Abstract
Abstract The crystal structure and lattice dynamics of copper selenide were studied in the superionic (α) and the non-superionic β-phase. X-ray and neutron powder diffraction confirmed space group Fm3m for Cu 1.78 Se with Cu atoms mainly in the tetrahedral 8( c ) sites and a small fraction in trigonal 32( f ) sites. Inelastic neutron scattering in polycrystalline α-Cu 1.78 Se and β-Cu 2 Se samples indicate the presence of a broad quasi-elastic component and low-energy peaks in the dynamic structure factor S ( Q , ω ). The quasi-elastic component is related to localised diffusion of Cu ions in the 32( f ) sites. The low-energy modes observed in Cu 1.78 Se and Cu 2 Se at energies of 4–5 meV are most probably due to acoustic phonons. The phonon dispersion curves measured in α-Cu 1.85 Se single crystal reveal the existence of low-lying acoustic transversal branches with no marked dispersion at wavevectors higher than 0.3–0.4 q / q m with an energy of about 4 meV.
Published Version
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