Cryolite structure

The ideahzed cubic cryolite-structure is characterized by the following positions of the space group xa3m (No. 225), (z = 4) ... 

The occupation of lattice sites differing widely in size and coordination, by equally sized A-ions, is the reason, why the ideal cryolite structure appears strained and therefore tends to distort. This distortion, as Bode and Foss (44) described in detail, results in various rotations of the MeFe-octahedra, whereby the fluoride ions always move away from the edges of the unit cell and thus enlarge the space available for the octahedrally coordinated A-ion. In addition a further distortion may shift the atoms from the originally face-centered arrangement, so that structures of lower S3unmetry may be observed. But the cubic cryolite-type is still to be seen as the basic form of these distorted modifications, which appear at lower temperatures only and become cubic if the temperature is raised (J52). 

In this cubic cryolite structure, investigated by Bode and Foss 44), the MeFe-octahedra again are oriented in such a way, that the fluoride ions lie somewhat off the cell edges. This fact explains that the observed lattice constants are always considerably smaller than the radii sum 2rMe + 2rA + 4rp, which accounts for a linear array of ions only. 

As for the structures of ammonium compounds apparently no certainty has been attained. Bode and Foss followed the views of earlier authors and accepted the (NH4)sFeF6-type 245) with a linear array of ions as an example of the ideal cryolite structure. The discrepancy between the observed lattice constants and the ionic radii sum is indeed smaller in the case of ammonium-compounds. But this applies also to the compound (NH4)3GaFe, of which a structure analysis has been performed recently by Schwarzmann 303). After a detailed discussion of the possibilities of arranging the fluoride ions in this compound, Schwarzmann shows, that there must be either micro-twinning in the space group Pa3 or a statistic occupation of the 192-fold position of space group FmSw by 24 fluoride ions, as characteristic of the KsFeFs-type. 

Fig. 2 Cryolite structural type adopted by K3C60. Shaded and open spheres represent ions residing in octahedral and tetrahedral sites, respectively...

Complexes with all the halide ions have been made, though the fluorides and chlorides are the most studied. Fluorides ALnp4, A2LnFs and AsLnFe exist (A = alkali metal) most have structures with 8- or 9-coordinate metals, though a few AsLnFe have the six-coordinate cryolite structure. 

Figure 17 (a) NaNiFa structure (GdFeOs-type) (b) Nas AlFe (cryolite) structure. Na-F bonds drawn for Na on former perovskite M sites only. Both structures in perspectives to recognize two parent perovskite subceUs. Na atoms as large spheres... 

Figure 14. Polyhedral representation of the cryolite structure (Na2(NaAl)Fe perovskite). Lightly shaded octahedra are A1[F]6, dark octahedra are Na[F] e, circles are 8-coordinated Na. The pseudo-tetragonal p-phase is orthorhombic (Immm) [Used by permission of the editor of Physics and Chemistry of Minerals, from Spearing et al. (1994), Fig. 1, p. 374, Springer-Verlag 1994.].

This account of these structures has been based on the mode of packing of the c.p. layers. For the alternative description in terms of the way in which the octahedral BX groups are joined together by sharing X atoms, see Chapter 5. The structures based on cubic closest-packing are normally illustrated and described in terms of the cubic unit cell. We noted earlier that the cryolite structure is a superstructure of perovskite the relation between the perovskite, cryolite, and K2PtQg structures is described in Chapter 10 (p. 388). 

From the structural standpoint it is preferable to write the formulae of compounds of the cryolite family in the form A2(B B")Xg rather than A3BX6 because in this structure one-third of the A atoms in the formula A3BX6 are, like the B atoms, in octahedral holes in a cubic close-packed A2X6 (AX3) assembly the other two-thirds, corresponding to the ions in the K2PtCl6 structure, are surrounded by twelve equidistant X ions. Accordingly the cryolite structure is very closely related also to the perovskite structure. In Fig. 10.7 the A and X atoms are... 

FIG. 10.7. Relation between perovskite, K2PtCl6, and cryolite structures. 

The cryolite structure is a superstructure of the perovskite arrangement with a... 

A stoichiometry of MsC o results from the occupation of all tetrahedral and octahedral gaps, the compounds feature the cryolite structure. 

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