Example for MgO

Calculations for other oxides for which the standard free-energy difference between amorphous and crystalline oxides is large yield correspondingly smaller critical thicknesses [9]. For example, for MgO on Mg the value is on the order of a few tenths of a nanometer, i.e. less than a lattice parameter, such that vitreous MgO would not be expected to form. 

Other methods of prepn include the heating of nitrates, as for Na20 of carbonates, as for CaO or of hydroxides, as for MgO. Of course, many inorganic oxides occur in nature as ores. Examples of these are Fe203, Fe304 and Sn02 For individual methods of prepn, see under the selected inorgenic oxides described below... 

Many metal hydroxides are insoluble. For example, when MgO is made into a slurry, the white suspension is known as milk of magnesia. Calcium oxide (lime) is produced by heating limestone (calcium carbonate) at high temperature. 

In the above equation, may include the flux of several species because there may be several species for the component. For example, the MgO component in a rock can be in the form of Mg2Si04 and MgSiOs, and the H2O component in a melt can be in the form of molecular H2O and OH groups. For the H2O component, because of two species, the variation of total H2O concentration (CH20,) with time may be expressed as... 

In host matrices with cubic structure (e.g., AO = MgO, CaO, NiO, or SrO), the coordination number of the guest cation is 6 when substituting, for example, for bulk Mg2+ ions but M5C, M4C, and M3c dopant cations are obtained when the substitution occurs with surface Mg2+ or Ca2+ ions located on terraces, steps, or corners (1, 25, 331). As has been mentioned, at low x values M cations are isolated in the matrix, but increasing x leads to pairs, triplets, etc. (25, 332). 

The same model can be applied to an ionic solid. In this case, for the example of MgO, Fig. 2.9 represents the transfer of electrons from anions to cations resulting in an electron in the conduction band derived from the Mg2+ 3s states and a hole in the valence band derived from the 2p states of the O2- ion. Because the width of the energy gap is estimated to be approximately 8eV, the concentration of thermally excited electrons in the conduction band of MgO is low at temperatures up to its melting point at 2800 °C. It is therefore an excellent high-temperature insulator. 

A few moments thought about the nature of the surface of an oxide leads to the conclusion that the surface oxide ion should have quite different properties than the bulk lattice ions. For example, consider a simple cubic oxide such as MO with a sodium chloride structure where each ion is sixfold coordinated if this is cleaved along a <100) plane, then the coordination of the ions in this plane is reduced from six- to fivefold. This new surface will not be ideal, and ions of still lower coordination will also be present where higher index planes are exposed at the surface. However, for MgO prepared by thermal decomposition of the hydroxide or carbonate, evidence from electron microscopy (130) indicates that these have high index planes that... 

These processes give rise to the electronic absorption bands of lowest energy observed in the pure undamaged single crystals which occur at 7.68 eV for MgO and 6.8 eV for CaO (142). Defects within the crystal structure are associated with optical absorption bands at reduced energies [for example, the anion vacancy band in the alkali halides (143)] because of the lower Madelung potential. The energy is still absorbed by the processes described in Eqs. (27) and (28), but the exciton is now bound to a defect and is equivalent to an excited electronic state of the defect. 

More interesting behavior is exhibited by the metal oxides when the loose powders are consolidated into pellets. Compression is carried out by application of pressure using a hydraulic press. For example, AP-MgO powder has a low density of 0.30 cm3/g. Upon compression at low pressure (3000 psi) this increases to 0.58 cm3/g, and at 20,000 psi to 1.0 cm3/g. The surface area of AP-MgO powder is 364 m2/g. Upon compaction at a low pressure of 1000 psi, the surface area was found to reproducibly increase to 370 m2/g.29 On compaction at higher pressures of 5000 psi and 20,000 psi, the surface area was found to be 366 m2/g and 342 m2/g, respec-... 

I. For any given cation and two different anions, the larger anion forms a stronger covalent bond, for example, MgS > MgO S2- 1.85 A crystalline radius > 02 1.40 A crystalline radius. 

According to Eq. 4.10, each of these reactions will be dominant at different pH that is determined by Eq. 4.4, and hence, is dependent on the speciation of H3PO4 in the aqueous solution. For example, for 7 s pH > 2, H3PO4 will produce H2PO4. In acidic solution, i.e., in the presence of H" ", MgO will also partially dissolve to form cations of Mg by the dissolution reaction... 

One of the most prominent examples for which this technique has been applied was oxidative coupling of methane catalyzed by doped MgO. It was shown clearly that this reaction involves the formation of CH3 radicals 46). Another example, the oxidation of propene catalyzed by bismuth- and molybdenum-containing oxides, is discussed in detail in Section III.C.l. However, it is emphasized that the total pressure in the catalytic reactor was usually <0.1 MPa, quite different from practically realistic catalytic reaction conditions. Nevertheless, MIESR is a useful tool for elucidating the mechanisms of radical reactions. 

An important aspect of empirical potential parameterization is the question of transferability. Are, for example, models derived in the study of binary oxides, transferable to ternary oxides Considerable attention has been paid to this problem by Cormack et al., who have examined the use of potentials in spinel oxides, for example, MgAl204, NiCr204, and so on in addition Parker and Price have made a very careful study of silicates especially Mg2Si04. These studies conclude that transferability works well in many cases. However, systematic modifications are needed when potentials are transferred to compounds with different coordination numbers. For example, the correct modeling of MgAl204 requires that the potential developed for MgO, in which the magnesium has octahedral coordination, be modified in view of the tetrahedral coordination of Mg in the ternary oxide. The correction factor is based on the difference Ar between the effective ionic radii for the different coordination numbers. If an exponential, Bom-Mayer, repulsive term is used, the preexponential factor is modified as follows ... 

Figure 8.5 shows example calculated DOS plots for MgO and Ti02 (ratile). In both cases the zero of energy is taken as the highest occupied state which occurs at the top of the upper valence band (UVB). The small tail on the UVB that indicates some states with positive energy is an artefact of the smoothing process used to construct the DOS plot from a calculation using finite number of Ic-points. In addition to the total density of states, a decomposition into states associated with... 

With the advent of synthetic procedures leading to the production of highly dispersed metal oxides [46, 47], access to a wide range of surface area is now possible (that is <3m g to >300 m g for MgO) An alternative approach to the generation, and stabilization, of high surface area MgO samples is the use of a support, for example MgO/y-Al203 [48]. 

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