Magnesium fluoride, structure

The Rutile Structure.—A large number of compounds MX crystallize with the tetragonal structure of rutile, TiCfe. In this structure the position of the ion X is fixed only by the determination of a variable parameter by means of the intensity of reflection of x-rays from various crystal planes. In accordance with the discussion in a following section, we shall assume the parameter to have the value which causes the distances between X and the three ions M surrounding it to be constant. With this requirement the inter-atomic distance R and the edges a and c of the unit of structure are related by the equation R = (a/4 /2) [2 + (c/o)2]. In this way the inter-atomic distances in Table XII are obtained. In the case of magnesium fluoride the agreement is satisfactory. 

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. 

In order to specify the structure of a chemical compound, we have to describe the spatial distribution of the atoms in an adequate manner. This can be done with the aid of chemical nomenclature, which is well developed, at least for small molecules. However, for solid-state structures, there exists no systematic nomenclature which allows us to specify structural facts. One manages with the specification of structure types in the following manner magnesium fluoride crystallizes in the rutile type , which expresses for MgF2 a distribution of Mg and F atoms corresponding to that of Ti and O atoms in rutile. Every structure type is designated by an arbitrarily chosen representative. How structural information can be expressed in formulas is treated in Section 2.1. 

XPS is able to distinguish between metal ions in a mineral structure and those adsorbed on the surface with the same oxidation state. As an example, the Mg Is photoelectron- and Auger electron-spectra of Mg montmorillonite reveal considerable differences in the electronic states of the exchangeable and skeletal Mg (45), the former being similar to typical ionic compounds, such as magnesium fluoride, whilst the latter resembles magnesium oxide. 

With the aid of the data of Appendix F, predict the crystal structure of magnesium fluoride. (The observed structure is of the rutile type.)... 

FIG. 15-6. The structure of magnesium fluoride this substance has very high melting point and boiling point. 

The relative number of cations and anions also helps determine the most stable structure type. All the structures in Figure 12.27 have equal numbers of cations and anions. These structure types can be realized only for ionic compounds in which the number of cations and anions are equal. When this is not the case, other crystal structures must result. As an example, consider NaF, Mgp2, and SCF3 ( FIGURE 12.28). Sodium fluoride has the sodium chloride structure with a coordination number of 6 for both cation and anion. Magnesium fluoride has a tetragonal crystal structure called the rutile structure. The cation coordination number is stiU 6, but the fluoride coordination number is now only 3. In the scandium fluoride structure, the cation coordination number is stiU 6 but the fluoride coordination number has dropped to 2. As the cation/anion ratio goes down, there are fewer cations to surround each anion, and so the anion coordination number must decrease. We can state this quantitatively with the relationship... 

Dental caries result from a process that locally destroys the structure of the tooth. Organic acids demineralize the enamel, which liberates enamel ions such as calcium, phosphate, carbonate, magnesium, fluoride, sodium, and other trace elements. 

Of substances MX.j, silicon dioxide (radius ratio 0.29) forms crystals with tetrahedral coordination of four oxygen ions about each silicon ion, magnesium fluoride (radius ratio 0.48) and stannic oxide (radius ratio 0.51) form crystals with octahedral coordination of six anions around each cation (the rutile structure, Figure 18-2), and calcium fluoride (radius ratio 0.73) forms crystals with cubic coordination of eight anions around each cation (the fluorite structure. Figure 18-3). The ligancy (coordination number) increases with increase in the radius ratio, as indicated in Figure 18-1. 

The structure of magnesium fluoride (stereo) this substance has high melting point and boiling point. (This structure is usually called the rutile structure it is the structure of the mineral rutile, Ti02.) The large spheres are fluoride ions and the small spheres magnesium ions. 

S. Wuttke, G. Scholz, St. Rudiger and E. Kemnitz, Variation of sol-gel synthesis parameters and their consequence for the surface area and structure of magnesium fluoride, J. Mater. Chem., 17, 498(E4988 (2007). 

The influence of preparation procedure on structural and surface properties of magnesium fluoride support and on the activity of ruthenium catalysts for selective hydrogenation of chloronitrobenzene... 

The effect of preparation conditions on structural and surface properties of magnesium fluoride was studied in the aspect of its use as a catalyst support. Amorphous and spherical polyciystaUine Mgp2 supports were prepared and characterised by BET, XRD, TEM, and FTIR (pyridine adsorption) techniques. The influence of Mgp2 properties on the performance of Ru/MgF2 catalysts in selective reduction of ortho- and para-chloronitrobenzene to respective chloroanilines is reported as well. 

Table 1. Textural and structural properties of magnesium fluoride samples obtained by different methods.

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,... 

Fig. 13-11.—The structure of the cubic crystal KMgF3. Potassium ions are represented by large shaded circles. They are at the corners of the unit cube. The fluoride ions, represented by large open circles, are at the face-centered positions, and the magnesium ions, represented by small circles, are at the center of the cubes. This structure is often called the perovskite structure perovskite is the mineral CaTiOj.

Fluoride and hydroxide ions are saturated by bondB of total strength 1. This is achieved by two aluminum octahedral bonds, as in hydrar-gillite (Al(OH)t), with the structure shown in Figure 13-17, topaz (AljSiChF ), jmnyite, described below, and many other crystals, and also by three magnesium octahedra in brucite, Mg(OH)t, and other crystals. 

The sodium chloride structure is adopted by most of the alkali metal halides All of the lithium, sodium, potassium, and rubidium halides plus cesium fluoride It is also found in the oxides of magnesium, calcium, strontium, barium, and cadmium... 

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