Dynamics of silver ions in the superionic conductorAg3SI

The far-infrared reflectivity $R\left(\tilde{\omega }\right)$ of $\beta$-${\mathrm{Ag}}_3$SI and $\gamma$-${\mathrm{Ag}}_3$SI has been measured in the range $5<\mathrm{}\tilde{\omega }<400\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and at various temperatures between 90 and 300 K. The most interesting feature is the observation of a resonance near ${\tilde{\omega }}_0=60\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and a pronounced increase of $R\left(\tilde{\omega }\right)$ below 40 ${\mathrm{cm}}^{-1\mathrm{}}$; the latter disappears below $T_c=157\mathrm{}$ K, the $\beta \to \gamma$ phase-transition temperature. These data can be interpreted successfully on the basis of a microscopic model in which the silver ions move in a double-well potential. The oscillation of the silver ions in the wells gives rise to the resonance near ${\tilde{\omega }}_0=60\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ while the jumps across the potential barrier lead to the observed increase of $R\left(\tilde{\omega }\right)$ below 40 ${\mathrm{cm}}^{-1\mathrm{}}$. The height of the potential barrier is between 250 and 400 K. The interpretation of our optical data is supported by recent structural investigations.