Infrared and Raman spectra and lattice dynamics of the superionic conductorLi3N

The polarized infrared and Raman spectra of hexagonal ${\mathrm{Li}}_3$N (space group $D_{6h}^1$) are reported. The frequencies of the infrared-active phonons and the dielectric constants are determined from a Kramers-Kronig analysis and oscillator fits to the reflectivity data. A group-theoretical analysis has permitted an identification of the symmetries of the infrared- and Raman-active phonons. The frequencies of these phonons are used to determine the parameters of a rigid-shell model which includes the anisotropic polarizability of the nitrogen ions. The calculated dispersion curves are discussed in connection with the second-order Raman spectra. It is concluded that ${\mathrm{Li}}_3$N is the first known compound in which the nitrogen ions are close to an anomalous ${\mathrm{N}}^{3-}$ state.