Calcium fluorite

Wadsworth and coworkers (13, 14) have found considerable evidence for surface polarization in double-beam infrared spectroscopy. Not only do new differential peaks due to adsorption appear in the spectrograms but also the bands due entirely to the adsorbent are frequently appreciably shifted by adsorption. This occurred, for example, in calcium fluorite treated with oleic acid, in samples of bentonites taken from aqueous solutions of different pH, and in various minerals treated by flotation collectors. In fact, it is more the rule than the exception that the spectrograms of finely divided solids dispersed in the KI or KBr window exhibit distortion due to adsorption, whether adsorption occurs at the solid-aqueous solution or at the solid-vapor interface. For example, Eyring and Wadsworth (13) found that two (differential) peaks were produced by adsorption on willemite either from the vapor or aqueous solution of hexanethiol. These peaks were due to the influence of adsorption of the hexanethiol on the Si-O bands of the willemite and occurred at about 9.2 and 12.3 microns. 

Figure 3.1. Some common ceramic structures (a) rock salt, (ft) cesium chloride, (c) zinc blende, (d) wurtzite, (< ) calcium fluorite, (f) rutile.

AX2-type structures. Calcium fluorite (CaFi) and rutile (Ti02), shown, respectively, in Fig. 3.1c and/, are two examples of this type of structure. 

Normal achromats are made of a crown glass and a flint glass lens. Unfortunately, flint glass becomes nontransparent below about 360 run. Standard achromats therefore cannot be used in the UV. UV achromats can be made of fused silica and calcium fluorite. Their high price and low f numbers make them less useful for TCSPC optics. 

Fig. 3.51 Total density-of-states (DOS), partial DOS of Zn (in black), and COHP analysis for the Sn-Zn and Sn-Sn bonding in the calcium fluorite structure type of hypothetical Sn2Zn.

UO2 (as well as (U,Pu)02 for thermal reactors) form a crystal lattice of the calcium fluorite type showing a lattice constant of 0.5468 nm (5.468 A) at stoichiometric composition. As can be seen from Fig. 3.12., the structure consists of a cubic... 

In a Hall-Heroult process, the electrolyte is cryolite, Na3AlF6 (3NaF AIF3), with dissolved alumina and additives (up to 16-18 %) of aluminium fluorite, calcium fluorite, magnesium fluorite, and sometimes other fluorites [1-4]. The melting point of pure cryolite is 1,010 ""C (Fig. 2.1), but the temperature of electrolyte may be... 

Perovskite-type layered compounds are the intergrowth of perovskite l ers (P) ABO3 and slabs of the differ type of structure (rock salt, calcium fluorite type, cations of metals). Depending on the nature of slabs between perovskite blocks, layered compounds belong to three big groups Ruddlesden-Popper phases, Aurivillius phases, Dion-Jacobson phases. 

Fluorine occurs widely in nature as insoluble fluorides. Calcium fluoride occurs as jluospar or fluorite, for example in Derbyshire where it is coloured blue and called bluejohn . Other important minerals are cryolite NajAlFg (p. 141) and Jluorapatite CaFjSCaj (P04)2. Bones and teeth contain fluorides and some natural water contains traces. 

Calcium is a metallic element, fifth in abundance in the earth s crust, of which if forms more than 3%. It is an essential constituent of leaves, bones, teeth, and shells. Never found in nature uncombined, it occurs abundantly as limestone, gypsum, and fluorite. Apatite is the fluorophosphate or chlorophosphate of calcium. 

Eluorspar assay may be completed by fluoride determination alone, because the mineralogical grouping rarely iacludes fluorine minerals other than fluorite. Calcium can be determined as oxalate or by ion-selective electrodes (67). SiUca can be determined ia the residue from solution ia perchloric acid—boric acid mixture by measuriag the loss ia weight on Aiming off with hydrofluoric acid. Another method for determining siUca ia fluorspar is the ASTM Standard Test Method E463-72. 

The compounds of the MMe205F type, where Me = Nb or Ta M = Rb, Cs, Tl, crystallize in cubic symmetry and correspond to a pyrochlore-type structure [235-237]. This structure can be obtained from a fluorite structure by replacing half of the calcium-containing cubic polyhedrons with oxyfluoride octahedrons. 

Using the density of calcium fluoride (CaF2, found in nature as the mineral fluorite), which is known to be... 

In deriving theoretical values for inter-ionic distances in ionic crystals the sum of the univalent crystal radii for the two ions should be taken, and corrected by means of Equation 13, with z given a value dependent on the ratio of the Coulomb energy of the crystal to that of a univalent sodium chloride type crystal. Thus, for fluorite the sum of the univalent crystal radii of calcium ion and fluoride ion would be used, corrected by Equation 13 with z placed equal to y/2, for the Coulomb energy of the fluorite crystal (per ion) is just twice that of the univalent sodium chloride structure. This procedure leads to the result 1.34 A. (the experimental distance is 1.36 A.). However, usually it is permissible to use the sodium chloride crystal radius for each ion, that is, to put z = 2 for the calcium... 

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,... 

The structure can be instructively compared with that of fluorite, CaF2. In fluorite the calcium ions are arranged at face-centered lattice points, and each is surrounded by eight fluorine ions at cube corners. 

From the perspective of the fluoride ions, the fluorite stmcture can be seen as a body-centered cube with an F" anion in the center. Instead of eight comer ions, however, the alternating comers are vacant, placing calcium ions... 

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... 

Halides Metal ion(s) + halogen ion Common salt (sodium chloride), a component of animal diets Fluorite (calcium fluoride), a lapidary material and flux... 

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

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