Anionic conductors

Both types of ions can be carriers of electric current in ionic solids but the majority of fast ionic conductors discovered are cationic conductors, for the reasons previously explained. 

Between, the existing anions only, F and Cl are found to be good carriers of electricity [37,40], For example, rare-earth oxychlorides between them, lanthanum oxychloride shows anionic conduction however, its conductivity, is small then to increase it, this material was doped with Ca2+ in the trivalent, La3+, site of LaOCl in order to generate CL vacancies [19,40], 

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The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

Fleig reviews fundamental aspects of solid state ionics, and illustrates many similarities between the field of solid state electrochemistry and liquid electrochemistry. These include the consideration of mass and charge transport, electrochemical reactions at electrode/solid interfaces, and impedance spectroscopy. Recent advances in microelectrodes based on solid state ionics are reviewed, along with their application to measuring inhomogeneous bulk conductivities, grain boundary properties, and electrode kinetics of reactions on anion conductors. 

Oxygen anion conductors solid electrolyte Oxygen, chemisorbed - chemisorption of oxygen Oxygen conducting solid electrolyte - solid electrolyte... 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

Fluoride (F ) anion conductors fluorides with general formula A F (A = Ca, Sr, Ba, Pb, Cd, Bi, Ln, Sn and their mixtures), and also oxy- and chloro-fluoride phases containing these metal cations. In the latter case, however, both types of anions are often mobile. 

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

F guK22.1 Representative chemical structures of polyether/salt hybrids, (a) Cation conductor, and (b) anion conductor. 

The pyrochlore-type compounds, where the crystal structure is usually considered as a cation-ordered fluorite derivative with % vacant oxygen site per fluorite formula unit, constitute another large family of oxygen anion conductors [9, 33, 41—43, 84—88]. The unoccupied sites provide pathways for oxygen migration furthermore, the pyrochlore structure may tolerate formation of cation and anion vacancies, doping in both cation sublattices, and antistructural cation disorder. Regardless of these factors. 

Miura. N.. Yan. Y. Sato. M., Yao, S., Nonaka. S.. Shimizu. Y. and Yamazoe, N. (1995) Solid state potentiometric CO2 sensors using anion conductor and metal carbonate. Sens. Actuators B, 24-25, 260-5. 

Imanaka, N., Kamikawa, M. and Adachi, G.-y. (2002) A carbon dioxide gas sensor by combination of multivalent cation and anion conductors with a water-insoluble oxycarbonate-based auxiliary electrode. Anal. Chem., 74 (18), 4800-4. 

Now we can try to put this all together to try to predict where else to look for new superconducting phases. The main themes that arise from the preceding discussion can be condensed into four general characteristics 1) A large cation-anion mixing of the wavefunctions near the Fermi level, 2) metallic conductor, but close to a Mott transition, 3)fast anion conductor, 4) the electropositive cations do not play an essential role in the electronic properties. For the sake of discussion, we can arbitrarily break these materials into oxides and other anion compounds. 

Na P-alumina can be replaced by cations such as Li+, K+, Cs+, Rb+, Ag, Tl and H+. However, the conductivities of these materials are lower than that of Na P-alumina the match between the size of the Na ions and the interlayer channels in the host lattice leads to the most efficient cation mobility. The conductivities of Na P-alumina and selected cation and anion conductors exhibiting relatively... 

Figure 4 Dense solid oxide membranes (a) anion conductor with external circuit (b) mixed conductor with no external circuit...

Many other systems have subsequently been found to display anion conductivity. The electrical conductivity of lead fluoroborate and fluorogermanate glasses is similar to that of the lead fluorosilicate glasses. Calcium fluoroaluminate glasses also conduct by fluorine migration. Fluorophosphate glasses have also proven to be good anionic conductors. All of these systems are free of alkali ions. 

These substances have more sites then ions. This led to high conductivity values, which led to the name super ionic conductors. The disorder can concern the cation lattice (cation conductors) or the anion lattice (anion conductors). The transfer numbers t+ and t determine the classification with 1 for cation conductors or t 1 for anion conductors. Because of the usually small size of the cation compared to the anions, cation conductors are dominant. 

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