Crystal structure and lattice dynamics of superionic copper selenide Cu2-δSe

The crystal structure and lattice dynamics of copper selenide were studied in the superionic (alpha) and the non-superionic beta-phase. X-ray and neutron powder diffraction confirmed space group Fm3m for Cu1.78Se with Cu atoms mainly in the tetrahedral 8(c) sites and a small fraction in trigonal 32(f) sites. Inelastic neutron scattering in polycrystalline alpha-Cu1.78Se and beta-Cu2Se samples indicate the presence of a broad quasi-elastic component and low-energy peaks in the dynamic structure factor S(Q,omega). The quasi-elastic component is related to localised diffusion of Cu ions in the 32(f) sites. The low-energy modes observed in Cu, Se and Cu,Se at energies of 4-5 meV are most probably due to acoustic phonons. The phonon dispersion curves measured in alpha-Cu1.85Se 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. (C) 2003 Elsevier B.V. All rights reserved.