Perovskite anion conductivity

The perovskite structure is capable of high anion conductivity when oxide vacancies are introduced, as in, for example, Lai (Sr Co03 (/2 or in the perovskite-related superconductor phases, La2Cu04 and YBa2Cu307. The oxide ion transport number is not unity since such materials are often electronic conductors as well, due to the presence of... 

Fast anionic conduction is found mainly in sohds of the fluorite (CaF2) and fluorite-related structures. It is also observed in sohds with the perovskite, YF3, tysonate (LaFs), and simple cubic structures (for these structures, see Oxides Solid-state Chemistry aoA Fluorides Solid-state Chemistry). The smaller anions (r 1.4 A) and F (r 1.2 A) show the fastest conduction however, good anionic conductivity is also found for Cl (r 1.7 A), Br (r 1.8 A), I (r 2.1 A), and for (r 1.7 A). 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

Different kinds of non-oxide perovskite-type compounds have been known in carbides, halides, nitrides, and hydrides [2], Conjecturing from the oxide ion conduction in ABO3, it would be possible to expect anionic conduction, such as halide ionic or nitride ionic, in these non-oxide perovskite compounds ABX3. 

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Figure 46. Specific conductivity of a few selected anion conductors as a function of the temperature (02 F-).23 Very high O2 conductivities are met in some perovskites, typically in doped ferrates.188 These materials (cf. Sr

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

The synthesis of barium and strontium molybdates(IV) was first reported by Scholder, Klemm, and Brixnerd The compounds are of interest because they contain molybdenum in the 4-4 oxidation state. Both compounds have the undistorted cubic perovskite structure, which is ideal for stud3nng cation-anion-cation (superexchange) interactions. The compounds also exhibit metallic behavior, thus permitting study of the role of d electrons in the conduction process. 

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