Neutron diffraction study of diffuse scattering in Cu2−δSe superionic compounds

Crystal structure and short-range order in Cu2−δSe compounds were studied in superionic and non-superionic phases using high-resolution neutron diffractometer Echidna at ANSTO. In diffraction patterns of β-Cu1.98Se (ordered phase at ambient T), both Bragg peaks and diffuse background change sharply through the β → α structural phase transition at T = 414 K during heating. In case of α-Cu1.75Se (disordered superionic phase at ambient T) the changes are monotonic, showing gradual shifts of Bragg peaks and increased intensity of the diffuse background as a function of temperature. On cooling, both compounds undergo a β → β′ transformation. Diffuse scattering in the α-phase shows an oscillating dependence on wavevector, with broad peaks centred at Q ∼ 3, 5.5 and 8 Å−1. The measurements taken in energy dispersive mode show that the oscillating diffuse background arises from correlated thermal displacements of the ions. Diffuse scattering is higher for compositions close to stoichiometry and increases with temperature. Theoretical calculations show that the increase in diffuse intensity both with temperature and Cu content is related to correlated thermal vibrations of Se and Cu atoms, with Se–Cu(8c, 32f) and Cu(8c)–Cu(8c) correlations being the most important.

► Neutron diffraction study of crystal structure and short-range order in superionic and non-superionic Cu2−δSe compounds.
► Structural model with occupation of both 8c and 32f (x = 0.39/0.41) sites gives better agreement with experiment for Cu1.75Se and Cu1.98Se in superionic phase compared to the model with only 8c site occupied. Allowing Cu atoms occupy the 4b position does not improve the quality of refinement.
► Superionic α-phase of Cu1.75Se and Cu1.98Se show the presence of broad peaks of diffuse scattering centred at Q ∼ 3, 5.5 and 8 Å−1. Theoretical simulations indicate that the increase in diffuse intensity both with temperature and increasing Cu content is related to contributions from correlated thermal vibrations of Se and Cu atoms, with Se–Cu(8c, 32f) and Cu(8c)–Cu(8c) correlations being most important.