Fluorides, alkaline-earth

The alkaline earths, Be, Mg, Ca, Sr, and Ba, and the gas Ra are divalent and form some halogen crystals under STP conditions, but they also form gaseous, liquid, and unstable halogen compounds. 

The Chin-Gilman parameter for these compounds is about 0.11. 

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Properties. Lithium fluoride [7789-24-4] LiF, is a white nonhygroscopic crystaUine material that does not form a hydrate. The properties of lithium fluoride are similar to the aLkaline-earth fluorides. The solubility in water is quite low and chemical reactivity is low, similar to that of calcium fluoride and magnesium fluoride. Several chemical and physical properties of lithium fluoride are listed in Table 1. At high temperatures, lithium fluoride hydroly2es to hydrogen fluoride when heated in the presence of moisture. A bifluoride [12159-92-17, LiF HF, which forms on reaction of LiF with hydrofluoric acid, is unstable to loss of HF in the solid form. 

The Dow No. 7 treatment, popular in the USA, also falls within this class. The process differs from other chromate treatments in that the activator, magnesium fluoride, is formed on the metal surface by immersion in 20% hydrofluoric acid solution, the parts then being immersed in a 10-15% alkali dichromate solution with or without sufficient alkaline earth fluoride to saturate it. A slow action occurs on the surface and the fluoride film is replaced by a chromate or mixed chromate/fluoride film. 

Donor-acceptor interactions also lead to strangely bent geometries in heavier F— M—F alkaline earth fluorides. Such bending can occur when strong ionic attractions force a filled fluoride pz orbital into proximity with an orthogonal metal pj orbital, for in this case symmetry-forbidden (pz)p —(p l)m interactions can turn on only when the strict a/n symmetry of a linear F—M—F arrangement is broken. 

Borides The electrolysis (at 950-1100°C) of a mixture of a borate and a transition metal fluoride (or alkaline-earth fluoride with the transition metal oxide) resulted in the synthesis of several borides (Ca, Sr, Y, Th, Ti, V, etc.). 

Likewise, Bayer AG have patented [ 133] an improvement process which entails the addition of fluoride salts to the HF electrolyte enabling the ECF of alkyl-sulphonyl fluorides to be run for many months, giving high yields and using less energy than usual. Examples of the additives are alkaline earth fluorides, alkali tetrafluoroborates, alkali hexafluorophosphates, HPF6, HBF4 and BF3. 

There has been a long standing question about the nature of the defect equilibria in the fluorites (13-22) Measurements of the conductivity and diffusion in fluorites is commonly interpreted with simple mass action relationships but the site selective laser spectroscopy (as well as other techniques) has shown that the situation is more complex and that simple mass action relationships don t even describe the observed equilibria qualitatively (23-28). The principal reasons for the failure of standard relationships is either other equilibria (in particular, the f scavenging equilibria) compete for F, that F is an unusual ion that is known for forming associates with itself that could change the defect equilibria, or there are abnormally large nonideality effects. These effects were studied in all of the alkaline earth fluorides and we found the same anomalies were always observed. 

C, appreciable yields of products were not produced until 750°C. Reaction of ethylene with pure CuF2 or CuF2 dissolved in a eutectic melt of alkali and alkaline earth fluorides between 450°C and 700°C yielded vinyl fluoride and HF and Cu metal. A similar reaction with propylene at 400°C yielded 2-fluoropropene [19]. The reaction cycle is shown in equations (7a) and (7b) and the net reaction scheme is shown in equation (7c). 

The story gets better. C. H. Anderson and E. S. Sabisky, "The absence of a solid layer of helium on alkaline earth fluoride substrates," J. Low Temp. Phys., 3, 235-8 (1970), reported the thickness of helium liquid condensed from vapor onto ceramic substrates. Van der Waals attraction nicely explains film thickness vs. the chemical potential of helium in the vapor. 

The influence of the non-complex negative ion on the stability also becomes manifest in the anhydrous crystallization, for example, of the alkaline earth fluorides, but on the other hand sodium perchlorate is NaC104.H20 while the sulphate with the double-charged anion contains ioH20 (Glauber s salt). Here other factors play a part, since the perchlorate in question is hygroscopic and is also very readily soluble. 

The superposition model has also been applied to experimental crystal field parameters obtained for lanthanides [31] substituted into host lattices of oxides, zircons, anhydrous trihalides, oxysulphides, alkaline earth fluorides and some other cubic crystals. The intrinsic parameters obtained from the analysis are given in Table 8.23. The solution spectrum of Er3+ aquo ion is given in Fig. 8.29. 

As the previous section showed, in a variety of examples severe enhancements of the ionic conductivity has been found and successfully attributed to space charge effects. Typical examples are silver halides or alkaline earth fluorides (see Section V.2.). How significantly these effects can be augmented by a particle size reduction, is demonstrated by the example of nano-crystalline CaF2.154 Epitaxial fluoride heterolayers prepared by molecular beam epitaxy not only show the thermodynamically demanded redistribution effect postulated above (see Section V.2.), they also highlight the mesoscopic situation in extremely thin films in which the electroneutral bulk has disappeared and an artificial ion conductor has been achieved (see Fig. 

