RAMAN-SCATTERING STUDIES IN THE SOLID ELECTROLYTES OF THE RBAG4I5 FAMILY

We have studied the dynamics of the mobile Ag ions and their surrounding tetrahedral iodine cages in the solid electrolytes MAg4I5 (M=Rb, NH4, K) by Raman scattering. Polarized spectra of oriented single crystals have been measured from the melting point to the second-order phase transition T2 and to the first-order transition T1 for KAg4I5. We have observed (i) a broad peak at 105 cm-1 of mainly A1 symmetry which is believed to be due to the breathing modes of the crystalline cages of I- tetrahedra, (ii) a shoulder near 20 cm-1 of nearly equal F2 and E components and little A1, 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 Ag ions, (iv) a peak near 17 cm-1 having A1 symmetry, and (v) three E peaks having frequencies near 50, 27, and X cm-1, where X=15, 17, and 18.5 for M=Rb, NH4, and K, respectively. Except for the latter peak, the spectra are essentially identical for different M, and no new features appear below T2. © 1979 The American Physical Society.