Spinels cationic substitution

Two different cations may be substituted into Fe304 for a variety of purposes. The experiments of Lotgering and Van Diepen for example, provided an interesting example of how judicious substitutions could be used to establish reaction (32) for B-site ions, and Mn substitutions into LiFc50g to suppress dielectric loss illustrates the use of a second-cation substitution to optimize a material for a specific application. In this final section on spinels, we choose to discuss the solid solution between Zn[Fc2]04 and Ge[Fe2]04, which would appear to offer the opportunity to study the evolution of the character of the B-site charge carriers with Fes -ion concentration. Ideally, the system would be Znil,GeJ" [Fe2i2xFe2t2x]04-... 

ZnFe204 are structurally related, forming a family in which Fe + or Zn + ions occupy tetrahedral sites, while Cr + or Fe + ions are octahedrally sited. In nature, cation substitution occurs to produce, for example, black crystals of the mineral franklinite (Zn,Mn,Fe)(Fe,Mn)204 which has a variable composition. In the ceramics industry, spinels for use as pigments are prepared by heating together suitable metal oxides in appropriate stoichiometric ratios so as to control the cation substitution in a parent spinel lattice. In (Zn,Fe)(Fe,Cr)204, a range of brown shades can be obtained by varying the cation site compositions. For the commercial market, reproducibility of shade of colour is, of course, essential. 

Similarly to spinel ferrites, garnet ferrites present a wide variety of cation substitutions, which leads to a large range of magnetic properties. 

Magnetic properties can be modified widely by cation substitution. An illustrative case is substitution of Ni by Zn in Ni ferrite to form solid solutions Zni Ni Fe204. Ni ferrite is an inverse spinel, with Fe in A sites Ni and the remaining Fe share B sites. Zn " preferentially enters... 

Experience with Li4Mri50 2 showed that 3V Li-Mn-0 spinel structures can be stabilized (with respect to lithium inser-tion/extraction) by cation substitution to increase the manganese oxidation state in fully charged electrodes to Mn . This concept was recently extended to the spinel Li[Mn,5Ni(,5]04 in which all the manganese ions are tetravalent if the structure is synthesized with in the spinel... 

Magnetite, Fe304, crystallizes in the inverse spinel-structure (cubic), with Fe " " in the tetrahedral sites and both Fe " " and Fe " " in the octahedral sites (space group Fd3m). Many cationic substitutions occur. It is an important iron ore and occurs in many geological environments, as aggregates, veinlets, and inclusions. 

Some cations with an octahedral-site preference (such as Ni2+, Co3+, and Cr3+) are expected to occupy the 16d sites of the spinel with Mn, whereas cations with a strong tetrahedral-site preference (such as Zn2+) are expected to occupy the 8a sites and to dislodge corresponding lithium ions into the 16d sites. In cases where Mn is substituted by transition metal ions (such as Co, Ni, and Cr) that can partake in the electrochemical reaction, voltage plateaus between 4 and 5V have been observed [135, 136],... 

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. 

The ample diversity of properties that the spinel compounds exhibit is derived from the fact that the possibility of synthesis of multi-component spinels by partial substitution of cations in position A and B giving rise to compounds of formula (AxA i x)(ByB 2 y)04. This accounts for the variety of reactions in which they have been used as catalyst. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible, while maintaining the spinel structure. 

Ni-Co ferrites with the general formula Nii cCoxFe204 were tested for the methylation of pyridine [110]. It was observed that the systems possessing x values >0.5 are selective for 3-picoline formation, whereas the ones with x values 0 and 0.2 give a mixture of 2- and 3-picolines. Pyridine conversion increased with the progressive substitution of Ni ions by Co ions. The cation distribution in the spinel lattice influences their acidic and basic properties, and these factors have been considered as helpful to evaluate the activity of the systems. 

Ferrite compounds with the inverse spinel structure are similar to magnetite, with different ions substituting for the iron atoms. As with FeO (cf. Figure 6.62), the oxygen ions have no permanent magnetic moment. Tetrahedral sites in the FCC oxygen array are occupied by half of the trivalent cations, and octahedral sites are occupied equally by divalent cations and the remaining trivalent cations. 

The Na+ ions in the conducting plane have been substituted for by many other (mostly monovalent) cations, and Al3+ in the spinel block has also been substituted for by other di- and trivalent cations. This exchange results in a very complex crystal... 

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