Spinel ferrites cation distributions

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... 

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. 

Figure 1.41) have the oxygen ions in a nearly close-packed cubic array. The unit cell contains 32 oxygen ions, with 32 octahedral and 64 tetrahedral sites, of which 16 of the octahedral and 8 of the tetrahedral sites are filled. It is the position of these 24 cations within the unit cell that determines magnetic behavior. The distribution of cations in the sites is specific to the type of cations, and it must be determined experimentally. There are two idealized spinel structures. In the normal spinel, all the divalent ions are on the tetrahedral sites, as in ZnFe204. In the inverse spinel, the 8 occupied tetrahedral sites are filled with trivalent ions and the 16 occupied octahedral sites are equally divided between di- and trivalent ions (see Figure 6.63). The prototypical inverse spinel ferrite is magnetite, whose structure consists of an FCC oxygen array with Fe + and Fe + ions in the interstices.

Ferrites with the cubic spinel structure (see Section 5.3.10) are soft magnetic materials (see Section 12.4.3), widely used in electronic circuitry. The formula of all ferrites can be written as A +Fe +04, where can be chosen from a large number of medium-sized cations, for example Ni or Zn. The majority of the important ferrites are inverse spinels, in which the cations occupy the octahedral sites, together with half of the Fe + ions. The other half of the Fe cations is found in the tetrahedral sites. Thus nickel ferrite would be written (Fe +)[Ni Fe ]04, where the cations in octahedral sites are enclosed in square brackets and those in tetrahedral sites in parentheses. Lodestone, or magnetite, Fe304, described in Section 12.3.4, is an example in which the cations are Fe and Fe, and the cation distribution is (Fe +)[Fe2+Fe +]204. 

At this point, the spinel ferrite structure MFe204, where M refers to a metal, will be briefly discussed. As a typical representative, reference is made to the ZnFe204 structure, with Fe ions occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn [43]. Depending on the distribution of the and Fe cations between the tetrahedral... 

Table 4.1 Characteristics of main spinel ferrites with expected distribution of cations in tetrahedral (A) and octahedral (B) sites and theoretical values of magnetization...

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,... 

It turns out that crystal field theory accounts adequately for practically all the experimental results in spinels. The fact that all the known chromites have a normal distribution is consistent with the high octahedral field stabilisation energy value calculated for Cr (S = 3/2). In ferrites, the arrangement is very dependent on the divalent cation, since Fe has no crystal field stabilisation energy. When the divalent cation also shows no clear preference, ferrites with 3 values between zero and one (mixed ferrites) are obtained. 

According to the distribution of cations, there are normal, mixed and inversed spinels structures which depended on the fact what kind of ions and in what order A and B take empty spaces. In this review, we will investigate zinc ferrite (ZnFe204), manganese ferrites (MnFe204) and nickel ferrite (NiFe204) from normal, mixed and inverse structures respectively [12]. 

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