STRUCTURE OF NA2FE2F7 AND STRUCTURAL RELATIONS IN THE FAMILY OF WEBERITES NA2MIIMIIIF7

The mixed valence compound Na2Fe2F7 crystallizes in the monoclinic weberite structure of the Na2CuFeF7-II type (M-II), space group C2/c, Z = 16, a = 12.676, b = 7.422, c = 24.710 angstrom, beta = 99.97-degrees. Refinement on F2 led to wR(F = 2) = 0.117 for all 2 645 reflections, R = 0.057 for 1258 refl. with F(o) > 4sigma(F). On the basis of this and other recent single crystal investigations the structures of weberites Na2M(II)M(III)F7 are classified in four main structure types: 0-I is the ''classical'' orthorhombic weberite type (S.G. Imma) encountered e.g. in the mineral Na2MgAlF7, from which two lower symmetric variants 0-11 (Na2CuCrF7 type) and 0-III (Na2NiInF7) are deriving. The second M-1 and third M-II basic type are monoclinic (e. g. Na2CuFeF7-I and -II), the fourth type T is trigonal (Na2MnFeF7). The structural relations within the weberite family are discussed and the connections established, which allow to regard these weberites as well as the pyrochlores as subtraction derivatives of the fluorite type.