Diffusion paths formation for Cu+ ions in superionic Cu6PS5I single crystals studied in terms of structural phase transition

In order to understand the structural transformations leading to high ionic conductivity of Cu+ ions in Cu6PS5I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at T-c = (144-169) K Cu6PS5I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above T-c delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43c (a' = 19.528 angstrom, z = 32). Finally, above T-1 = 274 K increasing disordering of the Cu+ ions heightens the symmetry to F-43M (a = 9.794 angstrom, z = 4). In this work, the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions (R-1 = 0.0397 for F-43c phase, and 0.0245 for F-43m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism. (c) 2005 Elsevier Inc. All rights reserved.