Spinels normal

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

In the following, we start by assuming purely ionic structures. In spinel the oxide ions form a cubic closest-packing. Two-thirds of the metal ions occupy octahedral interstices, the rest tetrahedral ones. In a normal spinel the A ions are found in the tetrahedral interstices and the M ions in the octahedral interstices we express this by the subscripts T and O, for example Mgr[Al2](904. Since tetrahedral holes are smaller than octahedral holes, the A ions should be smaller than the M ions. Remarkably, this condition is not fulfilled in many spinels, and just as remarkable is the occurrence of inverse spinels which have half of the M ions occupying tetrahedral sites and the other half occupying octahedral sites while the A ions occupy the remaining octahedral sites. Table 17.3 summarizes these facts and also includes a classification according to the oxidation states of the metal ions. 

X = 0 normal spinel X = 0.5 inverse spinel The distribution of the cations among the tetrahedral and octahedral sites is then expressed... 

The parent phases for this series can be regarded as MgGa204 and MgMn204. Both are mixed spinels, but it is convenient to regard them as normal spinels initially. The replacement of Ga3+ by Mn3+ then only concerns the octahedral sites in the structure. Ignoring the MgO component, this is written ... 

The normal spinel Li[Ti2]04 is a metallic oxide with a superconducting transition temperature of 13.7 K. The nominal formula is Li+[Ti3+Ti4+]04, in which the Li+ ions occupy the tetrahedral sites while the octahedral sites contain titanium with an average charge Ti3 5+, although as the material is metallic at room temperature the electrons are delocalized in a partly filled 3d band. 

The normal spinel MgTi204 is a black semiconductor with a high conductivity. 

By forming solid solutions between two ferrites, it is possible to tune the properties magnetically. An example is provided by the solid solution Zn,Ni1 xFe204, which is between the normal spinel ZnFe2C>4 and the inverse spinel NiFe2C>4. Reaction to form a solid solution with NiFe2C>4 ... 

Both ordered and normal spinels should disorder at high temperature, since the entropy of the random spinel is larger. The disordering reaction for a normal spinel can be described by a quasi-chemical reaction... 

The spectra of the doped materials (Cr, Ni, Zn +, Li+, Co +, AP+) are similar to those seen for the nominally stoichiometric materials, and sets of resonances between 500 and 700 ppm are seen on cation doping in addition to that of the normal spinel environment (at ca. 500 ppm). Again, these resonances are assigned to lithium ions near manganese-(IV) cations. The lower intensity of the additional resonances seen on Cr + substitution, in comparison to Zn + or Ni + substitution, is consistent with the oxidation of fewer manganese ions near the depart ions. For the Li- and Zn-doped spinels, resonances at ca. 2300 ppm were also observed, which are assigned to lithium ions in the octahedral sites of the spinel structure. In the case of Zn doping, it is clear that the preference of Zn + for the tetrahedral site of the spinel structure forces the lithium onto the octahedral site. 

In a normal spinel structure, the formula is XY2O4 with 8 ions occupying the tetrahedral sites and 16 ions occupying the octa-... 

The point symmetry of the octahedral sites in the spinel-structure is Dsd- While the octahedra are perfect for an oxygen parameter u of 3/8, they are compressed along a threefold axis for u < 3/8, and expanded for u > 3/8. In the case of MgCr204 ( a0,385) and MgAl204 (21 = 0,387) ( normal spinels with Mg + in tetrahedral and Cr +, Al + in octahedral sites), the expansion is of the magnitude of about 4 to 5%. The corresponding splitting effects of the second absorption band in the remission spectra of the mixed-crystal powders (Mg)( > (Cr jAL-x) O4 (13) are about... 

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