Structural behaviour of β-Cu 2− δSe ( δ = 0, 0.15, 0.25) in dependence on temperature studied by synchrotron powder diffraction

Abstract

Superionic β-copper selenide, β-Cu 2− δ Se ( δ = 0, 0.15, 0.25), has been studied by synchrotron powder diffraction and calorimetric methods in the range 293 K < T < 823 K. Cu 1.75Se exists in the superionic phase at room temperature and remains stable in air up to 600 K. Cu 1.85Se exists at room temperature as a mixture of β and α phases, β + α → β phase transition is not detected in DCS experiment, however, pure β-phase was observed in diffraction pattern of Cu 1.85Se at 353 K; Cu 1.85Se remains stable in air up to 600 K. A copper release process starts in Cu 2Se at a temperature below the superionic–non-superionic phase transition. At T > 543 K the composition is close to Cu 1.95Se and remains stable up to 573 K. Time average structure of β-Cu 2− δ Se has been analysed in the whole temperature range of existence in air. A face centred cubic unit cell (space group F m 3 ¯ m ) is formed by the selenium anions with copper cations distributed over tetrahedral 8(c) and 32(f) ( x, x, x) split positions within the octahedral voids (1/3 < x < 1/2). The copper occupancy of the 32(f) sites increases with temperature with a simultaneous decrease in the 8(c) sites. Besides, the copper occupancy of the 32(f) lattice site also increases with the total copper content 2 − δ. A diffusion pathway of mobile Cu ions along the tetrahedral sites, in “skewed” 〈1 0 0〉 directions through the peripheries of octahedral cavities is proposed.