Raman scattering studies in the solid electrolytes of the RbAg4I5family

We have studied the dynamics of the mobile ${\mathrm{Ag}}^{+}$ ions and their surrounding tetrahedral iodine cages in the solid electrolytes $M{\mathrm{Ag}}_4{\mathrm{I}}_5$ ($M=\mathrm{Rb}, \mathrm{N}{\mathrm{H}}_4, \mathrm{K}$) by Raman scattering. Polarized spectra of oriented single crystals have been measured from the melting point to the second-order phase transition $T_2$ and to the first-order transition $T_1$ for K${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$. We have observed (i) a broad peak at 105 ${\mathrm{cm}}^{-1\mathrm{}}$ of mainly $A_1$ symmetry which is believed to be due to the breathing modes of the crystalline cages of ${\mathrm{I}}^{-}$ tetrahedra, (ii) a shoulder near 20 ${\mathrm{cm}}^{-1\mathrm{}}$ of nearly equal $F_2$ and $E$ components and little $A_1$, which is interpreted as being due to the attempt vibrations of the silver ions, (iii) a quasielastic peak which has been assigned to the diffusive modes of the ${\mathrm{Ag}}^{+}$ ions, (iv) a peak near 17 ${\mathrm{cm}}^{-1\mathrm{}}$ having $A_1$ symmetry, and (v) three $E$ peaks having frequencies near 50, 27, and $X$ ${\mathrm{cm}}^{-1\mathrm{}}$, where $X=15, 17, \mathrm{and} 18.5$ for $M=\mathrm{Rb}, \mathrm{N}{\mathrm{H}}_4, \mathrm{and} K$, respectively. Except for the latter peak, the spectra are essentially identical for different $M$, and no new features appear below $T_2$.