Corundum Covalent bond

As this review is intended to illustrate, the interplay between metal and oxygen leads to a richness of reactivity that is reflected in the surface structure of oxides. Much of this richness can be rationalised as varying proportions of ionic and covalent character in the metal-oxygen bonding, and is manifest in a variety of non-stoichiometric surfeces. We therefore focus on the prototypical transition metal oxide smface rutile Ti(>2 (1 1 0). This is contrasted with computational results for one of the most widely-studied p-block oxide surfaces - corundum Al2O3-(0 0 0 1) - and we refer also to computational surface studies on oxides of ruthenium, iron, vanadium, tin and silver, as well as ternary oxides. 

The majority of crystals can be classified as semi-ionic (with chemical bonding being intermediate between covalent and purely ionic limits). For such crystals the adequacy of selected basis sets must be carefully tested as is discussed in [458], for example, for semi-ionic compounds Si02 (quartz) and AI2O3 (corundum). The exponents of the outer shell for the two cations (Si and Al) used in molecular calculations prove to be too diffuse. For the Si atom in quartz, reoptimization in the bulk gives a = 0.15 (instead of the molecular value 0.09). Corundum is more ionic than quartz, and in this case it is better to eliminate the most diffuse valence shell of Al, and to use two Gaussians of the inner valence shells as independent functions (a = 0.94 and a = 0.3, respectively). 

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