B-site substitution

B-Site Substitution Acceptor doping of the normally insulating per-ovskite structure SrTi03 has been widely explored for the purpose of fabricating mixed conductors. Replacement of part of the Ti4+ by a lower valence acceptor cation such as Fe3+ leads to enhanced total conductivity with greatly enhanced O2- migration. 

Zuev A, Singheiser L, andHilpertK. Defect structure and isothermal expansion of A-site and B-site substituted lanthanum chromites. Solid State Ionics 2002 147 1-11. 

Patcas, F Buciuman, FC Zsako, J. Oxygen non-stoichiometry and reducibility of B-site substituted lanthanum manganites, Thermochimica Acta, 2000, Volume 360, Issue 1,71-76. 

Chen, T.Y. and Fung, K.Z., A and B-site substitution of the solid electrolyte LaGa03 and LaAlOj with the alkaline-earth oxides MgO and SrO, J. Alloys Compounds, 368, 106-115 (2004). 

As seen from Fig. 10.11, the value of (3-8) in Lai xSrxCo03 falls off with decreasing oxygen activity much more rapidly than for the other compounds shown. The general trend at which the perovskites become nonstoichiometric follows that of the relative redox stability of the late transition metal ions occupying the B-site, i.e. Cr " > Fe > Mn > Co ". The reductive nonstoichiometry of the cobaltites increases further by partial B-site substitution with copper and nickel. 

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

As possible B-site substituents, Sri. Tii.yM Oj+g (M = Nb, Ta) are appropriate with respect to an appearance of electrical conductivity. Ti ions with di system can be existed stably by the formation of A-site deficiency in the B-site-substituted Sri.xTii.yMyOg+g (M = Nb, Ta) perovskites at room temperature. From the results of crystallographic study, it is revealed that the B-site-substituted Sri xTii yMyOj4 g with cubic symmetry showed a single perovskite phase in the range of 0 y 0.2 for Nb and 0 y < 0.3 for Ta. 

The introduction of noble metal in B-site substitution does not have any influence on oxygen stoichiometry. 

There are a number of other structures similar to that of YBajCUjO, especially those in which the Y cations are replaced with lanthanoid ions Ln. In addition, phases with B-site substitution, for instance, YSr Cu CoO, are known, although many of these have structures that are not so obviously perovskite related. The compound YSrCUjCoO., for example, is built from tetrahedra and square pyramids and does not show a simple structural relationship with perovskite. 

Many other perovskites containing mixed valence cations behave in a similar way, and these can be modified with A- or B-site substitutions in order to improve either aspects of stabihty or conductivity. For example, the perovskite SrFej Sc Oj shows the same conductivity dependence as SrjFe Og ... 

The slope of Wa is found at high oxygen partial pressures and the slope of-Va at low oxygen partial pressures, following the same defect chemistry as that described previously. However, the B-site substitution of Sc prevents the transformation of the high-temperature disordered cubic phase into an ordered brownmillerite stmc-ture below a temperature of approximately 850°C. This improves the conductivity at lower temperatures compared to the un-substituted material. 

The situation can be illustrated with respect to the B-site substituted perovskite SrZrj Y 03 (,5, with dopant levels of x=0.05-0.2. The B-site substitution of two lower valence cations is balanced by the formation of one oxygen ion vacancy. 

Lanthanoid manganites, such as LaMnOj, NdMnOj and GdMnOj, are of potential value in solid oxide fuel cell cathodes. However, many of these phase show thermal contraction because of the diminishing Jahn-Teller distortion of the Mtf " cations as the temperature is increased. Such effects tend to rule out these materials for real cell applications, although A- and B-site substitution, as demonstrated for PbTiOj earlier, can ameliorate the problem. 

The Seebeck coefficient can be altered significantly by both A-site and B-site substitutions. For example, A-site doping with Sr to form Laj Sr CoOj forces one Co " ion to transform to Co to maintain charge neutrality. The value of the Seebeck coefficient remains positive, as the Seebeck current is still due to hole migration, but the number of Co defects will increase in proportion to the amount of substituent. The number of defects is equal to the number of impurity Sr ions (ignoring the small concentration of Co in the parent compound), and so the fraction c in the Heikes equation is equal to x in Laj Sr CoOj and as a consequence the value of the Seebeck coefficient will fall (Figure 9.6). 

An analogous situation occurs for appropriate B-site substitution. For example, the B-site substitution of Tb " for Co " in the parent LaCoOj structure forces the transformation of two Co " cations to Co " to maintain charge balance in the substituted perovskite LaTi COj Oj. Each Co " cation can be considered to be a Co " cation ion plus a trapped electron Co. Electronic conductivity can then be considered to occur by the migration of electrons from one Co " ion to a neighbouring Co " ion ... 

As for the BSCF family, B-site substitution improves the stability toward CO2-rich atmospheres. Sirman [74] established that the CO2 tolerance of perovskite... 

Eguchi and coworkers [86] investigated the influence of the A-site cation (A La, Pr, Sm, and Nd) on AMnAliiOi9 a. Both the specific area and the catalytic activity increased with increasing ionic radius of the lanthanides. La was found to have the most beneficial effect on catalytic methane combustion activity. The activities of the La-based catalysts were further enhanced upon substitution of Al with Mn and Cu. Interestingly, the activity of these compounds was found to be greater than the B-site substituted Ba-hexaaluminate catalysts. Based on this, the authors concluded that the A-site cations could exert a significant influence on the oxidation state of the catalytically active B ions. 

Singh, N. Lai, B. High surface area lanthanum cobaltate and its A and B sites substituted derivatives for electroeatalysis of O2 evolution in alkaline solution. Int. J. Hydrogen Energy 2002, 27,45-55. 

To overcome the technological problems associated with the LSC materials at high temperatures, B-site substitutions have been developed. One interesting example refers to the Lai- Sr Coi-yFeyOs (LSCF) composition, initially studied by Tai et In this work they identified the... 

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