Metal ions octahedral trivalent

The other structures of interest are those ivith the basic formula M2O3, O/M = 1.5. There are tv o of these structures found in CICPs, one being corundum, named after the a-alumina phase of AI2O3, and the other is hematite named after the mineral Fe203. There is a slight difference in spatial geometry betiveen the tv o structures, but both are quite similar. Metal ions are trivalent and octahedrally coordinated in both. 

F. Solvent Exchange on Octahedral Trivalent Transition Metal Ions... 

The unit cell of garnet contains eight formula units. It s silicon-oxygen tetra-hedra exist as independent groups linked to octahedral of the trivalent ions, while the divalent metal ions are situated in the interstices within the Si-Al network, each divalent ion being surrounded by eight oxygen atoms. 

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... 

The number of Bohr magnetons contributed by various divalent transition metal ions are summarized below. Since the trivalent ions are equally distributed between half of the occupied octahedral sites (8) and all the occupied tetrahedral sites (8), their moments cancel out, and the net magnetic moment of a ferrite can be predicted from the moment of the divalent ions that occupy the remainder of the octahedral sites (8). 

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. 

In contrast, 1 2 complexes contain one metal ion and two dye ligands and tend more towards dyeing the surface. This system is characterized by five- or six-membered rings, with the metal in the center of the octahedral structure. Metals are the trivalent, six-coordinated chromium, cobalt, and iron ions. Today, the principles underlying the structure and syntheses of metal-complex dyes are well known. Examples are C.I. Acid Blue 193, 15707 (1 2 Chrome) [12392-64-2] (11) and C.I. Acid Yellow 151, 13906 (1 2 Chrome) [12715-61-6] (12). 

The crystal field spectra and derived A0 and CFSE parameters for several garnets containing octahedrally coordinated trivalent transition metal ions are summarized in table 5.3. The values of A0 and CFSE reflect the variations of metal-oxygen distances in the garnet structures. 

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+... 

In ferromagnesian silicates, therefore, Ni2+ ions are expected to be enriched over Mg2+ in smallest octahedral sites, with the other divalent transition metal ions favouring larger sites in the crystal structures. Thus, based on the ionic radius criterion alone, the olivine Ml and pyroxene Ml sites would be expected be enriched in Ni2+, with the other divalent cations showing preferences for the larger olivine M2 and pyroxene M2 sites. Similarly, in aluminosilicates, all trivalent transition metal ions are predicted to show preferences for the largest [A106] octahedron. 

In aluminosilicates each high-spin trivalent transition metal ion has a larger ionic radius than the host Al3+ ion. However, cations such as Cr3+ (-249.9 kJ/mole), V3+(-182.8 kJ/mole) and Mn3+ (-150.8 kJ/mole), which acquire particularly large CFSE s in octahedral coordination (table 2.5), are induced to enter [A106] octahedra, and not five-coordinated [A105] (andalusite, yoderite) or tetrahedral [AlOJ (silhmanite) sites, by the enhanced stabilization bestowed in octahedral crystal fields. Although Co3+ ions have not been positively identified in silicate minerals, the strong enrichment of cobalt in natural and syn-... 

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. 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

Nickel spinel, NiAl204, was used for making synthesis gas by C02 reforming of methane. The spinel structure is based on a cubic close-packed array of oxide ions. Typically, the crystallographic unit cell contains 32 oxide ions one-eighth of the tetrahedral holes (of which there are two per anion) are occupied by the divalent metal ion (N +), and one-half of the octahedral holes (of which there is one per anion) are occupied by the trivalent metal ion (AP+). These spinels are usually very stable and have been used for high temperature catalytic reactions [12,13]. 

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