Anharmonicity and Structural Transitions

Temperature and pressure effects on azides have been studied with the emphasis being on the alkali-metal salts of relatively simple structure. Phase transitions have been observed, but studies of the dynamics of the transitions have only recently been started via Raman scattering experiments. These aspects are reviewed below. 

The thermal expansion coefficients of NaN3 and KN3 have been determined by Parsons and Yoffe [85] Weir etal. [84] determined the isothermal compressibilities of a number of metal azides. The results are summarized in Table XIII. 

The thermal expansion and compressibility of rhombohedral NaN3 and tetragonal KN3 are highly anisotropic. The largest values in both cases are perpendicular to the azide axis, suggesting that ionic forces within the azide layers are stronger than those between the layers. The results of Parsons and Yoffe [85] also indicate that the lattice parameters increase linearly with temperature between 300 and 600°K. 

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