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Structure of Na 2 Fe 2 F 7 and structural relations in the family of weberites Na 2 M II M III F 7

1993; Wiley; Volume: 619; Issue: 11 Linguagem: Inglês

10.1002/zaac.19936191115

1521-3749

O. V. Yakubovich, V. S. Urusov, Werner Massa, G. Frenzen, D. Babel,

Nuclear materials and radiation effects

Abstract The mixed valence compound Na 2 Fe 2 F 7 crystallizes in the monoclinic weberite structure of the Na 2 CuFeF 7 ‐II type (M‐II), space group C2/c, Z = 16, a = 12.676, b = 7.422, c = 24.710 Å, β = 99.97°. Refinement on F 2 led to wR(F 2 ) = 0.117 for all 2 645 reflections, R = 0.057 for 1 258 refl. with F o > 4σ(F). On the basis of this and other recent single crystal investigations the structures of weberites Na 2 M II M III F 7 are classified in four main structure types: O‐I is the ‘classical’ orthorhombic weberite type (S.G. Imma) encountered e.g. in the mineral Na 2 MgAlF 7 , from which two lower symmetric variants O‐II (Na 2 CuCrF 7 type) and O‐III (Na 2 NiInF 7 ) are deriving. The second M‐I and third M‐II basic type are monoclinic (e. g. Na 2 CuFeF 7 ‐I and ‐II), the fourth type T is trigonal (Na 2 MnFeF 7 ). 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.