Infrared Spectra of the Azide Ion in Alkali-Halide Lattices

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Abstract
The infrared‐active vibrational frequencies of the azide ion as an impurity in KI, KBr, KCl, and NaCl lattices have been measured as functions of temperature. Samples were prepared using the pressed‐disk technique. In several cases comparisons are made of the spectra of pressed disks with those of single‐crystal samples grown from aqueous solutions. Identical frequency shifts with temperature and lattice in the two types of samples offer strong evidence of lattice intermixing (solid solution) in pressed disks. A simple electrostatic model of impurity‐ion—lattice interaction is presented. It is shown that inductive, Coulombic, and short‐range forces all provide significant contributions to the lattice perturbation. The experimental data on the azide ion are used to make approximate calculations of the quadratic and cubic force constants for the hypothetical ``free'' azide ion and to provide tests of the electrostatic model being considered.