Rare-earth trifluorides

Trifluorides are known from several elements of all transition series. In most of the compounds there are again linked MeFe-octahedra. A C.N. of 8 + 1 is found in the orthorhombic YFs-type [347) that is adapted by some of the rare earths trifluorides and will not be discussed here. The LaFa-type which represents the structure of trifluorides of the actinides, does not display a C.N. of 5, as was reported earlier A reinvestigation 220, 348) proved also 9-coordination for the larger Me-ions. 

Fig. 9.4. XPE spectra of some rare earth trifluorides showing the doublet structure in the 4s region [50],...

The thermal reaction between Xep4 and rare-earth trifluorides succeeds only for CeFj and TbFa, which are converted into the tetrafluorides. Xenon(iv) fluoride was found to be more reactive than Xep2 for these fluorination reactions. 

TIF3 is isostructural with /S-BiFg, YF3, and a number of rare-earth trifluorides, having an orthorhombic unit cell, space group Pnma with a = 5.825, b — 7.024, c = 4.851 A Z = 4. The T1 atoms can be regarded as eight-co-ordinate 6 fluorines describe a distorted trigonal prism about each T1 (one at 2.09, two at 2.22, two at 2.23, and one at 2.24 A), with two additional fluorines placed (at 2.49 A) outside two of the faces of the prism. ... 

Fig. 1. Dimorphism of the rare earth trifluorides (left) after Thoma and Brunton (1966) from samples containing significant amounts of oxide fluorides (right) from pure trifluorides after the data of table 1.

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. 

Thermodynamic data for hydrated rare earth trifluorides, RFs-nHjO. 

Several nonaqueous solvent systems have also been investigated. Solubilities of the trifluoride in nitrosyl fluoride solutions are reported (Galkin et al., 1978), and the solubilities of a variety of fluorides, including several rare earth trifluorides, in UFe have been measured and the results correlated with then-crystal lattice energies (Nikolaev and Sodikova, 1975). 

It is found that for many Zd transition metal compounds the observed Zs splitting is about half that calculated for the free ions and a similar effect is observed for 4 electrons in the rare earth trifluorides, although here the 5s splittings seem to show good agreement with Hartree-Fock calculations ... 

With the smallest of the rare-earth cations, Sc, a third structure type is found for rare-earth trifluorides ScFs crystallizes with the ReOs (AIF3) type of structure (Losch et al. 1982) with octahedral coordination of the cations and all octahedra [ScF ] sharing common comers. 

Sobolev BP (2000) The rare earth trifluorides. Institut d Estudis Catalans, Barcelona... 

The thermochemistry of rare-earth trifluorides was summarized in Gmelin Hand-buch (1976) and the thermochemistry of rare-earth tribromides and triiodides was summarized in Gmelin Handbuch (1978). The thermochemistry of trivalent rare-earth trichlorides was critically assessed by Morss (1976). Enthalpies of formation of most of the lanthanide tribromides were determined by Hurtgen et al. (1980). Thermodynamic properties for europium halides were assessed by Rard (1985). Only enthalpies of formation of Sc, Y, Dy and Tm triiodides have been redetermined since the classical work of Hohmann and Bommer (Morss and Spence 1992). A recent set of literature values of enthalpies of formation of rare-earth solid and gaseous trihalides has been published, accompanied by Born-Haber cycle estimated values for all trihalides (Struck and Baglio 1992). 

Determination of the basic thermodynamic properties of the rare earth trifluorides remains incomplete. This is in part due to experimental difficulties, and only one direct calorimetric measurement has been reported. The enthalpy of formation, AH 29s, of YF3 (-410.7 0.8 kcal/mole) has been measured by fluorine bomb calorimetry (Rudzitis et al., 1965). In an expansion of earlier work, Polyachenok (1967) has obtained values for several trifluorides (La, Pr, Nd, Gd, Er) by an equilibrium exchange reaction RCbi -f-A1F3( ->RF3(0 +AICl3(g). Solid state emf data have been reported by Skelton and Patterson (1973) for the trifluorides of Nd, Gd, Dy and Er. Similar measurements have been described by Rezukhina et al. (1974) for the trifluorides of La, Pr and Y. The AHt-m values are 5-10 kcal/mole more negative than values reported earlier. 

Fig. 17. Dimorphism among the rare-earth trifluorides (Thoma and Brunton 1966). (Reprinted by permission of the publisher. The American Chemical Society)...

The same trend was observed from a corrosion viewpoint whereas the corrosion resistance of solid film lubricants is increased to some extent by the addition of rare earth trifluorides it was considerably improved by the addition of BUC while withstanding more than 500 hours of exposure in salt spray test. 

Rare-earth trifluorides crystallize in three structural types the tysonite structure type, the hexagonal Or-YF3 structural type and orthorhombic P-YF structural type [7]. 

B. P. Sobolev, The Rare Earth Trifluorides, Part 1, The High Temperature Chemistry of the Rare Earth Trifluorides, Part 2, Introduction to Materials Science of Multicomponent Metal Fluoride Crystals, Inst. D Estudis Catalans, Barcelona (2000-2001). 

O. Greis, M. S. R. Cader, Polymorphism of high-purity rare earth trifluorides, Thermochim. 

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