Oxygen vacancies layer

The structural relationships in Bi203 are more complex. At room temperature the stable fonn is monoclinic o -Bi203 which has a polymeric layer structure featuring distorted, 5-coordinate Bi in pseudo-octahedral iBiOs units. Above 717°C this transforms to the cubic -form which has a defect fluorite structure (Cap2, p. 118) with randomly distributed oxygen vacancies, i.e. [Bi203D]. The )3-form and several oxygen-rich forms (in which some of the vacant sites are filled... 

An example of a layer structure mixed conductor is provided by the cathode material L CoC used in lithium batteries. In this solid the ionic conductivity component is due to the migration of Li+ ions between sheets of electronically conducting C0O2. The production of a successful mixed conductor by doping can be illustrated by the oxide Cei-jPxx02- Reduction of this solid produces oxygen vacancies and Pr3+ ions. The electronic conductivity mechanism in these oxides is believed to be by way of electron hopping between Pr4+ and Pr3+, and the ionic conductivity is essentially vacancy diffusion of O2- ions. 

Jacobsen et al. have demonstrated a Schottky diode with a mixed ion conductor, Pt/CeO/SiC, which showed very high sensitivity to hydrogen at 500°C after annealing at 700°C [89]. It was proposed that the annealing introduced oxygen vacancies in the CeO layer, which were suitable for interaction with protons, thus causing... 

Figure 5.25 (a) Vacancy ordering in the ab plane of CajFejOj of brownmillerite structure (b) alternating sequence of octahedral and tetrahedral layers in the c direction. Small filled circles, iron large filled circles, oxygen open squares, oxygen vacancy. 

Typical point defects present at the Si02 surface are the so called E centres, holes trapped at oxygen vacancies, and Si dangling bonds. These latter defects are particularly important when present at the Si/SiOz interface because they markedly affect the electrical properties of electronic devices. These defects, which are also known as Pb centres, have been widely investigated in the past. Recently however, the microscopic origin of these defects has been unravelled by means of a sophisticated UHV-ESR system by Futako et al, 178 who elucidated the formation processes of interface dangling bonds (Pb centres) during the initial oxidation of a clean Si(lll) surface. After oxidation of one or two Si layer(s), the... 

The points representing the data plotted in the coordinates log P0l vs. mass of O in the surface layer lie on a straight line. Thus the logarithmic adsorption isotherm is obeyed. A particular feature of this system is that the state of free surface, 6 — 0, corresponds to a complete removal of oxygen from the monoatomic surface layer of the Fe304 lattice and the state of occupied surface, 0 = 1, corresponds to the oxide with the stoichiometric composition in the surface layer. Thus Z in (344) is an oxygen vacancy on the surface of a magnetite crystal. 

III-7. Hydrogen Photogeneration from Low-Pressure Water Vapor. Water vapor can react with oxygen vacancies of illuminated pre-reduced SrTi03 surfaces to yield hydrogen and lattice oxide.(14) Vacuum-prepared (111) surfaces of pre-reduced and stoichiometric SrTi03 were heated to 400°C in 10 Torr D2O in a UHV system equipped with a quadrupole mass spectrometer. Illumination of the pre-reduced crystal caused an increase in the D2 pressure of the system equivalent to D2 production of 3 mono-layers/hr. No such effect was seen on the stoichiometric crystal. 

The unit cell of pyrochlore can be considered as eight CaF2-type cells stacked as octants of a cube. There are two types of cells shown in Figure 6.21. They differ in the position of the oxygen vacancy and the relative positions of atoms A and B. Only two of the eight octants are shown to make it easier to visualize the relative positions of the three types of atoms. The octant on the lower left is type I and the other one is type II. There are four of each type, and they are not in adjacent cells. Atoms A and B are in ccp layers (a face-centered cubic structure) with oxygens in T layers. The type I cube has A ions located on face... 

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