Perovskite structure doubled

This type of orthorhombic perovskite structure appears, if the tolerance factor of Goldschmidt is smaller than t — 0.88. The example of the compound NaMnFs [t = 0.78), showing doubled lattice constants a and h (287), is likely to mark the lower limit of the field in which orthorhombic fluoro-perovskits of the GdFe03-t3q>e may occur. Fluoroperovskites which have a smaller tolerance factor than t = 0.78 never have been observed so far, nor do fluoride structures of the ilmenite type seem to exist, which might be expected for ya = Me, corresponding to 1=1/1/2=0.71. 

Ordered Perovskite-type Compounds, A2(BB )06 Systems Cubic Fmim A feature of the perovskite structure is that, with the proper substitutions, many types of ordered structures can readily be formed. This can be accomplished by the substitution of two suitable metal ions (with different oxidation states) in the octahedral sites of the structure. In this case the unit ceil is doubled along the three cubic axes to generate an 0.8 A unit ceil (Figure 15). Partial substitution of different transition metal ions in the octahedral sites is also possible the general formulation for these compounds would be A2(B2 xB x)06. The parentheses in this formulation enclose atoms occupying the octahedral sites in the structure. 

The simplest quaternary derivative with the perovskite structure would be one in which two different transition metals might occupy the B-site position. This can be formulated as A(B,1 2B1 2)03, or preferably A2(B B0O6. These compounds can then crystallize with a doubled unit cell, if ordering occurs on the octahedral metal sites. Further compositional and structural adaptions could be obtained, as shown below, all possessing an overall 1 1 3 ratio of A B 0 atoms. In all the following examples and formulations, the proper stoichiometry will be maintained, and oxygen will be the principal anionic species. 

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 ... 

K3M2F7 or Ruddlesden-Popper-type. If in the perovskite structure AMF3 the layer separation by addition of AF is performed every second layer only, the double-layer stmeture type K-iNi2 Fq is obtained, known in oxides as Sr3Ti207 or Ruddlesden-Popper stmeture (s.g. I4/mmm, Figure 21(b)). 

The chloro-complexes usually contain finite complex anions. None has the perovskite structure of KMgFg. The complex CsAuClg contains equal numbers of square AuCl4 ions and linear AuCl2 ions in addition to Cs+ ions. CS2CUCI4 contains finite CUCI42- ions, but CsCuClg has a double chain (Fig. 228). 

R642 K. Asai, Studies of Ferromagnetic Oxides with Ordered Double Perovskite Structure by Nuclear Probes , KURRI-KR, 2000, (KURRI-KR-49, Proceedings of the Specialist Research Meeting on New Developments in Solid State Physics with Probes of Radiations and Nuclei, 1999), 229... 

According to Campbell (1992), the double perovskite, LaCaMnCoOf, was of interest as a cyclic mode methane coupling catalyst due to its structure and redox properties. This mixed oxide has an ordered perovskite structure showing multiple occupations of both A (La, Ca) and B (Mn, Co) sublattices. The crystalline material presented some ordered domains, while in other areas the cations were distributed at random. For the ordered domains, the most probable structural model was an AB03 perovskite-type structure in which Mn4 and Co3+ ions occupy B positions in adjacent AB03 units while La3+ and Ca2+ ions alternate in A positions. Two ions of this structure can be reduced ... 

A novel system was introduced to the catalytic field by Matsuda et al. (1993). A solid of the general formula ALaNb207 is made up of the layer compound LaNb207 with double perovskite structure interleaved with A atoms. If the interlayer compound is water, they call the compound HLa2Nb707 to indicate the acidic character of this system. They report the results obtained when 1- and 2-butanol were reacted over this solid at temperatures between 453 and 623 K. The secondary alcohol was much more reactive, as expected for an acid catalyst. Consistent with this, no dehydrogenation product was formed when... 

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