Cryolite-type structure

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). 

Salts of hexafluorovanadate can be obtained by high temperature techniques or from solutions containing hydrofluoric acid. For instance, X-ray patterns and DTA were used for the characterization of 17 double fluorides obtained by solid state reactions in the systems VF3 + MF (M = Li, Na, K, Rb, Cs, Tl) and seven double fluorides in the systems VF3 + MF2 (M = Ca, Sr, Ba, Pb),296 and the lattice constants297,29s and magnetic properties298 of A2B[VF6] (A, B = Cs, Rb, Tl, K, Na, Li) were also reported. The structure of the high temperature j0 phase of Li3VF6 has been determined and compared with the cryolite-type stable a form. The vanadium atoms have an octahedral coordination.299... 

One of the most important salts is cryolite whose structure (Fig. 9-2) is important since it is adopted by many other salts containing small cations and large octahedral anions and, in its anti-form, by many salts of the same type as [Co(NH3)6]I3. It is closely related to the structures adopted by many compounds of the types M2[AB6]2- and [XY6]2 + ZJ. The last two structures are essentially the fluorite (or antifluorite ) structures (see Fig. -2=3,-page 51), except that the anions (or cations) are octahedra whose axes are oriented parallel to the cube edges. The unit cell contains four formula units. 

Cryolite, Na3 [AlFs] (see Section 13.2) occurs naturally but is also synthesized (reaction 13.46) to meet commercial needs. The sohd state structure of cryolite is related to the perovskite-type structure. 

Temperature dependent phase transitions between both structure types have been found for the respective chlorides and bromides as is indicated in fig. 26 (Wickleder and Meyer 1995a). Within the mixed cation series Ag3 cN YCl6, the cryolite type occurs when the Na content exceeds x=2 (Stenzel and Meyer 1993). With x<2 this structure is only stable at elevated temperatures, with x=0 (pure AgsYCle) the cryolite type of structure... 

Fig. 26. The dependence of the phase transition temperature from (be stuBed LiSbFj to the cryolite type of structure upon the size of the cation for ternary Na3RX5-type dilorides and bromides.

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. 

The often mentioned relations between the structure types of cryolite and perovskite (page 41) may be explained best with the example of the elpasoHte type. The elpasoUte structure is really a superstructure of the perovskite-lattice, generated by substituting two divalent Me-ions in KMeFs by two others of valency 1 (Na) and 3 (Me) resp. The resulting compound K (Nao.5Meo.5)F3 crystallizes with an ordered distribution of Na+ and Me + because of the differences in size and charge of the ions. Thus to describe the unit cell the lattice constant of the perovskite ( 4 A) has to be doubled to yield that of the elpasohte structure ( 8 A). 

Vanadium hi).—Halides and Oxyhalides. Solid-state reactions275 in the systems VF3-M1F and VF3-M2F2 (M1 = Tl1, M2 = Ca, Sr, Ba, or Pb) have yielded 17 double fluorides of five different structural types depending on the VF3 MF ratio. The chief structural type is the cryolite-like M3VF6. 

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

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... 

Fig. 25. Similarities between the cryolite (at right) and stuffed LiSbF types of structure.

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