Tysonite

Oftedal, I. Dher die Kristallstruktur von Tysonit und einigen kiinstlich dargestellten Lanthanidenfluoriden. Z. phys. Chem. B 5, 272 (1929). 

Lanthanide and actinide trifluorides adopt a different structure called the tysonite structure (DOg). In this structure, the metal atom has the unusual eleven (5 -I- 6) coordination. Covalent AB3 halides crystallize in layer structure (DO5) typified by CrCl3 and Bil3 these structures are related respectively to CdCl2 and Cdl2. 

Higher coordination numbers of 8 -F 1 are adopted in the LT-YF3 type by the trifluorides of the larger ions TP+, bP+ and the smaller rare-earth ions Sm to Ln. The tysonite or LaF3 type with CN 9 + 3 is found for the trifluorides of the larger 4f and the 5f elements (see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry). 

Ionic Conductivity (see Ionic Conductors). Pluoride anionic conductivity is observed mainly in derivatives of fluorite (Cap2) and tysonite (Lap3). If Cap2 is doped by a tervalent rare-earth metal ion, the additional fluoride ions are positioned in interstitials where they become mobile by a hopping mechanism. 

The early fluorides feature the tysonite (LaFs Figure 7) structure in which the lanthanide has nine nearest-neighbor fluorides in a tricapped trigonal prismatic array (like the aqua ions), with two rather more distant neighbors capping the trigonal faces. From SmFs onwards the stable form is the... 

Much of the structural data are based on powder X-ray diffraction, but structures are typically polymeric with the bismuth in a distorted nine-coordinate environment similar to that in BiF3 or the Tysonite structure. There has been considerable interest in these compounds as fast fluoride ion conductors. ... 

Colorless solid, insoluble in water. Hexagonal (tysonite) structure. 

Lattice parameters of the tysonite-related superstructure phases CajRTFaT after Sevan and Greis (1978). 

Space groups Cc or C2/c Z for the formula unit (Ca, R)F2.7o is 20 relations to the tysonite parent-structure, see the original paper. 

With respect to experimental aspects, it follows that equilibrium will be more and more difficult to achieve, the more diluted the clusters occur in their parent structures. In the case of tveitite, this equilibrium is not obtainable in the laboratory but occurs in nature. Therefore, for further superstructure phases one should not only search in phase studies on synthetic AF2-RF3 systems, but also in minerals of the yttrofluorite class (and tysonites for their corresponding superstructure phases). 

Mehlhorn and Hoppe (1976). This compound crystallizes with orthorhombic symmetry (a = 7.5 A, b = 10.9 A, c = 5.3 A, Z = 4) and in space group Cmma. It is isostructural to RbPaFe (Bums et al., 1968b), PbZrFs (Laval et al., 1974), and BaZrFft (Mehlhorn and Hoppe, 1976). EuSiF has been found to be isostmctural with BaSiFs (Latourrette et al., 1977). The tysonite-related phase EuThFg is also reported (Keller, 1967). The crystal chemistry of M M Fg compounds has been reviewed by Reinen and Steffens (1978). 

Hydrated rare earth trifluorides, RF3-nH20, having compositions with n =0.5 are described in the recent survey by Haschke (1979) and in the Gmelin Handbook (1976), but their thermodynamic properties have not been critically evaluated. An important question is whether these tysonite-type phases are true hydrated fluorides or simply anhydrous fluorides with adsorbed or occluded water. Insight into the problem is provided by the results of calorimetric studies (Kondrat ev et al., 1967 Storozhenko et al., 1975, 1976b Afanas ev et al., 1975). Measured enthalpies of formation of several trifluoride hydrates and values derived for the enthalpies of hydration of the anhydrous trifluorides are presented in table 40. 

Nagel, L.E. and M. O Keeffe, 1973, Highly Conducting Fluorides Related to Fluorite and Tysonite, in Van Gool, W., ed., Proc. NATO Advanced Study Institute on Fast Ion Transport, Solids, Solid State Batteries Devices (North-HoUand, Amsterdam) pp. 165-172. 

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