Defects transition metal oxides

Reactions involving the creation, destruction, and elimination of defects can appear mysterious. In such cases it is useful to break the reaction down into hypothetical steps that can be represented by partial equations, rather akin to the half-reactions used to simplify redox reactions in chemistry. The complete defect formation equation is found by adding the partial equations together. The mles described above can be interpreted more flexibly in these partial equations but must be rigorously obeyed in the final equation. Finally, it is necessary to mention that a defect formation equation can often be written in terms of just structural (i.e., ionic) defects such as interstitials and vacancies or in terms of just electronic defects, electrons, and holes. Which of these alternatives is preferred will depend upon the physical properties of the solid. An insulator such as MgO is likely to utilize structural defects to compensate for the changes taking place, whereas a semiconducting transition-metal oxide with several easily accessible valence states is likely to prefer electronic compensation. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

If majority point defect concentrations depend on the activities (chemical potentials) of the components, extrinsic disorder prevails. Since the components k are necessarily involved in the defect formation reactions, nonstoichiometry is the result. In crystals with electrically charged regular SE, compensating electronic defects are produced (or annihilated). As an example, consider the equilibrium between oxygen and appropriate SE s of the transition metal oxide CoO. Since all possible kinds of point defects exist in equilibrium, we may choose any convenient reaction between the component oxygen and the appropriate SE s of CoO (e.g., Eqn. (2.64))... 

Tomlinson, S. M. (1992) in Defect Processes in Transition Metal Oxides, Polar Solids Disc. Group, Oxford... 

Let us refer to Figure 5-7 and start with a homogeneous sample of a transition-metal oxide, the state of which is defined by T,P, and the oxygen partial pressure p0. At time t = 0, one (or more) of these intensive state variables is changed instantaneously. We assume that the subsequent equilibration process is controlled by the transport of point defects (cation vacancies and compensating electron holes) and not by chemical reactions at the surface. Thus, the new equilibrium state corresponding to the changed variables is immediately established at the surface, where it remains constant in time. We therefore have to solve a fixed boundary diffusion problem. 

Since these considerations are independent of the nature of the sample, the results are valid for all crystals which are exposed to a sudden change of intensive state variables. The meaning of the chemical diffusion coefficient D must, however, be carefully investigated in each case (see Section 5.4.4). At 1000°C, Dv for simple transition-metal oxides is on the order of 10 7 cm2/s. This gives for cubic samples of 10-3 cm3 a defect relaxation time of approximately 1 h according to Eqn. (5.86). 

A review of recent research, as well as new results, are presented on transition metal oxide clusters, surfaces, and crystals. Quantum-chemical calculations of clusters of first row transition metal oxides have been made to evaluate the accuracy of ab initio and density functional calculations. Adsorbates on metal oxide surfaces have been studied with both ab initio and semi-empirical methods, and results are presented for the bonding and electronic interactions of large organic adsorbates, e.g. aromatic molecules, on Ti02 and ZnO. Defects and intercalation, notably of H, Li, and Na in TiC>2 have been investigated theoretically. Comparisons with experiments are made throughout to validate the calculations. Finally, the role of quantum-chemical calculations in the study of metal oxide based photoelectrochemical devices, such as dye-sensitized solar cells and electrochromic displays, is discussed. 

The underlying motivation of the work presented in this paper is to provide a theoretical understanding of basic physical and chemical properties and processes of relevance in photoelectrochemical devices based on nanostructured transition metal oxides. In this context, fundamental problems concerning the binding of adsorbed molecules to complex surfaces, electron transfer between adsorbate and solid, effects of intercalated ions and defects on electronic and geometric structure, etc., must be addressed, as well as methodological aspects, such as efficiency and reliability of different computational schemes, cluster models versus periodic ones, etc.. 

J. Haber Crystallography of Catalyst Types Structural properties of metals and their substitutional and interstitial alloys, transition metal oxides as well as alumina, silica, aluminosilicates and phosphates are discussed. Implications of point and extended defects for catalysis are emphasized and the problem of the structure and composition of the surface as compared to the bulk is considered. 

Computer Modelling of the Defect Structure of Non-Stoichiometric Binary Transition Metal Oxides. 

The oxides containing cations in octahedral coordination, such as MgO and TiOz, seem to suffer little or no reconstruction of the crystal structure at the surface, but there are major changes in electronic structure. These effects have important implications for surface reactivity, especially when oxygen vacancy defects are considered and particularly in transition-metal oxides. The example of TiOj was discussed above more complex behavior is shown by species such as TijOj and NiO (Henrich, 1987). The latter, like other transition metals, shows increasing [Pg.415]

In defects on transition metal oxides DFT again fails, giving structures that show unlikely relaxations and tending to delocalize electrons associated with the defect into conduction band states. Hybrid functionals and DFT + U have also been used to correct the models in these cases, giving a localized picture of surface reduction. These methods are now able to give useful descriptions of reactions at these defect sites, including the transfer of electrons between surface and adsorbate required in redox chemistry. 

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