Site occupancy spinels

The fact that the site occupation in magnetite is opposite to that of spinel arises from the interaction of the d elecU ons on die cations with the suiTound-ing anions. The energy for the exchange... 

The electronic structure of the spinel type compound NiCo204 has been investigated by XANES, EXAFS, and Ni Mdssbauer studies. On the basis of the derived cation valencies, the octahedral and tetrahedral site occupancies as well as the formula in standard notation for spinel compounds could be delineated [25]. 

The physical properties of ferrospinels are sensitive to the iron-ion distribution on the two cation subarrays. Normally the Fe ion has a stronger A-site preference than the Fe " " ion, but the difference in site-preference energy depends upon the character of the counter cation, and in some spinels the Fe valence state competes with the Fe " valence state for tetrahedral-site occupation. 

In fact, the site occupancy of a normal spinel is abnormal. According to the radius ratio concept, larger divalent cations should occupy octahedral sites and smaller trivalent cations might be expected to favour the tetrahedral sites. The mineral spinel, Mg A1204 which is predominantly normal, does not conform with radius ratio criteria. 

One of the unusual features of spinel crystal chemistry is that some transition metals form normal spinels and others inverse. The spinel-types are summarized in table 6.2. The site occupancy patterns were considered to be anomalous until they were explained by crystal field theory (McClure, 1957 Dunitz andOrgel, 1957). 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

Fig. 9.13 Diagrammatic representation of site occupancy in (a) normal and (b) inverse spinels.

A crucial assumption of this technique is that the x-ray emission process is highly localized that is, the electron currents are the same for the unknown element X and for the standards A and B, as in Eq. (7.11). Early planar ALCHEMI determinations of site-occupancies of minor elements in spinels (Tafto 1982) and olivine (Tafto and Spence 1982), for example, suggested that the assumption is valid. However, more recent studies by Rossouw and Maslen (1987) and Rossouw, Turner, and White (1988) indicate that the x-ray generation is not perfectly localized and that Eq. (7.11) is, in general, not valid. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

FeGa204 shows two iron(Il) quadrupole doublets, indicating that it is partly inverse [56]. It is unusual in that reheating the product of the original reaction mixture causes an alteration in the site occupancy, apparently involving a change towards a normal spinel structure. 

As the crystallographic site occupancy is concerned, in inverse spinels all Li ions reside in octahedral coordination sites, while the transition metal ions (TMI) can occupy both octahedral and tetrahedral sites. The results obtained are reported in Table 1 they suffer a limitation due to the fact that the XRPD technique can discriminate between ion and Co ... 

In order to elucidate the energetics of the cation site occupation, we have applied ab-initio techniques of density functional theory within the local density approximation. Two possible configurations corresponding to normal and (partially) inverse spinel were considered (Figs. 3a and b). The results are given in Tablet, with the bulk moduli obtained by fitting the Birch equation of state. 

This class of ceramic is named after the mineral MgAl204 and many members have been fabricated because of the sensitive dependence of their electrical and magnetic properties on composition and temperature. Such a dependence arises from the variations in cation site occupancy that can be engineered. Spinels containing iron are particularly useful because of their magnetically soft properties that led to their application in computer hardware, memory devices, high-frequency transformers, and phase shifters. 

Crucial to the understanding of metal-substituted magnetite and their properties is the determination of the site occupancy of the metals. The structure of magnetite is very well known. It is a cubic, inverse spinel with one quarter of the tetrahedral (Tj) and one half of the octahedral (0 j) sites filled by Fe. The formula for magnetite is (Fe )[Fe Fe ]04 where Fe ... 

Eig. 2. Spinel stmcture where Q is oxygen, Q > A-sites, O, B-sites. A unit cell consists of eight fee subceUs with different cation occupants. Adapted from... 

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,... 

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. ... 

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... 

Commonly, many compounds with the same formula type have the same type of structure. Usually the most common compound is chosen as representative, but it could be the first structure of the type studied. For example, hundreds of binary compounds of the MX type have the same crystal structure as NaCl. Other compounds can be described as having the NaCl (or halite, the name of the mineral) structure. If the structure being considered has slight differences, these differences can be described in terms of the reference structure. One often sees statements such as a compound has a disordered spinel (MgAl204) type structure or an inverse spinel structure. This requires knowledge of the spinel structure because "inverse" or "disordered" terms describe variations of occupancies of octahedral and tetrahedral sites. 

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