Diffuse scattering and lattice dynamics of superionic copper chalcogenides

The energy-resolved neutron diffraction and inelastic neutron scattering study of diffuse scattering in copper chalcogenides was performed in order to clarify the role of static disorder versus low-energy phonons. Neutron diffraction patterns taken from Cu1.75Se, Cu1.98Se and Ag0.25Cu1.75Se powders in superionic phase show a broad maximum related to diffuse scattering. This diffuse background is suppressed in the energy-resolved experiment which indicates a strong contribution from inelastic scattering coming from correlated thermal displacements of the ions in the superionic phase. Diffraction experiments on a single crystal of α-Cu1.8Se have revealed an ordered structure with superstructure reflections at the G ± 1/2 <111> and G ± 1/3 <220> positions of reciprocal space at room temperature. In addition to superstructure spots, diffuse scattering was observed along the <111> direction which is considered as a possible diffusion path of mobile Cu ions. In inelastic neutron scattering measurements with this single crystal sample strong inelastic scattering was observed along <111>. This shows that the diffuse scattering found in conventional diffraction experiment is mainly inelastic and most probably comes from low-energy phonons.