Oxides cubic

Figure 7.2 The structural changes of zirconia as a function of temperature. The placement of the ions is shown only in the cubic oxide structure...

Recently Schwab (70) also emphasized the important role of electron defects for catalytic activities of solid oxides. Following this concept, he tried to develop a mechanism for the decomposition of H2O2 and for the oxidation of CO using inverse spinels as catalysts. Since the mechanism of the electron disorder and conductivity in spinels is much more complicated than in simple cubic oxides, it is not surprising that a satisfactory interpretation of these catalytic effects is still lacking,... 

With this electric potential Poisson equation (A

el = net charge density) to eventually obtain the concentration of electrons at the film surface (A ). It further follows that Ne(A ) varies with the film layer thickness as A -2. If we now assume that the (catalyzed) rate of dissociation of the adsorbed X2 molecules is proportional to the surface concentration of electrons, and that this dissociation process is rate determining, a cubic rate law for the film growth can be expected (A — At 2 At - t in). In fact, during the oxidation of Ni at temperatures between 250 and 400 °C, an approximately cubic rate law has been experimentally observed. We emphasize, however, that the observed cubic oxidation rate does not prove the validity of the proposed reaction mechanism. Different models and assumptions concerning the atomic reaction mechanism may lead to the same or similar dependences of the growth rate on thickness. 

In a second step, first results obtained for the JX / /iwg quantity for Cu2+ in cubic oxides [1-6] (MgO and SrO) are shown. This study sheds some light on the occurrence of dynamic vibronic effects in Cu2+-doped MgO [2-4,6], The present methodology can also be applied to systems where the distortion coordinate at equilibrium becomes comparable to interatomic distances. Along this line some results reached for d9 ions in crystal with fluorite structure [2,11] will also briefly discussed. 

OTHER JAHN-TELLER SYSTEMS EXPLORED THROUGH DFT 4.1. Cu2+ doped cubic oxides... 

An additional interesting property of cubic oxides is their ability to form solid solutions (25) that maintain the original cubic structure. In these solids the cation sites can be shared between the two competitive cations over a wide range of compositions. This is the case for the NiO-MgO system, for which the Mg Nii O solid solution can be prepared with 0 < x < 1 because of the very similar ionic radii of the cations jr(Mg2+) = 0.72 A and r(Ni2+) = 0.69 A], Another relevant case is CoO-MgO. 

NiO is a cubic oxide characterized by ionicity and lattice parameters very similar to those of MgO. Furthermore, the preparation procedures of the two oxides may be similar (i) Stoichiometric high-surface-area NiO is prepared (as is MgO) from the hydroxide precursor by decomposition under vacuum and (ii) low-surface-area materials are obtained by progressive sintering at high temperatures. The evolution of the microcrystal morphology on passing from high- to low-surface-area (sintered) NiO is also similar to that for MgO, as demonstrated by Escalona Platero et al. (73,265,266) the final habit of the microcrystals is represented by nearly perfect cubes predominantly defined by atomically flat (001) faces and terraces. 

It also became evident that a great variety of catalysts, potentially exhibiting a large flexibility, could be prepared via solid solutions. Three different degrees of freedom can be varied in a controlled fashion the chemical nature of the host matrix AO, the chemical nature of the guest cation M, and the dopant concentration x. Furthermore, solid solutions can be formed not only by cubic oxides but also by alumina, titania, zirconia, and others. Thus, another degree of freedom is added, namely, the different crystal structures. 

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... 

Theoretical work by Tasker 131) confirms that this type of surface, which is charged and possesses a dipole moment perpendicular to the surface, can only be stabilized by substantial reconstruction. Overall, for the simple cubic oxides, it is possible that the lower coordination ions are most likely to be associated with imperfections in the low-index surface planes. 

Figure 10. Electron diffraction pattern of thin cubic oxide film on iron...

Lest you believe things are too simple during "cubic" oxidation I ve chosen to show this slide (Figure 20) based on data by Bob Shannon at Hanford, Washington, which shows that individual specimens corrode in series of waves. Paul Pemsler once christened those the Picturesque Hills of Shannon during a meeting. What this illustrates is that we re not... 

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