The phase transition in sodium azide

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 correspon...

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Bibliographic Details
Published in:The Journal of chemical physics 1983-01, Vol.79 (9), p.4558-4563
Main Authors: AGHDAEE, S. R, RAE, A. I. M
Format: Article
Language:English
Subjects:
Condensed matter: structure, mechanical and thermal properties
Crystalline state (including molecular motions in solids)
Crystallographic aspects of phase transformations
pressure effects
Exact sciences and technology
Physics
Structure of solids and liquids
crystallography
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Summary: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.
ISSN:0021-9606
1089-7690
DOI:10.1063/1.446371