Variation in the Raman spectrum of thallium(I) nitrate with phase change:The anisotropic wing in solid and liquid phases

The Raman spectra are reported for the three solid phases and the melt of thallium(I) nitrate. Partial infrared spectra of the melt are also included. The internal frequencies are discussed in terms of known crystalline space groups and compared with spectra of isostructural salts. The changes in the external frequencies with change of crystal structure are also described and discussed. It is shown that a remarkable similarity exists in the low frequency spectrum of the disordered solid phase I and the melt. This similarity extends to both the anisotropic wing centred on the exciting frequency and the external lattice modes. The origin of the low frequency spectrum is discussed in terms of two-phonon scattering processes.