Cs4 W11O35 (CW) and Rb4 W11O35 (RW) belong to the class of hexagonal bronzes whose structure originates from the KxWO3 superconductor hexatungstate. Charge-imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second-harmonic generation. In this work, we report the polarized Raman spectra results for both CW and RW, as well as results of high-pressure Raman scattering experiments (0.0–11.0 GPa) for the Cs4 W11O35 system, in which we have observed two structural phase transitions at ∼4 and 7.5 GPa. We discuss these transformations and polarized Raman spectra on the basis of lattice dynamics calculation in the related system KNbW2O9. Polarized Raman spectra provide strong indication that the highest wavenumber modes observed in these systems originate from tungsten or oxygen vacancies. The observation of a soft-like mode indicates that the observed phase transitions exhibit a displacive-type behavior, thus further indicating that these transformations are likely related to reorientations of the octahedral units. The soft mode nature is discussed as well.
PACS: 77.80.Bh; 78.30.Hv; 78.30.-j. Copyright © 2010 John Wiley & Sons, Ltd.