Raman Scattering from RbAg4I5

Raman spectra of oriented single crystals of RbAg4I5 have been measured for all scattering geometries at temperatures up to near the melting point and down through the second order transition at −74C and finally down to near the first order transition at −151C. Above −64C this crystal has a 06 or 07 cubic structure and a very high ionic conductivity. It is optically active. Below −64C the crystal deviates from cubic symmetry and becomes trigonal and hence birefringent. There is always a broad, structureless peak at 105 cm−1 that is strongest in A1 symmetry. This suggests that the peak is due to the breathing modes of the iodine tetrahedra around the silver ions. The spectra also show low frequency scattering out to about 60 cm−1. At high temperatures the low frequency spectrum consists of a quasi-elastic peak about the laser line plus a shoulder. It has roughly equal T2 and E components with little A1 intensity. This behavior is consistent with the assignment of the shoulder to the attempt vibration of the Ag+ ions. At room temperature there are peaks of E-symmetry at 14 and 25 cm−1 superimposed on a broad shoulder at 20 cm−1, whereas in T-symmetry there is only the shoulder. The A1 spectrum shows a peak at 16.5 cm−1 and no shoulder. All three symmetries show the tail of the quasi-elastic peak. At −63C these features sharpen somewhat and are joined by a broad shoulder at 50 cm−1. Below the second order transition the optical polarizations become mixed due to the presence of multiple birefringent domains.