Reinvestigation and Structural Approach of the Ba–Pt–O System for <Y=Ba/Pt<

A reinvestigation of the Ba-rich region of the Ba–Pt–O system has been undertaken. The existence of a solid solution, already mentioned, has been confirmed, and its true formulation has been determined: Ba₁₂Ba_xPt_3−x]Pt₆O₂₇with 0≤x≤3. The structure of these phases can be schematically considered as being built up from the periodic stacking along thecdirection of one close-packing Ba₃O₉] layer, similar to those of the hexagonal perovskite, and three Ba₃A′O₆] layers, withA′=Ba, Pt. The stacking of these mixed layers is of hexagonal type, and then columns parallel tocare formed. They consist of octahedra and trigonal prisms, sharing faces, which are occupied byA′ cations. For 0≤x≤1, the structure appears to be incommensurate and should be described as a composite crystal based on two subsystems: Ba] and (Ba, Pt)O₃]. Forx>1, the X-ray diffraction pattern can be explained using a three-dimensional cell, but the structure refinement attempts as well as the electron diffraction patterns seem to indicate these phases are commensurate modulated structures.