Phonon-instability-driven phase transitions in ferroelectric Bi2WO6:Eu3+:: High-pressure Raman and photoluminescence studies

High- pressure Raman scattering and luminescence studies of Bi2WO6: Eu3+, which is a member of the bismuth layered Aurivillius family of ferroelectrics, are presented. These studies showed the onset of two reversible second order phase transitions near 3.4 and 6.2 GPa. The pressure dependence of Raman bands provides strong evidence that the first phase transition involves the loss of the WO6 tilt mode around the pseudotetragonal axis. This structural change may be the same as that observed at ambient pressure at 660 degrees C ( from the Pca2(1) to B2cb structure ). The second phase transition is associated with the instability of a low wave number mode, which behaves as soft mode. The discovery of the soft mode in Bi2WO6 that corresponds most likely to the E-u mode responsible for ferroelectricity in Aurivillius family of compounds proves that this transition is of displacive type. Our studies also suggest that the structure above 6.2 GPa is orthorhombic and centrosymmetric and, consequently, the phase transition can be classified as a pressure- induced ferroelectric to a paraelectric phase transition.