Raman Study of Metallic Tungsten Bronzes

Raman spectra of several metallic tungsten bronzes have been obtained. Cubic ${\mathrm{Na}}_{0.61}$W${\mathrm{O}}_3$ exhibits three weak features in the 200-400-${\mathrm{cm}}^{-1\mathrm{}}$ region which are correlated with W${\mathrm{O}}_3$ spectra; hexagonal ${\mathrm{Rb}}_{0.30}$W${\mathrm{O}}_3$ displays a single broad feature at 655 ${\mathrm{cm}}^{-1\mathrm{}}$; tetragonal ${\mathrm{Na}}_{0.28}$W${\mathrm{O}}_3$ has a rich spectrum of 11 features. The presence of a low-energy plasma edge in the bronzes, especially of ${\mathrm{Na}}_{0.28}$W${\mathrm{O}}_3$, is responsible for the strong signals obtained; in most metals the reflectivity limits Raman scattering to a tiny surface layer for laser frequencies below the uv plasma edge. The plasma frequencies for samples used in the present work are determined by reflectivity measurements on single crystals.