Cation interstitial pair modes in the vibrational spectra of mixedβ-aluminas

Raman and far-infrared measurements of the cation vibrational spectra of the mixed systems ${\mathrm{Na}}_{1-x}{\mathrm{K}}_x$ and ${\mathrm{Na}}_{1-x}{\mathrm{Rb}}_x$ $\beta$-alumina were carried out at 300 K. The composition dependences of the observed spectral features were used to assign those features to specific types of cation sites. The resulting assignments indicate that many of the main features of the vibrational spectra below 120 ${\mathrm{cm}}^{-1\mathrm{}}$ may be attributed to vibrations of interstitial cation pairs of the kind suggested by Whittingham and Huggins to account for fast ion diffusion in the $\beta$-aluminas (interstitialcy mechanism). Other regions of the vibrational spectra are attributed to unpaired cations. These results provide experimental evidence for the existence of these pairs and thus verify a central feature of the interstitialcy mechanism.