Rocksalt oxides, structure

Perhaps the most simple crystals in this class are the alkaline earth oxides. They are II-VI compounds and have rocksalt crystal structures. Data for their hardnesses versus their bond moduli (optical band gaps per molecular volumes) are displayed in Figure 11.4. 

Rocket propulsion oxidizers, 18 384-385 Rocks, weathering of, radiation and, 3 299 Rocksalt, crystal structure of, 2 6, 29 Rock-salt-type alkali halide crystals, dissolution process, 39 411 19 alkali chlorides, 39 413, 416 alkali fluorides, 39 413-415... 

Fig. 1. Structure of rocksalt oxides. Lattice constants from [15].

A majority of halides and oxides have the structures expected for largely ionic compounds, with the metal in octahedral coordination (see Topic D3. especially Fig. 1). Common oxide structures are rocksalt (e g. MnO, NiO), corundum (see Topic G5. e.g. Cr203, Fe203) and rutile (e g. Ti02, Cr02). 

Rocksalt NaCl structure (see Fig. 2.6) has the symmetry of symmorphic space group N225 and is typical of alkali halide crystals (Li, Na, K fluorides, chlorides and bromides) and some oxides (MgO, CaO, SrO). 

The cation-OH bond formation is very dependent upon the parameters which have been discussed above cation ionic radius adsorption site and Madelung potential (Fig. 6.8b). When the bond is weak, as on the rocksalt oxide surfaces, the LDOS on a surface cation is little perturbed and the states of the OH molecule are superimposed on the oxide band structure without noticeable interferences. When the bond is strong, which happens on more acid oxides such as rutile and quartz, a quasi-SiOH or... 

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). 

The NaCl structure is also found in compounds like TiO, VO and NbO, possessing a high percentage of cation and anion vacancies. Ternary oxides of the type MggMn 08 crystallize in this structure with of the cation sites vacant. Solid solutions such as Li,j )Mg Cl (0 x 1) crystallize in the rocksalt structure stoichiometric MgCl may indeed be considered as having a defect rocksalt structure with 50% of ordered cation vacancies. 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

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). 

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