Perovskite structure ordered systems

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

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. 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Microdomains within the perovskite-like slabs of layered perovskite phases in the Ba-Bi-O system illnstrate these ideas.The structures are layered perovskites with K2Nip4 related structures and gross changes in the Ba to Bi ratio are accommodated by the formation of a homologous series Ba +iBi 03 +i, made up of slabs of BaBi03 perovskite structure (see Section 8.4). In addition, subtle changes in the Ba to Bi ratio are accommodated by microdomains of ordered structure within the perovskite slabs themselves. These microdomains are ordered fragments of perovskite structure, and cation variation occurs at the interface between the microdomains in such a way that local excess or deficit of one cation over the other is accomplished. 

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. 

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). 

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