Oxygen-defect structure-property

A natural question to ask is whether this two-regime theory is consistent with the known properties of LSM. As recently reviewed by Poulsen, the defect structure of LSM has some similarities with other more reducible perovskites such as LSG and LSF. Like these other perovskites, LSM has electrical properties on the border between that of a p-type semiconductor and a metaP and becomes oxygen substoichiometric at high temperature and low as shown in Figure 35. However, unlike its more reducible cousins (which may have significant vacancy concentration at atmospheric Pq ), LSM maintains a nearly full perovskite stoichiometry above atm and in fact becomes superstoichio-... 

To sum up, the available data relative to oxygen adsorption or catalysis on nickel oxide show the existence of two types of chemisorbed oxygen, one of them being related to the ability of this oxide to accommodate excess oxygen. The evidence concerning this latter property will now be reviewed with emphasis on the defect structure of bulk nickel oxide. 

Figure 12. The conductivity of negatively doped SrTiOj as a function of oxygen partial pressure. Top model (D negative doping), Bottom experimental data from Ref.51. (Reprinted from G. M. Choi and H. L. Tuller, Defect Structure and Electrical Properties of Single-Crystal Bao.ojSro. TiOj. /. Am. Ceram. Soc. 71, 201-205. Copyright 1988 with permission from the American Ceramic Society.)...

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. 

Indeed, the experiements appear to be precise and carefully executed. The same may be said of the careful thermodynamic study of Picard and Gerdanian on slightly reduced rutile, where the results obtained are also treated in terms of point defects. Interpretation in terms of CS planes would seem to be at least as realistic an alternative as point defects, and it is a pity that the authors have not attempted such an analysis. Other recent papers on the physical and chemical properties of reduced rutile, such as that of Baumard on the chemical diffusivity of oxygen in oxygen-deficient rutile, or that of Izumi on dielectric properties, are also analysed in terms of point defects only. Similar criticisms therefore apply to these articles. In contrast, studies of oxygen-tracer diffusion in rutile and the Ti 02 -i phases by Bagshaw and Hyde are presented clearly, with no extrapolations made about the defect structure of the materials used. 

Y. Teraoka, M. Yoshimatsu, N. Yamazoe and T. Seiyama, Oxygen-sorptive properties and defect structure of perovskite-type oxides, Chem. Lett. (1984) 893-896. 

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