Reduction processes are frequently involved in doping of materials prepared for specific applications. Bai <5Sr(5MgF4 (< < 0.55) was doped with Sm2+ by addition of Sm metal to the charge for crystal growth [48], Eu2+ is the key ion in fluorescent lamp phosphors for emission of blue light. Respective reduction of Eu3+ is frequently achieved in H2 atmosphere, but in alkaline earth fluoride phosphates, Sn2+ may act as reducing agent [49]. 

The possibility of using the nd9(n + l)s -> nd10 transitions of Cu+ or Ag+ doped alkaline earth fluorides for obtaining a tunable UV laser emission (ex. SrF2 Ag+ 2max = 315 nm) is also investigated [34,35]. 

In discussing the states of aggregation of solid fluorides as solutes, there is some overlap between the structures of the transition metal pentafluorides, discussed above, and those of corresponding tetra-, tri- and difluorides of d- and /-transition metals. The intermediate and lower fluorides of the transition metals are fluorine-bridged polymeric lattices, whereas the alkali metal and alkaline earth fluorides are essentially ionic and the non-metal fluorides are molecular. 

Electrostriction in solids is important as the origin of piezoelectric effects. Von Sterkenberg has measured the electrostrictive coefficients in alkaline (earth) fluorides and found electrostriction there to be anisotropic. 

The necessary alkali and alkaline earth fluorides for such reactions are accessible by dissolving the respective carbonates in aqueous HF. After reducing to a small volume, the precipitated bifluorides or fluorides are heated up to about 500 °C to obtain the anhydrous fluorides, which are highly hygroscopic for the larger alkali ions K+, Rb+, Cs+. 

T able 1 influence of the host lattice on the nature of the luminescence of Eu + in alkaline earth fluorides. A 4f — 4f emission is observed when the first 4f 5d states are above the first excited level of the 4f configuration at 27 800... 

Other alkaline earth fluorides (BaF2, SrF ) have been added to the model. However, they are less likely than their respective sulfates or carbonates to be solubility limiting phases. 

Despite the strong environmental effects on satellite spectra of covalent systems, SW calculations on next-neighbour clusters of ionic alkaline and alkaline earth fluorides have been carried out with rather modest expectation. Then, we were surprised to be... 

In 1961 Hayes and Twidell (8) found that if calcium fluoride crystals containing trivalent thulium were irradiated with x-rays, some of the thulium was converted to the divalent state. This discovery was the first of many in the study of dilute solutions of divalent rare earth ions. Most workers prefer to study the alkaline earth fluorides since these materials are stable with respect to air and have more attractive mechanical properties than the alkaline earth chlorides, bromides, and iodides. Enough work has been carried out in these softer materials to show that reactions similar to those in the fluorides do occur. 

Friedman and Low (6) have shown that the trivalent lanthanides dissolved in the alkaline earth fluorides can be compensated by interstitial fluoride ions at either adjacent or remote sites. If the interstitial is adjacent, the crystal field of the trivalent is axial but if it is remote, the crystal field of the trivalent is cubic. Measurement of the crystal field splitting of radiation-produced divalent lanthanide ions indicate cubic symmetry 16). More recent measurements by Sabisky (20) have shown a small percentage of non-cubic sites. It is thought that the trivalent ions in the cubic symmetry are the species predominantly reduced by radiation. 

U(III) in an alkaline earth fluoride matrix was one of the earlier laser materials 49h). As might be expected with crystalline substances, the splitting of the various J levels can be completely resolved, especially at low temperatures. Thus the polarized absorption spectrum of in a lanthanum chloride matrix (Csh environment) has been measured from 5600 to 25,000 cm and analyzed in detail. The ground state yields 5 sublevels in this symmetry, the state yields 6 sublevels, and 20 lines result from the combination of these two levels (50, 51). 

Hu, Y., Veeramasuneni, S., and Miller, J.D., Electrokinetic behavior of selected alkali and alkaline-earth fluoride salts in organic solvents. Colloids Sutf. A, 141, 193, 1998. 

The only way to rationalize the transport data for nanocrystalline ionic materials is to consider the systems separately, from the viewpoint of the level of defects in the crystals and the nature ofthe samples. In the case of lowly defective systems, such as the alkaline earth fluorides, there is good evidence for a conductivity enhancement, and the data can be explained in terms of models based on the space-charge layer. A key experiment here was the observation of enhanced conductivity in very thin alternating Cafb/Bafb layers when measured perpendicular to the layers [298]. This was explained as being due to the space-charge layers overlapping and saturating the layers. However, this observation is difficult to explain in terms of a model based on surface mismatch. 

Formation and interaction of unstable FP defects, the dominant intrinsic defects in UO2, alkaline earth fluorides and related compounds generally plays an important role in many diffusion mechanisms... 

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