Kristallstrukturverfeinerungen an Natriumtrifluorometallaten NaMF3 (M  Mg,Co, Ni,Zn): Oktaederkippung und Toleranzfaktor orthorhombischer Fluorperowskite

Crystal Structure Refinements of Sodium Trifluorometallates NaMF₃ (M ? Mg, Co, Ni, Zn): Tilting of Octahedra and Tolerance Factor of Orthorhombic Fluoroperovskites Based on newly measured X-ray single crystal data the crystal structures of the orthorhombic fluoroperovskites NaMF₃ (M ? Mg, Co, Ni, Zn) were refined in space group Pbnm (GdFeO₃ type, Z = 4). The octahedra are but slightly distorted; the average distances (and bridge angles M? F? M) are: Mg? F = 197.8 pm (150.9°), Co? F = 203.8 pm (146.1°), Ni? F = 200.3 pm (148.0°) and Zn? F = 202.3 pm (147.4°). With respect to the axes of the pseudocell (Z = 1) the octahedra are rotated by tilt angles which vary, including NaMnF₃ and NaFeF₃, between 14.6 and 18.8°. The tilting becomes the more pronounced, the smaller the tolerance factor of the compound, so as to yield uniformly average values of Na? F = 232 pm for the four shortest distances within the NaF₈] coordination (distorted tetrahedron and bicapped trigonal prism, respectively). Measurements of magnetic powder susceptibilities show that tilting of octahedra is no sufficient cause to produce spin canting resulting in ferromagnetism: This property is confirmed only for NaNiF₃ (T_N = 150 K), whereas NaCoF₃ exhibits a sharp minimum in its χ^?l-T curve (T_N = 75 K) and remains uncanted antiferromagnetic then down to 4.2 K.