Inelastic neutron scattering and lattice dynamical calculation of negative thermal expansion compoundsCu2OandAg2O

Abstract

Negative thermal expansion (NTE) is known in ${\mathrm{Cu}}_{2}\mathrm{O}$ below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in ${\mathrm{Ag}}_{2}\mathrm{O}$ up to its decomposition temperature of about $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Inelastic neutron scattering measurements in ${\mathrm{Cu}}_{2}\mathrm{O}$ and lattice dynamics calculations of NTE in both ${\mathrm{Cu}}_{2}\mathrm{O}$ and ${\mathrm{Ag}}_{2}\mathrm{O}$ are reported. The phonon density of states from a polycrystalline sample of ${\mathrm{Cu}}_{2}\mathrm{O}$ is measured using the triple-axis spectrometer at Trombay. A lattice dynamical model is used for the calculations of phonon frequencies and their pressure dependence in the entire Brillouin zone. The experimental phonon spectrum is in fair agreement with calculations. The calculated Gr\uneisen parameters for ${\mathrm{Cu}}_{2}\mathrm{O}$ have lower negative values in comparison with ${\mathrm{Ag}}_{2}\mathrm{O}$. This results in a smaller value of negative thermal expansion coefficient in ${\mathrm{Cu}}_{2}\mathrm{O}$, in fair agreement with the experimental data from the literature. An important librational mode is identified that is related to NTE. Variance of bond lengths $(⟨{u}_{\mathit{\text{bond}}}^{2}⟩)$ and their temperature dependence have been calculated and compared with extended x-ray absorption fine structure data that illustrates the nature of the $M\text{\ensuremath{-}}\mathrm{O}$ and $M\text{\ensuremath{-}}M$ $(M=\mathrm{Cu},\mathrm{Ag})$ bonds in these compounds.