Spinel type crystal structure

Many of the mineralogically important transition-metal oxide phases contain more than one cation species, or more than one type of coordination site for the cations. Commonly, the cations are in more than one oxidation state. Examples include ilmenite (FeTiOj) and the family of minerals with the spinel-type crystal structure, including magnetite (Fe304), chromite... 

Several oxides of transition elements were tested for possible catalytic action both alone and with promoters. High yields of sodium sulfate were obtained when oxides were present which could combine in a spinel-type crystal structure. These same oxides alone possessed very little catalytic activity. 

Both compounds Mn304 (i.e., hausman-nite) and /-Mn203 crystallize with a tetragonally distorted spinel-type structure (see Fig. 17). 

Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Good-enough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This semiempirical model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. 

Spinels have a crystal structure in which there is a face-centered cubic arrangement of O2 ions. There are two types of structures in which cations have octahedral or tetrahedral arrangements of anions surrounding them. In the spinel structure, it is found that the +3 ions are located in octahedral holes and the tetrahedral holes are occupied by the +2 ions. A different structure is possible for these ions. That structure has half of the +3 metal ions located in the tetrahedral holes while the other half of these ions and the +2 ions are located in the octahedral holes. In order to indicate the population of the two types of lattice sites, the formula for the compound is grouped with the tetrahedral hole population indicated first (the position normally occupied by the +2 ion, A) followed by the groups populating the octahedral holes. Thus, the formula AB204 becomes B(AB)04 in order to correctly... 

The smallest A-ions, Li+ and Mg +, favour crystal structures in which they are octahedrally coordinated. This is the case in compounds of e.g. the LiSbFa-, LiaZrFe- and Na2SiF6-types. In the only fluoride spinel, Li2NiF4, half of the Li-ions is even tetrahedrally coordinated (page 34). In all cases these smallest A-ions, similar to the transition ions themselves, occupy the holes between the closed-packed fluoride layers. 

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). 

Still more surprising are certain crystals in which a set of equivalent positions is only partially occupied, some sites here and there at random being empty. The spinel group also provides examples of this type of structure. The cubic (y) form of Fe2Oa, for instance, gives an X-ray... 

Commonly, many compounds with the same formula type have the same type of structure. Usually the most common compound is chosen as representative, but it could be the first structure of the type studied. For example, hundreds of binary compounds of the MX type have the same crystal structure as NaCl. Other compounds can be described as having the NaCl (or halite, the name of the mineral) structure. If the structure being considered has slight differences, these differences can be described in terms of the reference structure. One often sees statements such as a compound has a disordered spinel (MgAl204) type structure or an inverse spinel structure. This requires knowledge of the spinel structure because "inverse" or "disordered" terms describe variations of occupancies of octahedral and tetrahedral sites. 

Figure 10.1.10 shows the crystal structure of spinel. The unit cell (represented by solid lines) is divided into eight small cubic octants (represented by broken lines) of two different types, as shown in (a) the structure of type I and type n octants and the connections of atoms are shown in (b). There are four O2- in each octant. The number of O2- in the unit cell is 32 (= 8x 4). The Mg2+ ions are located at the centers of type I octants and half of the vertices of type I and type II... 

Magnetite (FC3O4) is a component of the water-gas shift reaction that crystallizes to the inverse spinel structure. The general formula of the oxides known as spinel is AB204. In the normal spinel-type structure, A is an A2+ metal, and B is a B3+ metal. O2- forms a CCP anionic framework, where the A atoms occupy 1/8 of the tetrahedral sites and the B atoms occupy 1/2 of the octahedral sites (see Figures 1.6 and 1.7). An example of a normal spinel is Mg A1204. 

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). 

Europium doped barium aluminates of the type Bai xEuxMg2Ali6027 are used as a blue component in fluorescent lamps, and are also being investigated for possible use in flat panel displays. The crystal structure of this material is characterized by an intergrowth of spinel and magnetoplumbite units (See 3.4.2 and 3.11). 

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