Mixed Valences in Perovskites

One of the most useful properties of perovskites is their ability to incorporate mixed valences into their structure. It is worth noticing that conjugated mixed valence systems extensively occur in very important redox catalytic systems in nature, the most celebrated one perhaps being photosystem 11 in plants. This system contains mixed valence states of Mn and is responsible for water splitting and oxygen evolution, which is one of the most important reactions on our planet So the creation of mixed valences in perovskites bears strong elements of biomimicry. The routes for creating such mixed valences may be either structure deficiencies or isostructural substitution of cations into a mother structure. 

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Many properties of advaneed materials are determined by the elements with mixed valences in the structure [20], by which we mean an element that has two or more different valences while forming a compound. In the perovskite structured Lai cAxMn03 (A = Ca, Sr, or Ba) CMR materials, for example, the residual charges induced by a partial substitution of trivalent La + by divalent element A + are balanced by the conversion of Mn valence states between Mn " " and... 

Proton conduction at high temperatures occurs in certain perovskites such as doped strontium cerate, Sr Ceo.95Ybo.o503 t. In air, this material is primarily an electronic conductor due to the mixed valence of Ce. In the presence of moisture, water is absorbed by the reaction with positive holes to generate protons ... 

The compound Ba5[Ru2(0)n] has been synthesized by solid-state reactions of barium carbonate with ruthenium oxide. The X-ray structure shows that it contains peroxide ions and the ruthenium is in the +5 oxidation state, i.e., Ba5[Ru2(02)0g]. The novel mixed-valence Ru — Ru triple perovskites, Ba3M[Ru2(0)9] (M = Li, Na), have been grown from reactive hydroxide fluxes. ... 

In the case of the mixed-valence manganites with the perovskite stracture, Zener s double-exchange (DE) mechanism has been used to explain the CMR effect (Zener, 1951c). In this model, a strong Hund s coupling is presumed to exist between the itinerant... 

Additional attempts have been presented to render hosts with the fluorite and the related pyrochlore structure electronically conductive by doping with mixed-valence and/or shallow dopants. The list of dopant materials examined includes oxides of elements of, for example, Ti, Cr, Mn, Fe, Zn, Fe, Sn, Ce, Pr, Gd, Tb and U. In general, however, the extent of mixed conductivity that can be obtained in fluorite-type ceramics is rather limited, by comparison with the corresponding values found in some of the perovskite and perovskite-related oxides considered in the next section. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

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