Phonon Anharmonicity in Cuprite

Abstract Body: We report large anharmonicity of optical phonons in cuprite (cuprous oxide, Cu₂O). Phonon dispersions were measured at 10, 300, and 700 K by rotating a single crystal in the ARCS spectrometer at ORNL, and folding the data back into the first Brillouin zone. Ab initio calculations of harmonic phonons accounted for the dispersions at 10 K. At 300 K, however, broadenings of phonon energies made it impossible to use harmonic or quasiharmonic interpretations. The broadening was calculated from the cubic anharmonicity using the stochastic Temperature Dependent Effective Potential method (quasiharmonic phonons in an anharmonic potential), but the experimental broadening was considerably larger than calculated. Results will be compared from ongoing ab initio molecular dynamics calculations and experimental phonon linewidths with full background corrections. Cuprite is stable to a temperature of 1503 K, but the phonon spectrum at 700 K is already so broad that it may not be interpretable with phonons. Implications for vibrational entropy and chemical bonding will be discussed.