A neutron diffraction study of structure and thermal motion in several monovalent metal azides

The crystal structures of the monovalent metal azides β-NaN3, KN3, TlN3, and RbN3 were reinvestigated by means of neutron diffraction. All except β-NaN3 were studied at low temperature as well as at room temperature. These compounds form ionic crystals with spherical metal cations and linear azide anions. Because the azide ions in each case occupy centrosymmetric sites, their thermal motions can be treated, except for the small amplitude N–N stretching and the internal bending motions, as the motions of rigid bodies without screw correlations. All structures were, therefore, refined using rigid-body constraints, leading to final R values, both weighted and unweighted, ranging from 0.020 to 0.045. KN3, TlN3, and RbN3 all crystallize in the tetragonal space group I4/mcm, while β-NaN3 crystallizes in the space group R3̄m. The uncorrected N–N distances range from 1.159 to 1.183 Å, but when corrections for thermal motions are applied all distances except for β-NaN3 lie in the range 1.181–1.187 Å, with a weighted mean of 1.186 Å. In the β-NaN3 the corrected N–N distance is 1.180(1) Å. This apparently unusually short distance may be attributable to disorder, with the axis of the N3 group making a small angle with the c axis. In the tetragonal crystals, where the azide ions occupy sites with mmm symmetry, the anisotropy of libration is consistent with the steric hindrance in the structure. The librational amplitudes determined from the diffraction studies were compared with those determined by Raman scattering, and the agreement is satisfactory.