Structure of Na₂Fe₂F₇ and structural relations in the family of weberites Na₂M^IIM^IIIF₇

Abstract

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 Å, β = 99.97°. Refinement on F2 led to wR(F2) = 0.117 for all 2 645 reflections, R = 0.057 for 1 258 refl. with Fo > 4σ(F). On the basis of this and other recent single crystal investigations the structures of weberites Na2MIIMIIIF7 are classified in four main structure types: O‐I is the ‘classical’ orthorhombic weberite type (S.G. Imma) encountered e.g. in the mineral Na2MgAlF7, from which two lower symmetric variants O‐II (Na2CuCrF7 type) and O‐III (Na2NiInF7) are deriving. The second M‐I 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.

New Calcium Tantalate Polytypes in the System Ca2Ta2O7–Sm2Ti2O7

“…4, and involves a displacement of the tantalum atom toward the edge comprising the two terminal oxygens, O(1) and O (8). The displacement of Ti(3) is toward Ca(33) and as seen from Table 6, gives rise to alternating short and long M}M distances, 3.39 and 3.96 A s , respectively, along Ta(3)}Ca(33) chains in the Ca Ta layer.…”

“…The displacement of Ti(3) is toward Ca(33) and as seen from Table 6, gives rise to alternating short and long M}M distances, 3.39 and 3.96 A s , respectively, along Ta(3)}Ca(33) chains in the Ca Ta layer. An inspection of published structure data for weberite polytypes (8) shows that a strong perturbation to the metal atom array associated with the cis octahedra is a consistent feature of these structures, and it results in a macroscopic elongation of the structure in the direction of the chains. The strong polarization of the Ta}O bonds associated with the large Ta displacements may be at the origin of the measured high dielectric constant for Ca Ta O (4).…”

“…These are in a ciscon"guration in 3T weberite and in a trans-con"guration in the orthorhombic form (8). These are illustrated in Fig.…”

“…3. The 4M weberite polytype has an alternation of cis-and trans-forms in successive large-cation layers (8). The 6T polytype of Ca Ta O also has an alternation of cis-and trans-con"gurations for TaO in successive Ca Ta layers, as illustrated in Fig.…”

“…A solid-state preparation at 12003C was reported to give a phase with hexagonal symmetry (6) but a structure analysis was not made. An inspection of the reported unit cell dimensions and powder pattern intensities indicates that at 12003C, Ca Ta O has the trigonal weberite, Na MnFeF , structure (7) which is a #uorite-derivative structure closely related to that for pyrochlore (8). On this basis the structure can be described in terms of cubic stacking of closest-packed cation layers (,+111, ) where the layers alternately have the compositions Ta Ca and Ca Ta, as shown in Fig.…”