Rocksalt-perovskite structures

Knapp et al. (144) show that for oxides containing 3d elements in spinel, perovskite, rocksalt, or zircon-type structures, the K-edge XANES spectra are quite independent of 3d electron occupation but instead nicely correlate with the crystal structure type. Various studies of Ti K edges of titanium oxides and other titanium compounds have been reported (40,158,172,177, 297). 

The rocksalt structure and the perovskite structure are the only ones we shall discuss in detail here, but we should make a brief survey of other systems. Fora more complete account see, for example, Goodenough (1971) and for the bands themselves, see Calais (1977). 

Magnetic perovskite and rocksalt-structured oxides and fluorides. These are highly ionic compounds, in which the calculated coupling constants J indicate the degree of locaHsation of the unpaired electrons on the transition metal sites, and the range of the magnetic (spin-spin) interaction. 

The cubic AMO3 perovskite structure consists of an MO3 array of corner-shared MOe/2 octahedra with a larger A cation at the body-center position. As illustrated in Fig. 1, this structure allows formation of the Ruddlesden-Popper [1] rocksalt-perovskite intergrowth structures AO (AMOsJn. In these phases, the mismatch of the equilibrium (M—O) and (A—O) bond lengths is given by the deviation from unity of the geometric tolerance factor... 

Few oxide superconductors were known prior to 1985 and we shall now return to these so that we can discuss these materials in reference to their crystal structure classes. There are only three broad structural categories in which most of the oxide superconductors occur. The important structural types include sodium chloride (rocksalt, or Bl-type), perovskite (E2X), and spinel (Hlx). 

Figure 1.9 (a) The perovskite structure. Without the large A atom at the body centre position, the structure becomes that of cubic ReOj (b) The K2Nip4 structure consisting of rocksalt (KF) and perovskite (KNiFj) layers. The NiFg octahedra share equatorial corners restricting the Ni-F-Ni interaction to the xy-plane. 

In solids where cation-cation interaction is significant, by can be related to R, where R is the cation-cation separation. In cases where cation-anion-cation interaction is important, by is related to the covalent mixing parameter, X, of the cation-anion orbitals. For octahedrally coordinated cations, as in rocksalt and perovskite structures, the relevant mixing parameters are and in the following molecular wave functions... 

The SrTiOa (111) [211-214] and (110) polar surfaces [215-217] and the BaTi03(lll) surface [218] have been produced and studied. At variance with rocksalt polar surfaces, many of these investigations suggest that one can obtain non-reconstructed quasi-planar polar surfaces. It should be realized that the perovskite structure is such that there exist ordered configurations of vacancies in the surface layers compatible with (1x1) diffraction patterns. In addition, SrTiO3(110) displays a variety of reconstructions, such as c(2 x 6) [215,217], under reducing conditions. No precise determination of the layer stoichiometry has been performed. 

Figure 9. (a) The perovskite structure, (b) The alternating CuOj sheets and rocksalt sheets, (c) Size requirements for (b). 

Figure 93 Basic structural types of cuprate superconductors, (a) Perovskite structure (cubic) (b) Infinite- layered structure (tetragonal) (c) Rocksalt prototype structure (reduced cell) (d) Composite layer between infinite layer and rocksalt building blocks.

Cubic Structures Diamond, Rocksalt, Fluorite, Zincblende, Cesium Chloride, Cubic Perovskite... 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

---