Vibrational Spectrum of Sodium Azide Single Crystals

The infrared and Raman spectra of a single crystal of sodium azide have been intensively investigated at 298° and 90°K using grating spectrometers. Selection rules for internal and lattice vibrations are determined by application of the unit cell (factor group) method of Bhagavantam and Venkatarayudu. The unit cell modes of the crystal belonging to irreducible representations of the unit cell group have been constructed to within a good approximation. Spectra in the infrared are characterized by the occurence of medium‐strength absorptions symmetrically spaced about the principal bands of the spectrum with a separation of 122 cm^—1. The observation of the Raman‐active rotatory lattice mode at this frequency confirms the assignment of the medium‐strength bands as sum and difference frequencies involving rotational motion of the azide ion as a rigid unit. Tentative assignments were made for all bands observed in the spectra and deduced frequencies for unobserved infrared lattice modes are presented. The spectra were found to be in good agreement with the accepted structure as described by the space group ${\mathcal{D}}_{\text{3d}}^5\text{−R3̄m}$ .