Limits of the quasiharmonic approximation in MgO: Volume dependence of optical modes investigated by infrared reflectivity and <i>ab initio</i> calculations

Experimental and numerical investigation of phonon optical modes MgO as a function temperature (from 300 to 1400 K) pressure 0 21 GPa) are here presented. Infrared reflectivity measurements were performed probe energies widths the phonons, well multiphonon processes affecting spectral shape, over variation unit-cell volume exceeding 20%. Calculations within quasiharmonic approximation (QHA) account for dependence observed in high-pressure experiments, while they fail at larger volumes, corresponding highest investigated temperatures. Moreover, QHA calculations more closely predict transverse (TO) than those longitudinal (LO) ones. This can be ascribed known limitations modeling effective charges (${Z}^{*}$) dielectric constant (${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}$) that lead an underestimation LO-TO splitting. Based on comparison our experimental theoretical results, we propose empirical analytical expression ${{Z}^{*}}^{2}/{\ensuremath{\varepsilon}}_{\ensuremath{\infty}}$ atomic cell volume. Density-functional perturbation theory including phonon-phonon scattering up third order lattice potential expansion is used calculate widths. These reproduce explain remarkably nontrivial both TO LO phonons linewidths determined by experiments.