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
Abstract
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.