Structural behaviour of β-Cu2-δSe (δ=0, 0.15, 0.25) in dependence on temperature studied by synchrotron powder diffraction

Superionic beta-copper selenide, beta-Cu2-delta Se (delta = 0, 0.15, 0.25), has been studied by synchrotron powder diffraction and calorimetric methods in the range 293 K < T < 823 K. Cu1.75Se exists in the superionic phase at room temperature and remains stable in air up to 600 K. Cu-1.85 Se exists at room temperature as a mixture of beta and a phases, beta+alpha -> beta phase transition is not detected in DCS experiment, however, pure beta-phase was observed in diffraction pattern of Cu1.85Se at 353 K; Cu1.85Se remains stable in air up to 600 K. A copper release process starts in Cu2Se at a temperature below the superionic-non-superionic phase transition. At T > 543 K the composition is close to Cu1.95Se and remains stable up to 573 K. Time average structure of beta-Cu2-delta Se has been analysed in the whole temperature range of existence in air. A face centred cubic unit cell (space group Fm (3) over barm) 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-delta. A diffusion pathway of mobile Cu ions along the tetrahedral sites, in skewed < 100 > directions through the peripheries of octahedral cavities is proposed. (c) 2005 Elsevier B.V. All rights reserved.