Perovskite chemistry

Perovskite oxide materials possess the general stoichiometry ABO3. Conventionally, the A cation is larger than the B cation. In the archetype, the A cation has an oxidation state of -I- 2 and the B cation has the oxidation state -1- 4. These materials comprise three different ionic species, each with its own equilibrium defect concentration due to three different activation energies for defect formation, which, combined with the constraint of electroneutrality, make for diverse and potentially useful defect chemistry, particularly when considering electronic, hole, and ionic conduction under atmospheres of different oxygen partial pressures [13]. 

Perovskites containing a transition metal are of particular interest because of the availability of multiple oxidation states, which facilitate electrocatalytic processes and provide mechanisms for electronic conductivity. For example, under reducing atmospheres the transition metal ions change to lower oxidation states, effectively freeing up electrons to pass current. Typical examples of such species are titanium, niobium, and vanadium. SrNbOs has an electronic conductivity of 10 Scm under reducing conditions [14], and Petrie reported a conductivity of 10 cm for SrVOs under similar conditions [15]. Unfortunately, it was also found that these compounds could not be fabricated in air. 

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Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Kato, K. Zheng, C. Dey, S. K. Torii, Y. 1997. Chemistry of the alkoxy-derived precursor solutions for layer-structured perovskite thin films. Int. Ferro. 18(l-4) 225-235. 

The above techniques have a wide array of applications, including those that are both analytical and physicochemical (such as bonding) in nature. Typical examples of research include the surface chemistry of ferrite minerals (38) and the valence states of copper in a wide array of copper (39) minerals. Other areas of bonding that have been studied include the oxidation state of vanadium (40) in vanadium-bearing aegirities (also using x-ray photoelectron spectroscopy) and the. surface features of titanium perovskites (41). ... 

Crespin, M. and Hall, W. K. The surface chemistry of some perovskite oxides. J. Catal, 1981, Volume 69, Issue 2, 359-370. 

Crystal Chemistry of Superconducting Bismuth and Lead Oxide Based Perovskites... 

Perovskites also exhibit reaction relationships with phosphate minerals and exotic alkali-titanium-silicate minerals under late-stage magmatic conditions. In a systematic study of loparite mineral chemistry in the Lovozero alkaline complex, Russia, Kogarko et al. (2002) have... 

Mitchell, R. H. 1996. Perovskites a revised classification scheme for an important rare earth element host in alkaline rocks. In Jones, A. P., Wall, F. Williams, C. T. (eds) Rare Earth Minerals, Chemistry, Origin and Ore Deposits. Chapman and Hall, London, 41-76. 

FIGURE 2.11 Rietveld analysis of perovskite with partial substitution of Ti with Ca. (Courtesy of the Royal Society of Chemistry.)... 

Brown, I. D. (1991a). Internal strain in perovskite related materials. In P. K. Davies and R. S. Roth (eds), Chemistry of Electronic Materials. Washington US Department of Commerce, pp. 471-83. 

Perovskites AB)/3C2/303 (A = Ba, Sr, B = Zn, Mg, Co, Ni C = Nb, Ta) are promising compounds for microwave applications. It is important to synthesize these complex oxides as pure perovskite phases because the slightest admixture of a second phase hinders drastically the dielectric properties of ceramics, which sinter only at very high temperatures (1400 to 1500°Q. The precursor chemistry resembles greatly that of BaTi03 formation by alkoxide or alkoxide-hydroxide routes. Below we summarize the 3 approaches to the synthesis of these perovskites by the sol-gel method ... 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

Our work has applied these techniques to the study of the binary insulating materials including the fluorites, alkali halides, alkaline earth oxides, and perovskites. Many of these are simple materials that are commonly used as models for all solid state defect equilibria. Our work has had the goal of determining at the microscopic level the defect equilibria and dynamics that are important in understanding solid state chemistry as well as developing new tools for the studies of solid materials. 

The basic perovskite structure ABX3 forms the prototype for a wide range of other structures related to it by combinations of topological distortions, substitution of the A, B and X ions, and intergrowth with other structure types. These compounds exhibit a range of magnetic, electrical, optical, and catalytic properties of potential application in solid state physics, chemistry, and materials science. 

J. Wu, Defect chemistry and proton conductivity in Ba-based perovskites, PhD thesis, California Institute of Technology, Pasadena, CA (2005). 

N.C. Hyatt and K.S. Knight, in Synthesis, Properties and Crystal Chemistry of Perovskite-Based Materilas, W. Wong-Ng, A. Goyal, R. Guo, and A.S. Bhalla, (editors), Proceedings of the 106th Annual Meeting of the American Ceramic Society, American Ceramic Society, Westerville, OH, 2005, p. 151. 

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