Perovskite lattice structure

Perovskites and related compounds also have a three-dimensional structure. In perovskites of formula ABOj, the octahedra of BOj lie on a cubic lattice, and are joined at the corners. Between these octahedra are large sites for the A atoms. In ReOj, the A atoms are missing, so guests can be added to the A positions. Because adjacent octahedra are joined together by only one oxygen they can rotate relative to one another, changing the shape of the A site. In LijReOj, the rotation splits the large A sites into two smaller sites more suitable for Li ions (Cava et al, 1982). Bronzes of WO3 also have a perovskite structure. 

Hemley, R. J., M. D. Jackson, and R. G. Gordon (1987). Theoretical study of the structure, lattice dynamics, and equations of state of perovskite-type MgSiOj and CaSiOj. Phys. Ghent. Mineral. 14, 2-12. 

Zr) for NjO decomposition. The actual oxidation states of the metals are investigated with XPS [21]. The possible active sites for the reaction may also be related to lattice oxygen anion defects formed after cation incorporation [22]. For high Pb Zr ratio catalysts, such as in the case of Pb Zr = 1 1, the catalytic activity is substantially reduced. This can be attributed to change of ZrOj structure to perovskite type and thus to demolishment of cation pairs with multiple oxidation states that are essential for facilitating the decomposition of NjO gas [23-25]. 

Hemley, R. J., Jackson, M. D., and Gordon, R. G., (1987) Theoretical Study ofthe Structure, Lattice Dynamics, and Equations of State of Perovskite -type MgSiC ... 

Since BXe octahedra and AXn cubo-octahedra are the basic sub-structures of perovskite lattice, as shown in Fig. 8.1, it is easy to see that the stability of BXe octahedra and AX12 cubo-octahedra are also necessary conditions for the stability of perovskite lattice. It is obvious that the condition 0.75cubo-octahedral structure. It is necessary to find the suitable criteria for the above-mentioned stability requirements. 

The lattice structure of perovskites, ABO3, is shown in Figure 5.1. This oxide consists of three elements, namely the large cations. A ", the small cations. 

I idiire 5. / Schenialic representation of lattice structure of perovskite. ABOi... 

Modular structures are those that can be considered to be built from slabs of one or more parent structures. Slabs can be sections from just one parent phase, as in many perovskite-related structures and CS phases, or they can come from two or more parent structures, as in the mica-pyroxene intergrowths. Some of these crystals possess enormous unit cells, of some hundreds of nanometers in length. In many materials the slab thicknesses may vary widely, in which case the slab boundaries will not fall on a regular lattice and form planar defects. 

The perovskite structure is stable to relatively large amounts of dopant ions on either A or B sites. Oxygen vacancies are introduced into the lattice, either through transition-metal redox processes or by doping on the A or B sites with lower valence cations. 

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

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