similar to that exhibited by WO3. Our results also show that Bi2W2O9 belongs to the small group of compounds that show the presence of low wavenumber modes characterized by unusually small linewidths. (paper)