Crystalline metal fluorides

Zheng, A., Liu, S.B. and Deng, F. (2009) F chemical shift of crystalline metal fluorides theoretical predictions based on periodic structure models. J. Phys. Chem. C, 113(33), 15018-15023. 

The physical and chemical properties are less well known for transition metals than for the alkaU metal fluoroborates (Table 4). Most transition-metal fluoroborates are strongly hydrated coordination compounds and are difficult to dry without decomposition. Decomposition frequently occurs during the concentration of solutions for crysta11i2ation. The stabiUty of the metal fluorides accentuates this problem. Loss of HF because of hydrolysis makes the reaction proceed even more rapidly. Even with low temperature vacuum drying to partially solve the decomposition, the dry salt readily absorbs water. The crystalline soflds are generally soluble in water, alcohols, and ketones but only poorly soluble in hydrocarbons and halocarbons. 

Any pure gas, when cooled sufficiently, will condense to a liquid and then, at a lower temperature, will form a solid. There is great variance in the temperature at which this condensation occurs. Apparently there is a corresponding variance of the forces in liquids and solids. For example, lithium fluoride gas at one atmosphere pressure condenses when cooled below 1949°K. When the temperature is lowered to 1143°K, the liquid forms a clear crystal. In contrast, lithium gas at this pressure must be cooled to 1599°K before it forms a liquid and this liquid does not solidify until the temperature reaches 453°K. The solid is a white, soft metal, not resembling crystalline lithium fluoride at all. Fluorine gas is equally distinctive. At one atmosphere pressure it must be cooled far below room temperature before condensation occurs, at 85°K. Then the liquid solidifies to a crystal at 50°K. Why do these three materials behave so differently Can we understand this great variation Let us begin by finding a common point of departure. 

This calculation is still hypothetical, in that the actual substance formed when sodium metal reacts with difluorine is solid sodium fluoride, and the standard enthalpy of its formation is -569 kJ mol-1. The actual substance is 311 kJ mol-1 more stable than the hypothetical substance consisting of ion pairs, Na+F (g), described above. The added stability of the observed solid compound arises from the long-range interactions of all the positive Na+ ions and negative F ions in the solid lattice which forms the structure of crystalline sodium fluoride. The ionic arrangement is shown in Figure 7.5. Each Na+ ion is octahedrally surrounded (i.e. coordinated) by six fluoride ions, and the fluoride ions are similarly coordinated by six sodium ions. The coordination numbers of both kinds of ion are six. 

Crystalline Palladium was obtained by Joly 4 as the result of heating palladium ribbon, dusted over with finely divided topaz, to redness by means of an electric current. The topaz apparently decomposes, evolving fluorine, which attacks the palladium, yielding a fluoride, which in turn dissociates, leaving a residue of crystalline metal. The crystals resemble those of platinum obtained in a similar manner both in colour and lustre, and appear also to be isomorphous with them. 

The major use of C1F3 is in the nuclear industry which converts unclean spent fuel reprocessing, uranium metal into gaseous uranium hexafluoride. Other applications are low temperature etchant for single crystalline silicon [63,64], It is also used as a fluorinating reagent and in the synthesis of GIF and conversion of metals to metal fluorides such as tantalum and niobium metals to tantalum pentafluoride and niobium pentafluoride, respectively. 

Hexavalent. Uranium hexafluoride, UFe, is one of the best-studied uranium compounds in existence due to its importance for uranium isotope separation and large-scale production ( 70 000 tons per year). All of the actinide hexafluorides are extremely corrosive white (U), orange (Np), or dark brown (Pu) crystalline solids, which sublime with ease at room temperature and atmospheric pressure. The synthetic routes into the hexafluorides are given in equation (13). The volatility of the hexafluorides increases in the order Pu < Np < U in the liquid state and Pu < U < Np in the solid state. UFe is soluble in H2O, CCI4, and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The oxidative power of the actinide hexafluorides are in line with the transition metal hexafluorides and the order of reactivity is as follows PuFg > NpFg > UFg > MoFe > WFe. The UFe molecule can also react with metal fluorides to form UF7 and UFg. The same reactivity is not observed for the Np and Pu analogs. 

We may make two generalizations about crystalline metal halides. First, fluorides differ in structure from the other halides of a given metal except in the case of molecular halides (for example, Sbp3 and SbCl3 both crystallize as discrete molecules) and those of the alkali metals, all the halides of which are essentially ionic crystals. In many cases the fluoride of a metal has a 3D structure whereas the chloride, bromide, and iodide form crystals consisting of layer, or sometimes chain, complexes. (For exceptions, particularly fluorides MF3-MF5, see Table 9.9.) Second, many fluorides and oxides of similar formula-type are isostructural, while chlorides, bromides, and iodides often have the same types of structure as sulphides, selenides, and tellurides. The following examples illustrate these points ... 

The only binary halide of Mn(lV) is MnF4, prepared from the elements. It is an unstable blue solid which decomposes at ambient temperatures (equation 21.50). Crystalline MnF4 is dimorphic. The building blocks in ot-MnF4 are tetramers like those in VF4 and p-CrF4 (21.14). However, in these three metal fluorides, the assembly of the tetramers differs and in ot-MnF4, they are linked to give a three-dimensional network. 

Oxide/fiuoride composite thin films, where crystalline metal oxide nanoparticles are dispersed in a metal fluoride matrix, have been successfully fabricated by the... 

Fluorides represent another alternative to oxides, and may have higher redox potentials compared with similar oxides [54]. In comparison with the alkali fluoride salts used in the preparation of vanadium phosphates [62,63], Kohl et al. [54] prepared lithiated transition metal fluorides, LigMFg, using acetyl acet-onate salts as metal precursors and organic solutions of HF as a fluoride source. Extensive structural characterizations were presented for M = V, Mn, Cr, and Fe and the crystalline domain size was found to vary between 30 and 200 nm. However, no electrochemical characterization was reported. 

Metal fluoride nanomaterials can also be prepared by fluorination of oxide gels. The reaction of magnesium and aluminum alkoxides with HE in nonaqueous solvents led to the formation of amorphous or crystalline nanosized MgE2 and AIF3, respectively, with high specific surface areas [16, 17]. 

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