Crystal structure and magnetic properties of Ba10(MnFeF11–xClx)3FxCl2 –x(x= 0.85). Structural relationships with the apatite-type structure

Abstract

The crystal structure of the chlorofluoride Ba₁₀(MnFeF_11–xCl_x)₃F_xCl_{2 –x}(x= 0.85) has been determined from single-crystal X-ray data with the following parameters: a= 11.075(2)Å and c= 8.173(2)Å, and the trigonal P31m space group. The residual values (R values) are R1 = 0.0275 (based on F_o) and wR2 = 0.0584 (based on F_o²) for 1384 unique reflections and 103 parameters. The structure can be described in terms of infinite chains of face-sharing MBa₆(M = F or Cl) octahedra, 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 structure [Ca₅(PO₄)₃(F,OH,Cl)]. The structure is highly disordered with the Mn and Fe atoms forming either isolated dimeric [MnFeF_{11 –x}Cl_x] groups of corner-sharing octahedra (85%) or as two isolated units (15%). This result has been confirmed by the magnetic properties and a theoretical model is proposed to fit the thermal variation of the magnetic susceptibility. The homologous phase with Fe²⁺ and Cr³⁺ has been prepared. The corresponding unit-cell parameters are: a= 11.033(2)Å and c= 8.180(2)Å. The structural relationships with the apatite-type structure and related phases are discussed.