Low temperature phase transition in TlN₃. Infrared absorption and raman scattering study

The long wavelength modes of tetragonal (Phase I) TlN₃ in the frequency range 30-2500 cm^-1, have been observed and assigned. The low frequency spectrum becomes fairly complex at low temperatures due to an orthorhombic distortion of the Phase I lattice. The possibility of a non-centrosymmetric structure is indicated by an apparent breakdown of the rule of mutual exclusion for some of the lattice frequencies. The phase transition has been found to involve a distortion of the unit cell at ≈278°K followed by a distortion of the azide ions at ≈225°K. The latter temperature is close to the value observed by differential thermal analysis. The low frequency Raman spectrum has been measured at different low temperatures and the temperature dependence of the frequencies are discussed in terms of microscopic approaches to the phase transition problem.