Perovskite Mixed Metal Oxides

One attempt to solve these problems has been the development of S5mgas technology based on oxygen ion selective membranes [95]. The membrane materials are non-porous and are composed by mixed metal oxides (perovskite, etc.) that conduct oxygen ions and electrons through the oxygen-deficient lattice structure. At high temperature, 700-1100 C,... 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

Sadakane, M., Asanuma, T., Kubo, J. et al. (2005) Facile procedure to prepare three-dimensionally ordered macroporous (3DOM) perovskite-type mixed metal oxides by colloidal crystal templating method, Chem. Mater. 17, 3546. 

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

Chandler CD et al Precursor for perovskite-phase mixed-metal oxides 1993 [54]... 

The double inverse microemulsion method was also used to synthesize per-ovskite-type mixed metal oxides [ 155]. One microemulsion solution contained nitrate salts of either Ba(N03)2/Pb(N03)2, La(N03)3/Cu(N03)2 or La(N03)3/ Ni(N03)2, and the other microemulsion contained ammonium oxalate or oxalic acid as the precipitant. These metal oxalate particles of about 20 nm were readily calcined into single phase perovskite-type BaPb03, La2Cu04 and LaNi03. The calcinations required for the microemulsion-derived mixed oxalates were 100-250 °C below the temperatures used for the metal oxalates prepared by a conventional aqueous solution precipitation method. 

Bismuth oxide forms a number of complex mixed-metal phases with the divalent metal oxides of calcium, strontium, barium, lead, and cadmium, and these show a wide variety in composition. With transition metal oxides, mixed-metal oxide phases have been observed which are based upon a Perovskite-type lattice (10) containing layers of Bi202. It is notable that the high Tc superconducting materials which include bismuth also have this Perovskite-type of lattice with layers of copper oxide interleaved with bismuth oxide layers. 

The coordination numbers of metal ions range from I. as in ton pairs such as Na CI in the vapor phase, (o 12 in some mixed metal oxides. The k>v limit, 1. Is barely within the realm of coordination chemistry, since the Na Cr km pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not particularly important since k is rarely encountered in discrete molecules, and the treatment of sohd crystal lattices such as hexagonal BaTiOj and perovskite> as coordination compounds is not dc ie frequently. The lowest and highest coordination numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 being the most important. 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

To solve the problems associated with the instability of state-of-the-art NiO cathodes, research has been focused on the development of new stable cathode materials to replace NiO. Currently, perovskite types of compounds and mixed metal oxides such as LiFe02 and LiCo02 have been evaluated as cathode materials. " ... 

The various processes for the catalytic reaction are similar. The factor that makes the difference is the choice of catalyst, which in turn affects the temperature regime needed to trigger the decomposition of nitrous oxide. In the literature, numerous works illustrate the several classes of catalysts appropriate for this reaction [9a, k] noble metals (Pt, Au), pure or mixed metal oxides (spinels, perovskite-types, oxides from hydrotalcites), supported systems (metal or metal oxides on alumina, silica, zirconia) and zeolites. 

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

Compounds formally containing [Fe03] are actually mixed metal oxides CaFe03, SrFe03 and BaFe03 crystallize with the perovskite structure Figure 5.23). 

Table 27.5 Electronic applications of selected perovskite-type mixed metal oxides.

A considerable number of materials called titanates are known, some of which are of technical importance. Nearly all of them have one of the three major mixed metal oxide structures (page 54), and indeed the names of two of the structures are those of the titanium compounds that were the first found to possess them, namely, FeTi03, Hmenite, and CaTi03, perovskite. Other titanites with the ilmenite structure are MgTiOa, MnTi03, CoTi03... 

With the exception of a few insoluble lanthanide niobates and tantalates, e.g., ScNb04,5 which contain discrete, tetrahedral MO ions, the coordination number of Nbv and Tav with oxygen is essentially always 6. The various niobates and tantalates are really mixed metal oxides. Thus, for example, the M XO, compounds are perovskites (page 55). 

Mixed Metal Oxides with the Perovskite Structure... 

Smith, A. J., Welch, A. J. E., Some mixed metal oxides of perovskite stracture, Acta Crystallogr., 13, (1960), 653-655. Cited on pages 386, 387, 662. 

There is a quite extensive chemistry associated with the so-called mesoperrhenates and orthoper-rhenates, which are derivatives of the [ReOj] " and [ReOg] anions, and various mixed metal oxides. These phases have been surveyed in some detail in Ae review by Rouschias. More recent developments have included studies on rhenium-apatites containing the square pyramidal [ReOj] unit, hexagonal perovskites with cation vacancies that contain [ReOs] , and the double oxides M3Rc2O 0 (M = Sr or Ba) and MsRe20 2 (M = Ca or Sr). ... 

Because the majority of industrial heterogeneous catalysts are based on mixed-metal oxides, it is reasonable that perovskites have also been examined. The preparation of specific, tailor-made mixed oxides able to perform complex physico-chemical functions is one of the main topics of research in the field of catalysis. To achieve this goal, ample information on the physical and solid-state chemical properties of the catalytic materials should be accumulated. 

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