CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF BA-10(MNFEF11-XCLX)(3)FXCL2-X (X=0.85) - STRUCTURAL RELATIONSHIPS WITH THE APATITE-TYPE STRUCTURE

The crystal structure of the chlorofluoride Ba-10(MnFeF11-xClx)(3)FxCl 2-x (x=0.85) has been determined from single-crystal X-ray data with t he following parameters: a=11.075(2) Angstrom and c = 8.173(2) Angstro m, and the trigonal P31m space group. The residual values (R values) a re R1=0.0275 (based on F-0) and wR2=0.0584 (based on F-o(2)) for 1384 unique reflections and 103 parameters. The structure can be described in terms of infinite chains of face-sharing MBa(6) (M=F or Cl) octahed ra, located at the origin of the unit cell and parallel to [001]. Two other Ba atoms occupy the(1/3,2/3,0) and (2/3,1/3, ca. 0.5) positions. This network is similar to that encountered in the apatite-type struc ture [Ca-5(PO4)(3)(F,OH,Cl)]. The structure is highly disordered with the Mn and Fe atoms forming either isolated dimeric [MnFeF11-xClx] gro ups of corner-sharing octahedra (85%) or as two isolated units (15%). This result has been confirmed by the magnetic properties and a theore tical model is proposed to fit the thermal variation of the magnetic s usceptibility. The homologous phase with Fe2+ and Cr3+ has been prepar ed. The corresponding unit-cell parameters are: a=11.033(2) Angstrom a nd c=8.180(2) Angstrom. The structural relationships with the apatite- type structure and related phases are discussed.