Lattice Mode Anharmonicity in Sodium Nitrite

The anharmonicity of the infrared active lattice modes of sodium nitrite is studied from measurements of the infrared reflectivity from 60 to 400 cm −1 , in the temperature range 80 to 460 K. The infrared reflectivity spectra are analysed with the four‐parameter dispersion model based on the factorized form of the dielectric function. The comparison between the results of the quantum theory and the dependence observed of damping shows that cubic anharmonicity drives the lattice dynamics of sodium nitrite when the crystal structure is ordered, and that disorder occurs below the phase transition temperature. The results confirm the breakdown of the selection rules in the paraelectric phase and suggest the existence of dielectric dispersion in the low frequency region.