Librational motion and phase transition in sodium azide

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Abstract
The librational motion of N3− ion in the α and β phases of sodium azide is described in terms of an effective field treatment for rotational excitations. A rigid lattice model incorporating both electrostatic and short‐range interactions is employed. The order–disorder aspects of the phase transition in sodium azide are discussed in terms of the pocket state model. The estimated q=0 librational frequencies are compared with the results of Raman scattering measurements. The calculated tilt angle for the azide ion in the α phase is found to agree with experiment. The model provides additional evidence for a small amount of order–disorder character of the α–β phase transition in sodium azide superimposed on its structural aspects.

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