Tysonite fluorides

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. ... 

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. 

Tysonite-type fluorides 148 7.4. Sensors based on protonically ... 

Rare-earth fluorides exist either as hexagonal (Lap3) or as orthorhombic structures ((3-YP3), as shown in fig. 17 for rare-earth fluorides with La to Lu (Thomas and Brunton 1966). Pluorides of the rare-earth elements La to Nd crystallize in the tysonite structure at all temperature ranges below the melting point. Smp3 to Gdp3 are orthorhombic at lower... 

Class 5 structures without anionic groups - fluorides and simple oxides FLUOCERITE-(Cc) (=tysonite), CeFj FLUOCERlTE-(La), LaFj GAGARINITE-(Y), (Y,Ca)2(Na,D)F6 TVEITITE-(Y), Cai4Y5p43... 

Schubert (1989) points out that radius ratio considerations in LaFj would suggest coordination near LaFg, and the LaFj structure should therefore be homeotypic with CaFj. He then relates the LaFj structure to that of CaFj, compares the displacements to those of related structures, e.g., CaFj-YFj superstructures, and discusses crystal energetics. LaFj crystals were examined by DSC from 100 -500 K an anomaly which occurs in the Cp versus T curve is related to a phase transition in which the fluoride sublattice fuses (Aliev and Fershtat 1984). This phenomenon appears related to the high ionic conductivity of the tysonite-type fluoride phases. 

Ravez J. The inorganic fluoride and oxyfluoride faroelectrics. J. Phys. HI 1997 7 1129-1144 Reau J.M., Hagenmuller P. Fast ionic conductivity of fluorine anions with fluorite- or tysonite-type... 

Cerium trifluoride is hydrolyzed to cerium oxide in humid air at high temperature. Of the tysonite-type fluorides, cerimn oxide mixed with calcium fluoride (5 mol%) shows the highest fluorine ionic conductivity as presented in fig. 18 (Takahashi et al. 1977). 

Table 14.1 Conductivity of some fluoride-ion conductive solid electrolytes with fluorite- and tysonite-like structures [2] (references to original papers are In Reprinted with permission from [2]). Copyright (2007) Pleiades Publishing Inc.

In Figure 14.3 the conductivity of various fluoride-ion solid electrolytes with tysonite-and fluorite-like structures is presented as a function of the ionic transfer activation energy (Ea). It can be seen that a suitable fluoride solid electrolyte with optimal properties for any applications could be chosen or its targeted search could be carried out. 

Recently a lot of reviews on ionic conductivity of inorganic fluorides were published. A short history of these investigations starting with Faraday s basic work of 1834 up to their current applications is presented in [2]. The structure and ionic conductivity of Pbi xAlxF2+x. Mi x(U or Th)xF2+2x (M = Ca, Sr, Ba, Pb) fluorites and CeFa, Cei yCdyF3 y tysoiutes are described in [5]. In a detailed review [6], various fluoride-conductive electrolytes perovskites MPbF3 (M = K, Rb, Cs), B-deficit perovskites - tysonites... 

In this review we will consider anion-conductive fluorides and oxyfluorides because the occurrence of oxygen in these phases is an additional parameter that controls the defect structure of fluorite- and tysonite-like solid solutions by increasing the anion vacancies or... 

Fluoride ion replacement by oxygen in some oxyfluoride conductors, for example in Bii xBax(0,F)3 6 tysonite-Uke (Figure 14.5) [11] or Bii xTex(0,F)2+5 fluorite-like phases (Figure 14.6) [12] leads, together with the effects mentioned above, to a decrease in the mobile fluoride ion concentration. 

The name of the tysonite structural type comes from the mineral tysonite, a solid solution of cerium and other RE fluorides in LaF3. There are two modifications of the tysonite... 

Functionalized Inorganic Fluorides 14.7J2 Ordered Tysonite-like Phases... 

To determine the ionic transfer mechanism it is necessary to know which structure positions are occupied by anion vacancies. It was shown earlier (see 14.7.1) that there are two different modifications of tysonite. The main difference between them consists in the fact that the modification I contains two sublattices, FI and F2, and the modification II contains three sublattices, FI, F2 and F3. For example, the F MAS NMR spectra of LaF3 and Lai 99Sr0.01F2.99 demonstrate that the three crystallographically distinct fluoride ion sites have very different mobilities, and the activation energies, for the different fluoride ion jump pathways, increase in the order Fl-1 < Fl-3 < Fl-2. [99] (Figure 14.32b). 

Studying the mechanism of charge transfer in tysonite-like structures researchers combine sometimes F2 and F3 positions together and consider only two fluoride... 

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