Multiphonon anharmonicity of MgO

The anharmonic lattice dynamics of MgO has been studied at ambient conditions and at high temperatures by infrared spectroscopy and inelastic x-ray scattering measurements combined with density functional perturbation theory calculations. The agreement between the measured phonon energies and widths with ab initio calculated values provides a direct and pertinent test of the validity of advanced theoretical methods. Long observed anharmonic features in the infrared reflectivity find a clear explanation in terms of well-defined multiphonon scattering processes and lattice dynamics peculiarities, also responsible for a significant and sharp reduction of the longitudinal optical phonon lifetime at critical finite wave vectors. Our work highlights the importance of multiphonon scattering processes on the collective dynamics and related material properties.

Figure 1

Experimental (symbols) and theoretical (lines in the upper inset) optical conductivity of MgO at 300 and 873 K. The bottom inset shows the corresponding measured infrared reflectivity spectra (symbols) together with the theoretical reflectivity foreseen in the harmonic approximation (HA) (black lines). Stars indicate the position of the ESW.

Figure 2

Inelastic x-ray scattering spectra of the longitudinal optical phonon of MgO at 873 K along the (a) [100] and (b) [111] direction. The * indicate longitudinal acoustic phonons sometimes visible in the probed energy range.

Figure 3

Calculations of MgO phonon dispersion at 873 K along selected directions together with the phonon density of state (right panel) and the expected spectral density at Γ (left panel). The phonon branches are represented with a variable-width solid band: corresponding to the anharmonic width multiplied by a factor 10. The open dark gray circles indicate the reduced $q$ position at which an anomalous enlargement of the phonon width is expected for the high-energy LO branches. The dotted line across the LO branches is a guide for the eyes projecting at Γ the energy of the LO phonon at the $q$ where the width exhibits the maxima. This energy matches the ESW in the computed spectral weight σ(0,ω). The shadow areas show the reciprocal space region where the phonon density of state is particularly high. (a) Final state decomposition of the ESW at Γ and (b) of the LO phonon at $q=\left(1/400\right)$. Colored surfaces contain (a) 10% and (b) 20% of the Brillouin zone volume, and around 90% of the contribution to the linewidth.

Figure 4

LO experimental (symbols) and computed (lines) energy dispersions at (a) 300 K and (b) 873 K. The blue curves are results of DFPT calculations corrected by tuning the infinite value of the dielectric function to match the experimentally measured LO-TO splitting. (c) The measured (symbols) and computed (lines) LO width dispersions along the main crystallographic directions at 873 K (red) and 300 K (black).