Light-scattering studies of soft external lattice modes in metallic NaₓWO₃

Soft vibrational excitations associated with successive structural phase transitions in metallic NaₓWO₃ with $0.5<x<1$ have been studied over the temperature range $3<T<500$ K. The Raman spectra were recorded in the low-frequency region 150ω10 cm^-1 using an iodine filter for the attenuation of parasitically scattered light. Transition temperatures determined in the Raman measurements agree quantitatively with the results of other investigations in which the phase transitions in NaₓWO₃ were also detected, especially the x-ray work of Clarke who proposed a unique sequence of octahedral tilt structures for the four distinct phases in NaₓWO₃. A comparison of the group-theoretical selection rules for Raman scattering with the temperature-dependent light-scattering data serves not only to confirm the space-group assignments due to Clarke, but also provides evidence of the rigidity of the WO₆ units, a physical property that dominates the thermodynamic, structural, and vibrational properties of NaₓWO₃.