Spinel cation distribution

An interesting study of oxidic spinel ferrites of the type CO cNi5/3 xFeSbi/304 was reported [21], where three different Mbssbauer-active probes Fe, Ni and Sb were employed on the same material. The results have been interpreted in terms of the cation distributions over spinel A- and B-lattice sites, magnetic moments and spin structure, and the magnitude of the supertransferred hyperfine... 

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

In mixed spinels, electron transfer between tetrahedral and octahedral Mn3+ ions is hampered. As the cation distribution is temperature sensitive, preparation of materials for NTC use must optimize the amount of Mn in octahedral sites. 

The spinel family of oxides with composition AB2O4 has the A and B cations distributed in octahedral and tetrahedral sites in a close-packed oxygen structure (Supplementary Material SI). Impurity doping can take place by the addition of a dopant to octahedral or tetrahedral sites. In this, the spinel family of compounds is quite different from the A2B04 perovskite-related phases of the previous section in that both cation sites are similar in size and can take the same cations. 

In reality, very few spinels have exactly the normal or inverse structure, and these are called mixed spinels. The cation distribution between the two sites is a function of a number of parameters, including temperature. This variability is described by an occupation factor, X, which gives the fraction of B3+ cations in tetrahedral... 

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. 

The general cation distribution in the spinel structure of the iron oxide-chromium oxide catalyst (denoted Cr-Fe304 subsequently) is shown below ... 

According to the cation distribution over the 8(a) and 16(d) sites, the spinels of formula AB2O4 can be divided into normal and inverse classes. Examples of normal spinels are MgA CU and MgTi204, in which the oxidation states of A1 and Ti are both +3 their respective structural formulas are Mg2+ t A]2+ 004 and [Mg2+]t[Ti2+]o04, in which the subscripts t and 0 indicate tetrahedral and octahedral sites, respectively. 

Just as vivianite is regarded as the simplest example of a mineral with isolated clusters of Fe2+-Fe3+ octahedra showing IVCT transitions, so too is magnetite considered to be the classic example of a structure-type with infinite chains of Fe2+ -Fe3+ octahedra exhibiting electron delocalization. Magnetite, Fe304 or Fe2+Fe3+204, has the spinel structure illustrated in fig. 4.18 with an inverse cation distribution ( 6.4). Thus, half the Fe3+ ions occupy isolated tetrahedral sites,... 

Table 6.2. Experimental and theoretical cation distribution in spinels...

Glidewell, C. (1976) Cation distribution in spinels Lattice energy versus crystal field stabilization energy. Inorg. Chim. Acta, 19, L45-7. 

Navrotsky, A. Kleppa, O. J. (1967) The thermodynamics of cation distribution in simple spinels. J. Inorg. NucL Chem., 29,2701-14. 

O Neill, H. St C. Navrotsky, A. (1983) Simple spinels crystallographic parameters, cation radii, lattice energies, and cation distribution. Amer. Mineral., 68,181-94. 

With these energies, it is possible to calculate not only the preferred cation distribution over two types of available anion interstice, as in the spinel lattice, but also the valency distribution given two atoms on the same site, each with multiple-valence alternatives. This point... 

The thermal decomposition of hydrotalcites also is intriguing. They decompose on calcination to nanocrystalline spinel-like phases, and eventually, to crystalline spinels. This pathway initially produces spinels which are hydrated in their large surfaces and poorly ordered in their cation distributions. These materials are very reactive and analogous to precursors sought in ceramic synthesis. The thermochemical study of the evolution of MgAl204 from nitrate precursors (McHale et al. 1998) may in fact have encountered hydrotalcite as an intermediate. Further study of the thermal evolution of hydrotalcite nanomaterials may be applicable to both earth and materials sciences. 

---