Ab-initio crystal structure of Mo4+xRu9-xSi5 (0≤x≤1) by synchrotron powder diffraction and electronic properties calculation (KKR method)

The crystal structure of the new phase Mo26Ru47Si27 (beta phase) discovered during the study of the Mo-Ru-Si phase diagram performed in the aim of finding materials for high temperature applications, was determined by ab-initio methods from powder synchrotron diffraction data. Distribution of Ru and Mo atoms in the structure is specified by total energy electronic structure calculations using Korringa-Kohn-Rostoker method. The compound crystallizes in P4/mmm space group with a=9.2164(1) angstrom, c=2.88721(1), angstrom, D-x = 9.69(1) g/cm(3). The structure has been refined by the Rietveld method down to R-p = 1.1%, R-B = 12.0%, R-F = 8.7% for 6814 observed data and 271 independent reflections. The structure consists of four distorted cubes 'Mo4Si4' centered by Ru atoms, sharing edges in the (a,b) planes to form cross-like units linked each others by the lateral faces of the branches in the (a,b) planes and stacked by faces along the c direction. The center of the crosses is occupied by Mo or Ru atoms (disorder 1d site) in an octahedral coordination. The spacing between the connected crosses is filled by four others distorted cubic units 'Mo4Si4' centered themselves by Ru atoms. The composition of the beta phase, determined by microprobe analysis, presents an homogeneity domain, which can be expressed by the unit formula Mo4+xRu9-xSi5 (0 <= x <= 1) compatible with a partial or fully occupation by Mo or Ru of the Wyckoff position Id of the structure. Electronic properties of ordered and disordered models of Mo4+xRu9-xSi5 are also discussed. (C) 2005 Published by Elsevier Ltd.