Infrared absorption spectrum of KN₃ crystal: Selection rules and analysis of internal± internal and internal ±external mode absorptions

In this paper we report the measurement of the polarized infrared spectrum between 400 and 5000 cm^-1 of single crystal KN₃, at 80° and 298°K. A large number of absorptions due to internal, internaldb internal, and internal±external mode transitions were observed. Selection rules for internal+internal and internal±external phonon pairs along different symmetry sections of the Brillouin zone have been worked out. The observed band structure is assigned in many cases to critical points along symmetry lines and at points in the Brillouin zone, in terms of these selection rules. In the Γ, ∑, M, A, and H regions of the zone the spectral observations concerning the external translatory modes are compared with the rigid ion model calculation and neutron data of Rao et al.