Infrared and Raman Spectra of Single-Crystal α-Lead Azide

The polarized transmission infrared spectra between 400 and $\text{4000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ and the $\mathrm{He–Ne}$ laser excited Raman spectra between 30 and 1500 cm⁻¹ of an oriented single crystal of α‐lead azide (PbN₆), have been measured and the long‐wavelength vibrational modes have been assigned by factor‐group analysis. The observed splittings are in essential agreement with the reported space group $D_{\text{2h}}^{16}-$ Pnma. The Raman‐active lattice modes are observed in the frequency range $\text{30–210\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . The appearance of peaks in the $\text{1330–1360}{\mathrm{-cm}}^{\text{−1}}$ region due to the symmetric stretching vibration of the azide ion, in both the infrared and Raman spectra, confirms the presence of fairly asymmetric azide ions in the lattice. A probable relationship between a component of the bending mode frequency of the azide ion and the thermal instability of α‐lead azide and related metal azides is pointed out.