S. R. Aghdaee, A. I. M. Rae
1983
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0
Influential Citations
14
Citations
Journal
Journal of Chemical Physics
Abstract
Sodium azide undergoes a transition from a high‐temperature rhombohedral phase to a low‐temperature monoclinic phase at 292.2 K. Results of x‐ray diffraction studies of the low‐temperature phase are reported which indicate that the transition is of the elastic type with the order parameter corresponding to a linear combination of the strains e13 and (e22−e11) along with a rotation of the azide ions in the xz plane. The temperature dependence of the order parameter in the vicinity of the transition is consistent with Landau theory for a weakly first‐order change, and this model is also shown to explain a wide range of experimental data previously obtained on the transition. A microscopic model is developed on the basis of the quasi‐harmonic approximation and this is shown to be in quantitative agreement with the experimental data on the transition.