Autocompensation

A set of simple physical and chemical principles can be used to understand and predict the surface reconstruction or faceting, which occurs on oxide surfaces.8,9 These include (1) autocompensation of stable surfaces, (2) rehybridization of the dangling bond charge density, (3) formation of an insulating surface, (4) conservation of near-neighbor bond lengths (which minimize the formation of surface strain fields), and (5) surface kinetic factors. We shall briefly discuss all of these factors governing the surface reconstruction of metal oxide. 

The autocompensation model states that the energetically most stable surfaces are those for which all the cation-derived dangling bonds are completely empty and all anion-derived dangling bonds are completely full. Thus, this model predicts which rearrangement of atoms and which surface terminations will be stable and exist.10 Surface autocompensation is a necessary but insufficient condition for a stable structure. This means that there may be several autocompensated surfaces that are stable but not observed, presumably because some other autocompensated surfaces are more stable energetically. However, the main drawback of this model is that it cannot predict interlayer... 

Once a particular surface structure has been determined to be stable (i.e., autocompensated), the primary factor determining the nature of the surface reconstruction is the energy that can be gained by rehybridizing the surface dangling bond charge density in response to the reduced coordination at the surface. As a consequence, a charge transfer between the atoms at the surface takes place and this results in the formation of new bonds between surface atoms adsorbed to the surface (also known as adatoms). The formation of new bonds on the surface leads to different chemical and physical properties at the surface.11... 

A variety of commercially available autocompensating spcctrophotoflu-orometers (photoluminescence instrumentation) have been employed to measure and analyze the photoluminescence spectra of catalysts. The three principal components of aU spectrophotofluorometers are the excitation light source, the chamber to set the sample, and the emitted photon detector (Fig. 7). The light source is usually either a mercury or a xenon arc lamp. 

There is only one way to cut a rutile crystal in (001) direction (Fig. 16.) Although this creates a non-polar, autocompensated surface, it does not represent a low-energy configuration. This becomes clear immediately when reviewing the coordination of the surface atoms. All the Ti atoms are 4-fold coordinated, and all the O atoms 2-fold coordinated. Hence the number of broken bonds on this surface is higher than on the other low-index rutile surfaces discussed so far. Consequently, the (001) surface has a high surface... 

As shown in Fig. 19, the stable, autocompensated anatase (001) surface exhibits exclusively 5-fold coordinated Ti atoms, as well as two-fold and threefold coordinated oxygen atoms. Calculations show that the corrugation increases somewhat upon relaxation, from 0.82 A to 0.92 A [6]. 

Although the surface atomic stmcture varies little from the ideal, the redistribution of electrons at the surface can be expectedly significant because of the covalent nature of Fe-S bonding in pyrite. An ideal starting point is to consider the predictions of the electron counting principles of autocompensation, which uses the formal charges on the atoms. Each Fe atom contributes two valence electrons to six Fe-S bonds, or 1/3 e per bond. Each S atom contributes six valence electrons to three Fe-S and one S-S bond. Taking one electron from each S to form the S-S bond, 10 are left for the six Fe bonds... 

For various physical reasons, this effectively means the highest occupied and lowest unoccupied states at the surface are localized on Fe sites (Rosso et al. 1999a). This is in conflict with the autocompensation model which leads to a prediction that the S sublattice should be imaged at negative sample bias, and the Fe sublattice should be imaged at positive sample bias. 

Hemai6(001) surface. By examination of the bulk structure, a low surface energy, non-polar bulk termination is found along (001) planes between the iron atoms in Fe-Fe bilayers (Fig. 42). The planar division creates two equivalent surfaces composed of a 1/3 monolayer of iron over a close-packed oxygen layer. Along a single [001] vector, the minimum repeat unit of 18 atomic planes (conventional cell) can be seen to have no net dipole moment and is thereby predicted to be stable from an electrostatic perspective. Similarly, using the covalent perspective, autocompensation predicts that this surface... 

Fig. 119. Situation provided by deficiency or loss of the standard pathways and autocompensation by neuromodulators which generate fast mode and double search. Giving neuroleptics eventuallv retrieves the standard pathway (if it is only deficient). If this is not possible the therapy abolishes fast mode and double search by reducing the strength of the...

Aqueous processing, precursors for ferroelectric thin films, 95-104 Autocompensated surface structure of GaN film on sapphire experimental description, 26-27 experimental procedure classical ion trajectory simulations, 28 GaN sample, 27 first-layer species... 

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