Titanium-oxygen interstitial defect

MgO (Figure 9.26). Figure 9.25 shows that the hardening rate due to Ti is about sixfold that due to Ti [203]. There are no equivalent data for Fe compared to Fe in MgO, but it is certainly true that the hardening rate due to Fe is also about sixfold that due to Ni in MgO [204]. As stated above, this is due to the formation of chargedefect complexes - three substitutionals with one Al cation vacancy (ora titanium-oxygen interstitial defect [207]) in the sapphire case and... 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

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