Non-stoichiometric reconstructions

Contrary to metal and semi-conductor surfaces which present many intrinsic reconstructions, most observed reconstructions on oxide surfaces may be assigned either to an ordering of oxygen vacancies, characteristic of a surface sub-stoichiometry, or to a manifestation, at the surface, of a structural change in the bulk of the material. 

This latter case is exemplified by the doubling of the two-dimensional surface cell observed on the (001) face of SrTiOs, which is associated with the bulk anti-ferrodisplacive structural phase transition at 105 K. We will come back to this phase transition in Chapter 4 when discussing soft phonons. 

On the (0110) surface of a-quartz, (3x1) and (1x3) LEED patterns have been observed after annealing at 1173 K. They were correlated either to the bulk a—transition which occurs in Si02 at 846 K or to a tridymite phase present on the surface (Bart et ai, 1992 1994). On the (0001) surface, a reconstruction was also observed by LEED, after heating in air above 873 K. A preliminary interpretation in term of the a—phase transition was proposed (Bart and Gautier, 1994). 

The under-stoichiometric a-alumina surfaces present rich reconstructions associated with oxygen vacancies. After annealing at increasing temperatures, a succession of reconstruction patterns was observed on the (0001) surface (1 x 1), (V5x )R30°, (2 x2 )R30°, (3V3x3V3)R30 and i /31 x. 31)1 +9° (Chang, 1968 French and Somoijai, 1970 Baik 

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A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

Fig. 3.8. Non-stoichiometric reconstructions of a rocksalt lll face, (a) The stoichiometric surface (b) every other atom is missing in the surface plane (c) three out of four atoms are missing in the surface plane and one out of four in the underlying layer. Notice the 100 and 110 facets in (b) and the 100 facets in (c).

As far as charges are concerned, on non-polar surfaces the electrostatic effect approximately balances the reduction of hybridization. On polar surfaces, however, electrostatic effects are more drastic. Depending upon the compound, they may lead to surface instabilities, spontaneous desorption of atoms followed by non-stoichiometric reconstructions, or to surface metallization. 

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