Anharmonic lattice dynamics of cuprite, Ag2O, studied by inelastic neutron scattering and first principles molecular dynamics simulations

Tian Lan1, C.W. Li2, J.L. Niedziela2, H. Smith1, D.L. Abernathy2, G.R. Rossman3, B.Fultz1

1Department of Applied Physics and Materials Science
California Institute of Technology, Pasadena, California 91125, USA

2Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831, USA

3Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, California 91125, USA


Inelastic neutron scattering measurements of cuprite silver oxide (Ag2O) were performed at temperatures from 40 to 400 K, and Fourier transform far-infrared spectra were measured from 100 to 300 K. Even over this range of low temperatures, the measured phonon densities of states and the infrared spectra showed unusually large softenings with temperature, and large linewidth broadenings. First principles molecular dynamics (MD) calculations were performed at various temperatures, successfully accounting for the negative thermal expansion (NTE) and local dynamics. Using the Fourier-transformed velocity autocorrelation method, the MD calculations reproduced the large anharmonic effects of Ag2O, and were in excellent agreement with the neutron scattering data. The quasiharmonic approximation (QHA) could not account for much of the phonon behavior, but the QHA could account for some of the NTE below 250 K, although not at higher temperatures. Strong anharmonic effects were found for both phonons and for the NTE. The lifetime broadenings of Ag2O were explained by anharmonic perturbation theory, which showed rich interactions between the Ag-dominated modes and the O-dominated modes in both up- and down-conversion processes.