Effect of applied pressure on phonons in sodium nitrate crystal: a lattice dynamics study

Abstract

Abstract A simple semi-rigid model of the sodium nitrate NaNO 3 crystal was developed for phonon calculations at equilibrium volume and under external pressure. This model uses Buckingham and Lennard-Jones potentials to describe intermolecular interactions, as well as five many body potentials for bonded intramolecular interactions. The influence of pressure on the vibrational properties, including phonon frequencies at the Γ point, phonon dispersion curves along the Γ → B direction, and the phonon density of states, was examined at pressures up to 10 GPa. Notably, for the first time, the pressure dependence of the ν 2 ( A 2 u ) mode revealed a clear structural phase transition that occurs between 4 and 4.5 GPa. The phonon dispersion curves with the total and partial phonon densities of states, were analysed at the critical pressure of 4.5 GPa. A slight phonon softening was observed at the Γ point for the A 2 u transverse optical mode and at the B point for the lower-frequency transverse acoustic mode. The model was validated through comparison with available experimental data, and the calculated pressure dependence of various phonon modes served as a theoretical prediction.