Other Ordered Perovskites

A different ordering pattern in the phases K CaUjO and K SrUjOjj leads to the cubic structures space group Im3m (229), a=0.8483nm (Ca ) and a=0.8582nm (Sr ). In these latter phases the U cation shares the octahedral B-sites with Ca or Sr.  

The superconducting double perovskite (NaQjsKg jlBajBi Ojj has a complex ordering of A-site cations. The structure is cubic space group Im3m (229), a=0.8550nm. The A-sites are subdivided so that one set is occupied by (Na V), where V represents an A-site vacancy, while the other (A -sites) are fully occupied by Ba. The Bi cations occupy the B-site octahedra. 

In the unusual double perovskite CaMnTi Og, the Mtf cations occupy the A-sites. The Mn + and Ca are ordered in columns along the c-axis, with structural distortions allowing the Mn to take tetrahedral and square planar positions in the cage sites. The phase CaFeTi Og appears to be similar, but with Fe only occupying tetrahedral positions. These phases are structurally related to the AA B Oj -related phases described in the following section. 

Note that other ordered structures can occur. The minerals cryolite NUjAIFg (better written NUjNaAlFg for the present purposes) and elpasolite K NaAlF can be regarded as perovskite superstructures. 

A surprising example of A-site ordering is provided by a group of cubic perovskite oxides with a general formula AA B Ojj (Table 2.2). In these phases, A is an alkali metal, alkaline earth, lanthanoid, Pb or Bi A is a 3d transition metal ion and B can be a transition metal such as Ti, V, Cr, Mn, Fe, Ru, Rh, Ir or Pt, as well as the nontransition metals Ga, Ge, Sb or Sn. Ideally the ionic charge states are A Af B Ofj or A Af B B Ofj and the cubic unit cell is of side approximately 2a.  

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These results can be contrasted with results obtained using a less stringent equilibrium criterion of = 0.0005 S cm-1 min-1 as shown in the open circle data in Fig. 6. A large discrepancy is observed in the middle- o2 region. A difference between the measured values of Po2 by the sensors at the top and bottom of the cell of more than one order of magnitude in p0 was found under these conditions. In general, the p0 difference between the two sensors is slow to converge due to the slow equilibrium kinetics and consequently, both conductivity and log p0 difference criteria are needed to ensure that equilibrium is reached. The open circle data in Fig. 6 are similar to results in other ferrite perovskite oxides and reflect nonequilibrium behavior.14 17... 

Fig. 25. Part of the crystal structure of ordered perovskite AaBWlUlOs. Center black ion W(U), other black ions B, white ions O, grey ions A.

Further, crystal structures containing isolated octahedra are not very common. We mentioned already the ordered perovskite structure. In the system La2MgSni xTixO6 with ordered perovskite structure the isolated TiO complex can be studied as has been reported in Ref. 13. Another possibility is the monoclinic yavapaiite structure where BaTi(PO4)2 is an example of ). In other materials, however, condensation of octahedra occurs in the efficient luminescent system Mg2Sni xTixO4, for example, the TiO octahedra join edges with the SnO octahedra (for which they are substituted). We will now consider the luminescence of these systems more in detail to illustrate the state of our knowledge of the titanate octahedron. 

Compared with other uranium-doped tungstates the luminescence spectra of Ba2MgW0g—U show well resolved vibrational structure. Therefore the results that have been obtained for Ba2MgW06—U will be used in this section to illustrate the phenomena observed in the luminescence spectra of uranium-doped tungstates with ordered perovskite structure. 

The luminescence from octahedral uranate groups has also been reported for other uranium-doped oxidic compounds (see e.g. Ref. 7). Like in uranium-doped compounds with ordered perovskite structure isolated UOg octahedra are present in several other host lattices. In this type of compounds e.g. Y3Li3Te20i2-U" LigWOs-LT I and Mg3TeOg—, the luminescence properties of the octahedral uranate group are similar to the properties which have been observed for uranium-doped ordered per-ovskites. Due to symmetry lowering the vibrational structure in the luminescence spectra is more complicated, and also the luminescence decay time is shorter than in ordered perovskite systems (c.f. Sect. 2.1). 

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 ... 

In this article, we discuss the -site cationic ordering and disordering effects on magnetic and electron transport properties for rare earth cobaltites. Perovskite cobaltites have two possible forms of the -site cations distribution depending on the type of cations or the synthesis procedures [9, 12]. The first reported compounds on perovskite cobaltites are the A-site disordered structure [2, 13], which have been investigated for last few decades, and the other one is -site ordered perovskites possessing a layered 772-type structure [3]. The latter... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

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