Oxygen ordering

The disordered structure can be stabilized to room temperature by inclusion of substitutional impurities on the In sites. Thus the oxide formed when Ga is substituted for In, Ba2(ln1 xGaJt-)205+s to form Galn defects has a disordered cubic perovskite structure even at room temperature for values of x between 0.25 and 0.5, and the similar Ba2iln1 vCox)205+3 with Coin defects has a disordered cubic perovskite structure at room temperature when x lies between 0.2 and 0.8. The defects present in the In sites hinder oxygen ordering during the timescale over which the samples cool from the... 

Consider a fuel burning in inert airs and oxygen where the combustion requirement is only 0.21 moles of oxygen. Order the following mixtures as to their adiabatic flame temperatures with the given fuel. 

Acrolein order Concn. range, moles/liter Oxygen order Pressure range, mm. Hg... 

The existence of intermediate structures with various schemes of short-range oxygen ordering on the basal plane, indicates that the... 

Au/TiC>2 catalysts made by deposition-precipitation have been examined for the PROX reaction 27 they show some differences from Au/Fe203 catalysts. Hydrogen now interferes with the oxidation of carbon monoxide, perhaps by competing for the same adsorption sites this was shown by a marked increase in the order of reaction when hydrogen was present, but not in the oxygen order (see Table 7.2). However, there is no reason to expect that gold particles on these two supports differ in any fundamental... 

Key words 199Hg NMR MAS High-Tc superconductors oxygen order... 

Superstructures refer to secondary ordering derived from the basic unit cells of oxide crystals. The formation of superstructures in oxides can be due to cation ordering, oxygen ordering or both. 

In some oxides, formation of superstructures is merely due to oxygen ordering. A good example was demonstrated by Reller and coworkers in 1984, by showing many distinct superstructures in perovskite-type oxides CaMn03, t with oxygen contents of 2.5, 2.556, 2.667, 2.75 and 2.80 [38]. 

Fig. 1.7. Phase diagram for oxygen ordering in the YBCO class of high-temperature superconductors (after Ceder (1994)).

Case Study Oxygen Ordering in High Tc Superconductors... 

Fig. 6.28. Schematic illustrating the various effective interactions that are invoked in order to capture the energetics of oxygen ordering (adapted from Ceder et al. (1991)). Open circles correspond to O atoms and filled circles are Cu atoms.

Fig. 6.29. Phase diagram for oxygen ordering in YBa2Cu30 as obtained using Monte Carlo calculations (adapted from Aukrust et al. (1990)). Tetra refers to the disordered tetragonal phase, while ortho-1/4 and ortho-1 /2 are ordered orthorhomhic phases. The filled circles are experimental data points, and diamonds and crosses correspond to different Monte Carlo schemes. The dotted lines correspond to cluster variation calculations of the same phase diagram.

Ceder G., A Computational Study of Oxygen Ordering in YBa2Cu30 and its Relation to SupexconAuctiwity, Molecular Simulation, 12, 141 (1994). 

Lattice-Gas Model of Oxygen Ordering in YBa2Cu30 with Realistic Interactions, Phys. Rev. B52, 9784 (1995). 

Detailed electron diffraction evidence moreover shows that the hypothesis of oxygen ordering does not hold, and we will propose here evidence for an alternative model. The X ly/la phase is reproducibly obtained by... 

The absence of a transition state and the sharpness of the reflections at the very high oxygen deficiencies are untypical of the other oxygen ordered phases in YBa2Cu307 (5. However, since most models reported for the... 

Because of the small likelihood for the presence of oxygen vacancies in the Cu02 layer, and therefore also of oxygen ordering in this layer, the only reasonable model for the superstructure common to the compounds discussed is one of a displacive modulation in the CUO2 layers. A plausible deformation is one of the CuOs pyramids in between the BaO and the Y layers. A possible mechanism could be of Jahn-Teller type. Simple models were constructed with displacements within the (001) plane. 

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