Infrared and raman spectra of single-crystal α-Lead Azide

The polarized transmission infrared spectra between 400 and 4000 cm ^-1 and the He-Ne laser excited Raman spectra between 30 and 1500 cm^-1 of an oriented single crystal of α-lead azide (PbN ₆), 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_2h ¹⁶-Pnma. The Raman-active lattice modes are observed in the frequency range 30-210 cm^-1. The appearance of peaks in the 1330-1360-cm ^-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.