Titanates and niobates

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Pechini, M., Method of preparing lead and alkaline earth titanates and niobates and coating method using the same form a capacitor, U.S.Pat. 3330697, July 11, 1967. 

Layered compounds provide unique character for electron-transfer processes owing to their low dimensionality. Especially layered materials with ion-exchange and/or intercalation capabilities show behavior that is not seen in so-called bulk-type materials. Layered materials, which have been often used in studies of photoelectron transfer as well as photocatalysis, may be classified into two groups compounds in which the host layers work as an active component for the photoexcitation and electron-transfer reactions, and materials in which the layers are inert for electron-transfer processes. Examples of the former are layered titanates and niobates and of the latter are clays. In the latter case, photoactive materials are intercalated in the interlayer spaces. Recently, the exfoliation of various layered compounds has become possible and artificial assemblies consisting of these exfoliated sheets have been formed. Electron transfer in such assemblies is also an attractive subject in this field. 

No structures have been observed in the calcium-rare earth titanate and niobate phases that have not been observed in the Na-Ca-Nb-O system, but as the two are closely parallel, other stacking sequences or polytypic variations may well be found here, if sought for. The crystallography of the n = 4.5, 5, and 6 terms in the... 

The Pechini method refers to an original process developed by Pechini (95) for the preparation of titanates and niobates for the capacitor industry. The method has since been applied to many complex oxide compositions (96,97). Metal ions from starting materials such as carbonates, nitrates, and alkoxides are complexed in an aqueous solution with a-carboxylic acids such as citric acid. When heated with a polyhydroxy alcohol, such as ethylene glycol, polyesterification occurs, and on removal of the excess liquid, a transparent resin is formed. The resin is then heated to decompose the organic constituents, ground, and calcined to produce the powder. The typical steps in the method are illustrated in Figure 2.34 for the preparation of SrTiOs powders (98). 

The Pechini method was originally developed for preparing lead and alkaline-earth titanates and niobates and combinations thereof by way of resin intermediates [34]. On ignition of the resin the organic portion is removed, leaving the selected composition of mixed oxides chemically combined. The oxides are then sintered into dense ceramic bodies. This method is also applicable to solid electrolytes, allowing a very intimate mixture of the starting materials. 

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