Infrared Vibrations of Crystalline Potassium Azide

The infrared spectrum of crystalline potassium azide has been investigated over the region from 4000 to 550 wavenumbers by using a grating spectrometer. Selection rules for internal and lattice vibrations and their combinations were calculated by using the ``unit-cell'' method of Bhagavantam and Venkatarayudu. The spectrum was characterized by the splitting of fundamental absorptions and the occurrence of numerous lattice combinations symmetrically spaced about active and inactive fundamentals. From observed lattice combinations, tentative frequency assignments were made for the lattice modes. Polarized spectra indicate that most of these combinations are oriented in the xy plane of the crystal. The spectrum shows good agreement with the accepted crystal structure as described by the space group D4h18—I4/mcm.