Spinel arrangement

Only few structure types have been observed for compounds of this composition. The spinel structure type of the compound Li2NiF4 (a = 8.313 A) 267) is unique. It crystallizes in the inverse spinel arrangement containing all the Ni and half of the Li cations in octahedral sites. 

There are, of course, very many structures (such as the perovskite, ilmenite and spinel arrangements) in which octahedral co-ordination by oxygen is observed, but these are not structures of compounds which can be described as salts or in which the B 06 group can be regarded as a discrete ion. 

In order to make a more critical test of the theory, we must look for a structure in which the transition-metal ion has the possibilities of both octahedral and tetrahedral coordination, and preferably one in which a given transition-metal ion may be exchanged for another without substantial alteration in the atomic arrangement. Such a structure is the spinel arrangement in which a large number of XY2O4 compounds crystallize, where X is a divalent metal ion and Y is Al3+, Cr3+, Mn3+, Fe3+, or Co3+. This structure is also of importance in the discussion of distortions from cubic symmetry and we shall now consider it in some detail. 

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. 

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. 

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 beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

In P", (a), the oxide packing arrangement comprises close packed layers in a cubic close packed,. .. ABC. .., stacking sequence, which continues throughout the structure. The spinel blocks are four oxide layers thick and contain AP" ions (and stabilising Li, Mg ions) distributed over tetrahedral and octahedral sites, as in spinel, MgAl204. These sites... 

Spin density, Fe—S dimers, 38 443-445 Spin-dependent delocalization, 47 39 Spin echo studies, photosystem II, 33 233-234 Spin eigenfunctions, 38 427 Spinels, 4 155, 20 143, 144 arrangement, 2 29-33 crystal structure of, 4 153 neutron diffraction studies on, 8 249-250 substitution of oxygen by fluorine, 20 145, 146... 

Further adding to the complexity of the spinel structure are three possible arrangements of the metal ions in the cubic close-packed anions. The ordering of divalent metal ions (such as Mg2+) on the proper tetrahedral sites and all the trivalent ions (as Ai3+) in the correct octahedral sites, will give rise to the normal spinel structure. If the divalent ions occupy some of the octahedral sites and half of the trivalent ions move to the tetrahedral sites, the structure is then referred to as the inverse spinel structure. The last case exists when the tetrahedral sites and the octahedral sites are occupied by a mixture of di- and tri-valent ions. This type is known to generate the random spinel structure, and the exact composition and populations in the... 

In addition, there are 2N tetrahedral sites, and the divalent ions (Mg +) occupy one-eighth of these. In the inverse spinel structure, the oxide ions are also in a cubic close-packed arrangement, but the divalent metal ions occupy octahedral sites and the trivalent ions are equally divided amongst tetrahedral and octahedral sites. 

The structure of so-called hexagonal ferrite is based on the spinel (MgAljOJ structure. Arrangement of oxygen in spinel is CCP, and Mg and... 

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