Superstructural ordering in low-temperature phase of superionic Cu 2Se

Abstract

Single crystal X-ray and electron diffraction investigation of stoichiometric cuprous selenide shows that the structure of β-Cu 2Se below 413 K is monoclinic with a unit cell of space group C m·Z = 12, and cell parameters: a m = 0.7115 ± 0.0005 nm, b m = 1.234±0.001nm· c m = 0.7119±0.0005 nm, β m = 108.0±0.4 °, at temperature. The monoclinic cell is a supercell spanning the third neighbouring interstitial sites of an ordered cation subsystem within a slightly deformed cubic cage sublattice ( F 43m, α c = 0.581 ± 0.001 nm ). Its a m and b m axes lay in the close packed cage planes perpendic ular to the direction of cage distortion, while its c m axis extends over two close packed cage layers along one of the three 〈112〉 c sublattice axes closest to the distortion direction. The weak extra spots indicate also the four layer periodicity along the distortion direction, suggesting the interpretation of the room temperature structure of β phase in terms of a “block cell”: a M = a m, b M = b m, c M = 1.381 ± 0.001 nm, β M = 80.0 ± 0.04 °. The ⋉ock cell” implies that the basic chemical unit of “stoichiometric” β phase is Cu 47Se 24 rather than Cu 2Se.