Neutron diffraction study of diffuse scattering in Cu sub(2-[delta]Se superionic compounds)

• Published in: Journal of alloys and compounds 2011-05, Vol.509 (18), p.5460-5465
• Main Authors: Danilkin, SA, Avdeev, M, Sakuma, T, Macquart, R, Ling, C D
• Format: Article
• Language: English
• Subjects: Beta; Copper; Correlation; Diffusion; Oscillating; Phases; Scattering
• ISSN: 0925-8388
• DOI: 10.1016/j.jallcom.2011.02.101

Crystal structure and short-range order in Cu sub(2-[delta]Se compounds were studied in superionic and non-superionic phases using high-resolution neutron diffractometer Echidna at ANSTO. In diffraction patterns of [beta]-Cu) sub(1).98Se (ordered phase at ambient T), both Bragg peaks and diffuse background change sharply through the [beta] --> [alpha] structural phase transition at T = 414 K during heating. In case of [alpha]-Cu sub(1.75Se (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 [beta] --> [beta]' transformation. Diffuse scattering in the [alpha]-phase shows an oscillating dependence on wavevector, with broad peaks centred at Q [inline image] 3, 5.5 and 8 Aa[super]-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.)