Temperature dependence of Raman-active phonons and nature of the phase transition in lithium and sodium azide

The Raman spectra of single crystal NaN₃ and polycrystalline LiN₃ have been recorded in the temperature range 100 to 300 °K. The lattice mode Raman spectrum of NaN₃ below T_c ∼284 °K is consistent with previous reports of a reversible second order rhombohedral to monoclinic phase transition in NaN₃. Below T _c the Raman active librational mode at 122 cm^-1 splits into two components, with the higher frequency component displaying a large increase in frequency with decreasing temperature. The separation between the two components has been related to the degree of the temperature dependent tilt of the anion with respect to the c axis and to the order parameter of the system. The temperature dependence of the order parameter was found to essentially follow the predictions of conventional mean field theory. In the case of LiN₃, the librational modes are found to be accidently degenerate at 300 °K, with a frequency centered at 122 cm^-1. However, at 262 °K a phase transition apparently occurs as shown by the onset of splitting of the band at 122 cm^-1. The temperature dependence of the librational modes suggests that the phase transition in LiN3 is probably analogous to that in NaN₃.