Calcium fluoride structure

Crystals of the intermetallic compound magnesium stannide, MgjSn, have been prepared and investigated by means of Laue and spectral photographs with the aid of the theory of space-groups. The intermetallic compound has been found to have the calcium fluoride structure, with dwo = 6.78 0.02 A. U. The closest approach of tin and magnesium atoms is 2.94 0.01 A. U. 

FIGURE 7.5 The calcium fluoride structure (also known as the fluorite structure). 

Three oxide halides T10X (X = F, Cl, Br) are known.1 Preparations must be carried out at low temperatures1 387 since the heavier oxyhalides in particular readily decompose. There is no structural information on either TlOCl br TlOBr, but X-ray crystallography shows that TlOF has a calcium fluoride structure, with Tl3+ ions in a cube of F" or 02 ions.388... 

Efremova and Matizen (11) reported that the phase transition for SrF occurs over a short temperature interval (1421-1484 K). There are no discontinuities in their enthalpy data in this temperature range. Other alkaline earth dihalides (1J ), which have the calcium fluoride structure, are known to exhibit similar behavior, and this fact would seem to rule out the possibility that the observed transition resulted wholly from impurities. We speculate that the two crystalline forms, a and 0, of SrPg are practically... 

Goldschmidt predicted from his empirical rule that calcium chloride would not have the fluorite structure, and he states that on investigation he has actually found it not to crystallize in the cubic system. Our theoretical deduction of the transition radius ratio allows us to predict that of the halides of magnesium, calcium, strontium and barium only calcium fluoride, strontium fluoride and chloride, and barium fluoride, chloride,... 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

The fluorite structure is named after the mineral form of calcium fluoride, CaF2, which is found in the U.K. in the famous Derbyshire Blue John mines. The structure is illustrated in Figure 1.39. It can be described as related to a ccp array of calcium ions with fluorides occupying all of the tetrahedral holes. There is a problem with this as a description because calcium ions are rather smaller than fluoride ions, and so, physically, fluoride ions would not be able to fit into the tetrahedral holes of a calcium ion array. Nevertheless, it gives an exact description of the relative positions of the ions. The diagram in Figure 1.39(a) depicts the fourfold tetrahedral coordination... 

An ionic compound typically contains a multitude of ions grouped together in a highly ordered three-dimensional array. In sodium chloride, for example, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions (Figure 6.11). Overall there is one sodium ion for each chloride ion, but there are no identifiable sodium-chloride pairs. Such an orderly array of ions is known as an ionic crystal. On the atomic level, the crystalline structure of sodium chloride is cubic, which is why macroscopic crystals of table salt are also cubic. Smash a large cubic sodium chloride crystal with a hammer, and what do you get Smaller cubic sodium chloride crystals Similarly, the crystalline structures of other ionic compounds, such as calcium fluoride and aluminum oxide, are a consequence of how the ions pack together. 

The methods of synthesis of fluorapatite have been widely dis cussed (J ). It is for example possible to obtain fluorapatite by substituting the hydroxyl ion for the fluoride ion, either in a-queous solution at room temperature, or through a solid state reaction at 800°C. It can also be prepared by the action of 6-tricalcium phosphate on calcium fluoride at about 800°C. Its solubility and thermal stability have already been established. While much is known about fluorapatite, many questions still exist concerning the mechanism of their formation, their composition and the structure of some of them. Two of these problems are dealt with here. First, we discuss the formation mechanism of fluorapatite by a solid state reaction between calcium fluoride and apa-titic tricalcium phosphate. Then we present the preparation and the structure of a carbonated apatite rich in fluoride ions. 

The fiaorite structure Calcium fluoride crystallizes in the fluorite structure cubic F/n3m (Fig. 43). The coordination numbers are 8 for the cation (right fluoride ions form a cube about each calcium ion) and 4 for the anion (four Ca- ions tetrahedrally arranged about each F ion). 

Complete descriptions of the particle beam, its operation, its experimental setup, and its utility in protein structural studies have been previously described. (8, 12). Relevant PB dimensions include a 25 pm diameter fused silica capillary for production of the aerosol spray, a 22 cm length desolvation chamber to remove solvent, a single stage momentum separator, and a nozzle-substrate distance of 5 mm. Particle beam deposits ranged in size from 20 pm to 100 pm in diameter, and averaged approximately 50 pm. Deposit were made onto a water insoluble calcium fluoride (CaFj) window (25 mm dia. x 2 mm) from International Crystal Laboratories (Garfield, NJ). 

When radiation energy is absorbed by crystals of certain materials (e.g. lithium fluoride, lithium borate and calcium fluoride), the absorbed energy is trapped (stored) as displaced electrons within the crystal structure. If the material is heated later after the exposure, the trapped electrons are released and the stored absorbed energy is released in the form of visible light. This process is called thermoluminescence. Materials having this characteristic are called thermoluminescent. 

The crystal structure of sodium chloride (a) is not the same as that of calcium fluoride (b) because of the differences in the sizes of their ions and the cation-anion ratio making up each salt. 

Crystals of the ionic compound calcium fluoride have a different structure from that... 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

In this paper we are considering the metastable states of some of the chemical compounds with formula RX2 which crystallize in a lattice similar to that of calcium fluoride (Cap2) known as fluorite Hayes). In such structure crystallizes pure stoichiometric UO2 as well as its non-stoichiometric coimterparts known as hyperstoichiometric UO2+X and h q)ostoichiometric... 

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