Perovskite structure binary oxides

II. In some complex oxide structures the environments of the different kinds of metal ion are so different that the structure is not possible for a binary oxide. The size difference between the ions necessary for the stability of the structure may be too large (as in the perovskite and related structures) or the two (or more) oxidation states required for charge balance in the structure may not be possible for one metal. (The scheelite structure, for example, calls for equal numbers of 8- and 4-coordinated atoms in oxidation states totalling 8.) It should be noted that positions of different coordination number in complex oxide structures are not necessarily occupied by atoms of different metals. Just as one kind of ion occupies coordination groups of two kinds in certain exceptional binary oxides, as noted above, so we find the same phenomenon in some complex oxides. In the garnet structure (p. 500) there are positions of 4-, 6-, and 8-coordination for metal ions. In... 

The cubic perovskites BaNbO.N and BaTaO.N arc prepared from binary oxides or carbonates, and they have been characterized by neutron diffraction [172]. Yellow, orthorhombic BaSiN, (spacegroup One ) possibly has a superstructure of wurtzitc [173], The structures of tetragonal Ba3Ge3N [174] and orthorhombic Ba0M3Nn) (M = Os, Re) [155] are not known. I he ditfraction pattern of Ba,NI() is known but has not been indexed [165. 175]. Ba3B,N4 is known by elemental analysis IR measurements suggest linear NBN ions [139]. 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

The total DOS and PDOS give rich information on the chemical structure of a system, connecting the calculated band structure with the atomic states. We demonstrate this by considering total and projected DOS for binary oxides MgO, Ti02 and ternary oxides SrTiOa and SrZrOs with cubic perovskite structure (see Figures 9.5-9.S). 

Vanadium forms at tetravalent state an oxoanion V2O7 which can combine with rare earth, forming the R2V2O7 vanadites (Shin-ike et al., 1977). Trivalent vanadium forms rare earth compounds having the formula RVO3. These compounds, however, cannot be considered as complex compounds but are binary oxides with perovskite structure (Wold and Ward, 1954 Reuter and Wollnik, 1963 Palanisamy et al., 1975). 

As has been mentioned, ternary compounds having polyatomic ions such as NO3-, C032-, NFLt+, and so on often have the same types of structures as binary compounds in which a polyatomic ion occupies a lattice site as a unit. The mineral perovskite, CaTiOs, calcium titanate, however, is a somewhat different type of ternary compound that has the structure shown in Figure 3.8(a). Most ternary compounds are oxides, and the general formula ABO3 corresponds to many compounds because A = Ca, Sr, Ba, and so forth, and B = Ti, Zr, Al,... 

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