Rare earth halides fluoride

Among the rare-earth halides, fluorides represent the most extensively studied systems with respect to the thermochemical properties of their vapors. Most of the studies took place in the period from the late 1960s to mid-1970s. 

SmCl3 resulted in the reduction only to SmC. From NdCl3 + Ca with the addition of Fe powder, the alloy Nd2Fei7 was obtained. In a discussion of the results it was observed that the products obtained at ambient temperature by mechanical alloying are the same which result from the conventional metallothermic reduction of the rare earth halides. However, the metallothermic reduction requires a temperature of 800-1000°C for the reduction of the chlorides and 1400-1600°C for the fluorides. The products of the mechanical process, on the other hand, are fine, amorphous or microcrystalline, highly reactive metal powders mixed with CaCl2. 

The anhydrous fluorides are by far the most important halides of the rare earth elements. This results mainly from their chemical and thermal stability in comparison to the other halides and, therefore, to their advantageous application in research and industry. The chemistry of the rare earth fluorides has been reviewed by Batsanova (1971), in the Gmelin Handbook (1976), and partially together with the other rare earth halides by Haschke (1979) in chapter 32 of this Handbook. With respect to the importance of the fluorides, it seems to be appropriate to devote a separate chapter to this class of compounds, especially because many new and exciting results have been found more recently which are not covered in the above reviews. This review deals mainly with preparation, phase relationships, structural chemistry, and thermodynamic properties of RF3, RF2, RF2+6, RF4, and mixed fluorides of the systems AF-RF3 and AF2-RF3, A(I) being alkali and A(II) alkaline earth elements. Special regard is paid to aspects which are omitted from or inadequately covered in the Gmelin Handbook (1976) and by Haschke (1979). 

Because stable tetravalent rare earth halides are formed only for the fluorides of Ce, Pr and Tb, the number of MX-MX4 complexes is much lower than for the MX-MX3 systems. Early reports of stable complex fluorides of Nd(IV) and Dy(IV) have not been confirmed. The complex tetravalent fluorides are predicted by radius ratio correlations (Thoma, 1962) and are reviewed by Brown (1968). An extensive investigation by Delaigue and Cousseins (1972) has greatly expanded the data for the cerium systems. Seven complex fluorides and the corresponding metal systems are shown in table 32.11. As expected, these phases are similar to the corresponding complexes of the actinides (Brown, 1968). 

Rare earth halides, especially the chlorides, have been extensively studied both in solution and in the solid state. Fluoride is an exceptional non-oxygen-donor ligand as it competes effectively with water molecules and enters the primary coordination sphere of the cation in aqueous solutions. The stoichiometry of the precipitated fluoride shows it to be nearly anhydrous, whereas the other halides prepared this way usually contain 6-9 water molecules. The removal of water from haUde hydrates is difficult as oxyhalides are easily formed. Iodides are especially sensitive to decomposition. 

Among the rare-earth halides, bromides constitute the group of systems which are studied to a much lesser extend compared to fluorides, chlorides and iodides. It is noteworthy that after Makhmadmurodov et al. (1975a,b) published their results on vaporization thermodynamics of some rare-earth bromides it was very recently that an extensive well-documented Knudsen effusion mass spectrometric investigation of the DyBr3 vaporization appeared in the literature (Hilpert et al. 1995). The successful characterization of the thermochemical properties of the dimer homocomplex Dy2Br6(g) by Hilpert et al. is taken as an indication that further vaporization studies are required for most rare-earth bromide systems with a view to establish the probable existence of vapor dimer homocomplexes and determine their thermochemical properties. Table 8 summarizes the vapor pressures and vaporization thermodynamics of rare-earth bromides. Most likely the vapor pressures reported so far could be in considerable error since the formation of dimers has not been taken into account. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Neodymium, along with lanthanum, cerium and praseodymium, has low melting points and high boiling points. The fluorides of these and other rare earth metals are placed under highly purified helium or argon atmosphere in a platinum, tantalum or tungsten crucible in a furnace. They are heated under this inert atmosphere or under vacuum at 1000 to 1500°C with an alkali or alkaline earth metal. The halides are reduced to their metals ... 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and halides. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

The change of halide ion results in weaker acidic properties for LnCl3 as compared with LnF3. This means that equilibrium (1.1.41) with the participation of alkali metal halide should be shifted to the left as compared with the fluoride complexes. That is, lithium chloride does not react with chlorides of the rare-earth elements with the formation of any compounds the binary phase diagrams are characterized by one simple eutectic. The same situation is observed for the binary diagrams for lithium- and rare-earth bromides. 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

Reisfeld, R., 1987, Optical properties of rare earths and transition metal ions in fluoride glasses, in Halide Glasses for Infrared Fiberoptics, ed. R.M. Almeida (Martinus Nijhoff, Dordrecht) pp. 237-251. 

Earths, alkaline Erden, seltene Earths, rare Fluorwasserstoffsaure Hydrogen fluoride Gallensaurederivate Bile acid derivatives Glycerine Glycerols Halbacetale Hemiacetals Halogenide Halides... 

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