Structural variants in the low-temperature β phase of stoichiometric cuprous selenide

Symmetry lowering from T 2 d(F 43m) to C 3 s ( Cm) at α → β first-order phase transition of the superionic conductor Cu 2Se is described by two nonequivalent wave vectors: k R = 2π a c ( 111 222 ) and k m= 2π a c ( 11 33 0) Four orientation variants of the rhombohedrally deformed cage sublattice are equally probable equilibrium structures which correspond to four rays in the star of k R. Such twinning was observed using a microfurnace built for the Weissenberg goniometer. Three monoclinic β-phase structure variants displaying together pseudorhombohedral symmetry can couple to each body cube diagonal as a cage distortion direction, giving a total of 12—the number of rays in the star of k m .