Spinel crystal field

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. 

Not considered in this review are the removal of energetic equivalence by an applied field or stress where, as in a spinel, the energetically equivalent sites have differently oriented crystal-field axes. This lifting of site degeneracy coupled with charge transfer between mixed-valence states leads to such phenomena as magnetic after effect and photoinduced anisotropy ... 

Although crystal field theory quite successfully rationalizes observed structures of the spinels of the first transition series, it must be applied with care to other examples. In comparing structures in which other factors (ionic radii, covalency, etc.) are more dissimilar, d orbital splittings alone generally do not explain the observations. In these cases, a broader analysis is required. 

Coordination symmetry of iron and cobalt in staurolite The crystal field spectra of Fe2+ ions surrounded by oxygen in regular octahedral sites normally contain absorption bands centred near 1,000 nm or 10,000 cm-1 (see fig. 3.2). By changing from octahedral to tetrahedral coordination, absorption bands for tetrahedral Fe2+ ions would, according to eq. (2.7), be predicted to occur at (% x 10,000) or 4,444 cm-1 (2,250 nm) if the iron-oxygen distances remain identical in the two coordinations. Tetrahedrally coordinated Fe2+ ions in spinel, MgAl204, for example, produce an absorption band near 4,830 cm"1 (2,070 nm) ( 5.3.3). 

The crystal field spectrum of Fe2Si04 spinel contains a broad, slightly asymmetric band centred around 11,430 cm-1 (Mao and Bell, 1972b Bums and Sung, 1978), leading to the crystal field parameters for Fe2+ of... 

The reflectance spectrum of Ni2Si04 spinel contains intense bands with absorption maxima of 9,150 cm-1, 14,780 cm-1 and 22,550 cm-1 at atmospheric pressure (Yagi and Mao, 1977). These led to initial estimates of the crystal field parameters for Ni2+ in silicate spinel of A0 = 9.,150 cm-1 and CFSE = 10,980 cm-1. However, there is also a prominent shoulder in the reflectance spectra around 8,000 cm-1 attributable to trigonal distortion of the octahedral site in the spinel structure (Bums, 1985a). This led to revised estimates for Ni2+ in Ni2Si04 spinel of... 

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). 

Measurements of absorption spectra of oxides, glasses and hydrates of transition metal ions have enabled crystal field stabilization energies (CFSE s) in tetrahedral and octahedral coordinations to be estimated in oxide structures (see table 2.5). The difference between the octahedral and tetrahedral CFSE is called the octahedral site preference energy (OSPE), and values are summarized in table 6.3. The OSPE s may be regarded as a measure of the affinity of a transition metal ion for an octahedral coordination site in an oxide structure such as spinel. Trivalent cations with high OSPE s are predicted to occupy octahedral sites in spinels and to form normal spinels. Thus, Cr3, Mn3, V3+... 

Crystal chemistry of spinels. A classic example showing that transition metal ions display distinct site preferences in oxides stems from studies of spinel crystal chemistry. The spinel structure contains tetrahedral and octahedral sites normal and inverse forms exist in which divalent and trivalent ions, respectively, fill the tetrahedral sites. The type of spinel formed by a cation is related to its octahedral site preference energy (OSPE), or difference between crystal field stabilization energies in octahedral and tetrahedral coordinations in an oxide structure. Trivalent and divalent cations with large site preference energies (e.g., Cr3 and Ni2+) tend to form normal and inverse spinels, respectively. The type of spinel adopted by cations with zero CFSE (e.g., Fe3+ and Mn2+) is controlled by the preferences of the second cation in the structure. 

The views of Williams were extended by subsequent writers. Curtis (1964) considered the stability of ions in tetrahedral coordination in minerals as well as in octahedral sites. While this approach is applicable to spinels crystallizing from the magma, it does not apply to silicate minerals in which the transition metal ions occupy only six-coordinated sites. Bums and Fyfe (1964) presented crystal field spectral data indicating that cations such as Ni2+ are present in both octahedral and tetrahedral sites in silicate glasses assumed to approximate... 

Bums, R. G. Sung, C.-M. (1978) The effect of crystal field stabilization energy on the olivine - spinel transition in the system Mg2Si04-Fe2Si04. Phys. Chem. Minerals, 2, 349-64 [see also Phys. Chem. Minerals, 2,177-97 (1978)]. 

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

Mao, H.-K. Bell, P. M. (1972a) Optical and electrical behaviour of olivine and spinel (Fe2Si04) at high pressure. Interpretation of the pressure effect on the olivine absorption bands of natural fayalite to 20 kb. Crystal field stabilization of the olivine-spinel transition. Ann. Rept. Geophys. Lab., Yearb. 71, 520-8... 

Syono, Y., Tokonami, M. Matsui, Y. (1971) Crystal field effect on the olivine-spinel transformation. Phys. Earth Planet. Ineriors, 4,347-52. 

There has been a considerable effort in the physics and chemistry communities to use INS methods to study magnetic dynamics, which can often be described as spin waves. Measurements of spin wave dispersion curves can provide information about the interactions between atomic magnetic moments, the so-called exchange interactions. There have been comparatively few INS measurements on magnetic minerals. INS methods have been used to produce spin wave dispersion curves for hematite. Crystal field magnetic transitions in cobalt bearing cordierite, and spinel phases have also been studied by INS. ... 

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