REINVESTIGATION AND STRUCTURAL APPROACH OF THE BA-PT-O SYSTEM FOR 4 3-LESS-THAN-Y=BA/PT-LESS-THAN-5/2/

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: Ba-12[B axPt3-x]Pt6O27 with 0 less than or equal to x less than or equal to 3. The structure of these phases can be schematically considered as bein g built up from the periodic stacking along the c direction of one clo se-packing [Ba3O9] layer, similar to those of the hexagonal perovskite , and three [Ba(3)A'O-6] layers, with A' = Ba, Pt. The stacking of the se mixed layers is of hexagonal type, and then columns parallel to c a re formed. They consist of octahedra and trigonal prisms, sharing face s, which are occupied by A' cations. For 0 less than or equal to x les s than or equal to 1, the structure appears to be incommensurate and s hould be described as a composite crystal based on two subsystems: [Ba ] and [(Ba, Pt)O-3]. For x > 1, the X-ray diffraction pattern can be e xplained 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 structufes. (C) 1998 Academic Press.