Phonon spectra and lattice dynamics of lithium nitride

Lithium nitride, Li₃N, belongs to the hexagonal structure with the symmetry point group D_6h. The structure is ionic with four atoms per unit cell and N^3- coordinated in a regular way with eight Li⁺ ions. Polarized Raman scattering and infrared reflectivity spectra are reported. The frequencies of zone center phonons are determined from a Kramers-Kronig analysis and the oscillator fits to the reflectivity data. A rigid shell model with eight parameters, including the anisotropic polarizability of nitrogen ions, yields an excellent fit to the observed phonon frequencies.