Rayleigh-Brillouin spectra of the solid electrolyte RbAg4I5

The low-frequency Raman spectra and the Rayleigh-Brillouin spectra of the crystalline electrolyte, rubidium silver iodide have been measured. At 90°C the longitudinal-acoustical phonon was observed by Brillouin scattering with a triply-passed Fabry-Perot interferometer to have a frequency consistent with known and measured physical properties of the crystal. The absolute intensity was compared with that of fused silica, yielding a value for the Rayleigh ratio of 2 × ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and for the Pockels' coefficient of 0.15. The Brillouin linewidth was instrumental and less than 0.3 GHz. The Rayleigh-Brillouin spectra were dominated by an intense Rayleigh peak of width less than 10 MHz and Rayleigh ratio of 0.14 ${\mathrm{cm}}^{-1\mathrm{}}$. Removal of this feature with an iodine filter revealed a dynamic central peak wider than the 350-MHz half-width of the filter. Analysis and comparison between data from an interferometer and a double monochromator lead to the conclusion that the central peak may be described as a sum of two Lorentzians. The narrower peak has a half-width of roughly 9±3 GHz and a Rayleigh ratio of 4 × ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Its width is consistent with a relaxation rate estimated from the diffusivity. The broader peak has a half-width of 40±6 GHz and a Rayleigh ratio of about 9 × ${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. Its origin is less certain, but it may be a measure of the time of flight of mobile silver ions.