Spinel structures

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Tricoball teiroxide, C03O4. Black solid (ignition of CoO in air). Has spinel structure and other spinels MC02O4 are also known. 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (o) instead they adopt the inverse spinel structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B)t[AB]o04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects. ... 

The sensitive dependence of the electrical and magnetic properties of spinel-type compounds on composition, temperature, and detailed cation arrangement has proved a powerful incentive for the extensive study of these compounds in connection with the solid-state electronics industry. Perhaps the best-known examples are the ferrites, including the extraordinary compound magnetite Fc304 (p. 1080) which has an inverse spinel structure (Fe )t[Fe Fe ]o04. 

Zirconates and hafnates can be prepared by firing appropriate mixtures of oxides, carbonates or nitrates. None of them are known to contain discrete [M04]" or [MOs] ions. Compounds M ZrOs usually have the perovskite structure whereas M2Zr04 frequently adopt the spinel structure. 

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

Nickel sulfides are very similar to those of cobalt, consisting of NiS2 (pyrites structure, p. 680), Ni3S4 (spinel structure, p. 247), and the black, nickel-deficient Nii-j S (NiAs structure, p. 555), which is precipitated from aqueous... 

The lithium-titanium oxides are prepared by heating a mixture of anatase (Ti02) and LiOH at a high temperature. The product heated at 800-900 °C has a spinel structure of Li4/3Ti5y304. When the charge and discharge cycles are performed... 

In contrast, LiMn204 has a spinel structure. This material has the space group Fd3m in which the transition-metal and lithium ions are located at octahedral 8(a) and tetrahedral 16(d) sites, respectively, and the oxygen ions are at 32(e) sites. There are octahedral 16(c) sites around the 8(a) sites and lithium ions can diffuse through the 16(c) and 8(a) sites. As this structure contains a diffusion path for the lithium ions, these ions can be deinter-calated and intercalated in these compositions. 

By contrast, lithium extraction from the tetrahedral sites in Li[Mn2]04, i.e., for 0cubic symmetry of the spinel structure [105, 114, 120]. It is difficult to extract all the lithium electrochemi-cally from Li[Mn2]04, at least at practical voltages, without causing decomposi-... 

It is possible that all three phenomena contribute to the capacity loss of 4V Lix[Mn2]04 electrodes. Nonetheless, all three can be at least partly circumvented by slightly modifying the composition of the spinel electrode. For example, cell performance can be improved by increasing the amount of lithium in the spinel structure [107, 123, 130] and, in particular, by substituting a small amount of manganese on the B sites with lithium [107], which drives the composition a small way down the stoichiometric spinel tie-line, towards... 

Electrodes that are prepared from acid-leached LT-LiCo, xNix02 compounds (0< x<0.2) show significantly enhanced electrochemical behavior over the parent LT-LiCo1 xNix02 structure. The improved performance has been attributed to the formation of compounds with a composition and cation arrangement close to the ideal Li[B2]04 spinel structure (B = Co, Ni) [62]. These spinel-type structures have cubic symmetry, which is maintained on lithiation the unit cells expand and contract by only 0.2 percent during lithium insertion and extraction. 

Although Li[Ti2]04 and Li4Ti,Ol2 have low theoretical capacities (161 and 175 mAhg-1, respectively), they have excellent structural properties for an insertion electrode. The spinel structures are excep-... 

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