Double perovskite stability

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

Moreover, the environmental systems demonstrate unique diversity and versatility of the processes depending on the actual conditions, viz. the reaction directions and rates are sensitive to many diverse parameters, sometimes even difficult to be perceived. The example may be the dependence of the photocatalytic activity of Sr—A1—Nb—0 double perovskite on the cation ordering in the oxides (267), or the effect of the in-plane twist of the quinoline-based co-ligand on the thermal stability and yield of NO photorelease from the [Ru(NO)] nitrosyl complexes (96). 

I. Zvereva et al., Complex aluminates RE2SrA1207 (RE = La, Nd, Sm-Ho) Cation ordering and stability of the double perovskite slab-rocksalt layer P-2/RS intergrowth. Solid State Sci. 5(2), 343-349 (2003). 

More recently, the maximum oxygen non-stoichiometry in SMMO was successfully increased through partial substitution of Mo(Vl) by Nb(V), without compromising the stability of the double perovskite [85]. The increase in oxygen non-stoichiometry suggests that more Mo /Mo " redox centers are created, which are expected to increase both oxygen ion conductivity and fuel oxidation rates. 

Figure 2.33 A phase diagram indicating the range of stability of B-site ordered double perovskites, denoted as -I—I—I—I—I- rock salt, in terms of differences in ionic radii and formal charge. Reprinted with permission from Anderson eta/., 1993 [80], Copyright (1993) Elsevier...

Whereas the larger rare-earth ions form the perovskites LnFeOs, the smaller ions (Ln = Ho-Lu, Y) stabilize a series of structures Lni+ Fe2+n04+3 containing the Ln " " ion in an octahedral site and the iron in trigonal bipyramidal sites. The end member, LnFe204, contains rare-earth basal planes alternating with a double-layer of edge-shared iron sites, see Fig. The other members of the series incorporate intergrowths of a similar... 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed ... 

The valence stability itself is defined as the thermodynamic properties, so that it is natural to expect that the chemical stability of perovskite oxides is related to the valence stability in addition to the stabilization energy of double oxides from the constituent oxides. Particularly, the reaction of perovskite oxides with YSZ has been well examined experimentally as well as thermodynamically. 

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