Vacancies oxygen

Ionic conductivity is used in oxygen sensors and in batteries (qv). Stabilized zirconia, Zr Ca 02 has a very large number of oxygen vacancies and very high conductivity. P-Alurnina/72(9(9j5 -4< -(y, NaAl O y, is an excellent cation conductor because of the high mobiUty of Na" ions. Ceramics of P-alurnina are used as the electrolyte in sodium-sulfur batteries. 

The proposed diamagnetic species include the neutral oxygen vacancy, 6i—S , the doubly coordinated siUcon, —O—Si—O— and the peroxyl bond,... 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

When ahovalent, ie, different valence, impurities are added to an ionic soHd, the crystal lattice compensates by forming defects that maintain both electrical neutraUty and the anion to cation ratio of the host lattice. For example, addition of x mol of CaO to Zr02 requires the formation of x mol of oxygen vacancies. 

If this concentration is larger than the oxygen vacancies created by thermal effects, then the conductivity from the motion of the doubly charged ions is directly proportional to the concentration of CaO (eq. 15). 

Sohd solutions of ceria with trivalent ions, eg, Y and La, can readily be formed. The ions substitute for the tetravalent Ce and introduce one oxygen vacancy for every two ions. The dopant ions and the oxygen vacancies form charge associates. The resulting defect-fluorites have good oxide... 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

In a sense, a superconductor is an insulator that has been doped (contains random defects in the metal oxide lattice). Some of the defects observed via neutron diffraction experiments include metal site substitutions or vacancies, and oxygen vacancies or interstituals (atomic locations between normal atom positions). Neutron diffraction experiments have been an indispensable tool for probing the presence of vacancies, substitutions, or interstituals because of the approximately equal scattering power of all atoms. 

Another example of the use of neutron diffraction to understand the role of atomic vacancies in producing a superconducting metal oxide phase is work that has been performed on Bao Kq 4fii03. This work demonstrates that at the synthesis temperature (700° C), under the proper conditions, oxygen vacancies are created to allow the formation of the parent phase with bismuth largely in the +3 oxidation state. The presence of the vacancies allows the incorporation of potassium in the... 

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... 

Schematic illustration of shear-plane formation. Structure (a) with aligned oxygen vacancies shears to eliminate these vacancies in favour of an extended planar defect in the cation lattice as in (b). % cations oxygen ions are at the mesh intersections...

The oxygen vacancies then diffuse to the gas interface where they are annihilated by reaction with adsorbed oxygen. The important point, however, is that metal is consumed and oxide formed in the same reaction zone. The oxide drift has thus only to accommodate the net volume difference between the metal and its equivalent amount of oxide. In theory this net volume change could represent an increase or a decrease in the volume of the system, but in practice all metal oxides in which anionic diffusion predominates have a lower metal density than that of the original metal. There is thus a net expansion and the oxide drift is away from the metal. 

Unlike the PEM, the ionic conduction occurs for the oxygen ion instead of the hydrogen ion. SOFCs are made of ceramic materials like zirconium (Z = 40) stabilized by yttrium (Z = 39). High-temperature oxygen conductivity is achieved by creating oxygen vacancies in the lattice structure of the electrolyte material. The halfcell reactions in this case are... 

Chemical Resistance. Stabilized hafnia is resistant to oxidation but is particularly susceptible to oxygen diffusion, through oxygen vacancies in the lattice. In other words, it is not a good oxygen barrier. It is not attacked by most chemical reagents at room temperature. 

The defects of the matrix play an important role on luminescent performances in these materials. Taking into consideration the preparation process of these compounds with the solid-state reaction of mixtures of BaC03, H3BO3, and NH4H2PO4 at different molar ratio, non-equal evaporation during the sintering process of these powders is inevitable and thus results in the formation of intrinsic defects, such as cation and oxygen vacancies. Positional disorder of B and Vacant B (Vb)" have been reported in SrBPOs crystals on the basis of... 

Direct conversion of methane to ethane and ethylene (C2 hydrocarbons) has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuels industries [ 1 ]. CO2 OCM process provides an alternative route to produce useful chemicals and materials where the process utilizes CO2 as the feedstock in an environmentally-benefiting chemical process. Carbon dioxide rather than oxygen seems to be an alternative oxidant as methyl radicals are induced in the presence of oxygen. Basicity, reducibility, and ability of catalyst to form oxygen vacancies are some of the physico-chemical criteria that are essential in designing a suitable catalyst for the CO2 OCM process [2]. The synergism between catalyst reducibility and basicity was reported to play an important role in the activation of the carbon dioxide and methane reaction [2]. 

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