Far-infrared reflectivity spectra are measured to investigate the role of both phonon anharmonicity and disorder in ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ spinel at low temperature. Damping constants induced by phonon anharmonicity and disorder are investigated to tentatively discriminate these two effects. Using density-functional perturbation theory, ab initio one- and two-phonon density of states are computed for the nondisordered spinel. It is shown that both the frequency and temperature dependences of the experimental damping factors agree well with the calculated imaginary parts of the two-phonon density of states. Moreover, first-order phonon mode Grüneisen parameters are calculated and compared to experiments. Finally, the origin of the low intensity of the polar mode near $565{\mathrm{cm}}^{-1}$ of $T_{1u}$ symmetry is understood.

• Received 13 May 2005

DOI:https://doi.org/10.1103/PhysRevB.73.064305