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

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Abstract
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 Tc [inverted lazy s] 284 °K is consistent with previous reports of a reversible second order rhombohedral to monoclinic phase transition in NaN3. Below Tc 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.

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