Octahedral crystal field splitting parameter

Expressed as fractions of the octahedral crystal field splitting parameter, A0. 

Second, the octahedral crystal field splitting parameters, values of which are higher for smaller sites, are expected to decrease in the same order as eq. 

Expressed as fractions of the octahedral crystal field splitting parameter, A0 hs and Is are high-spin and low-spin configurations, respectively. 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

The geometries of the octahedral and tetrahedral coordination sites shown in figs 2.3 and 2.6a suggest that the value of the tetrahedral crystal field splitting parameter, A, will be smaller than the octahedral parameter, A0, for each transition metal ion. It may be shown by simple electrostatic arguments and by group theory that... 

Further resolution of the 3d orbital energy levels takes place within a transition metal ion when it is located in a low-symmetry site, including non-cubic coordination environments listed in table 2.4 and polyhedra distorted from octahedral or cubic symmetries. As a result, the simple crystal field splitting parameter, A, loses some of its significance when more than one energy separation occurs between 3d orbitals of the cation. 

The crystal field parameters of minerals containing Ni2+ ions are summarized in table 5.19. Note that the energy of the first transition, band u, for Ni2+ in octahedral coordination provides a direct measure of the crystal field splitting parameter A . Crystal field stabilization energies for Ni2+ derived from band u, decrease in the order... 

For an ion such as Cr in an octahedral site (in garnet or olivine) the lowest-energy spin-allowed transition gives an absorption band with frequency V that is equivalent to the crystal-field splitting parameter (Aq). The second spin-allowed transition gives rise to an absorption bemd V2 the energy of which is given by ... 

The energy level diagram for Ti3+ in fig. 3.4 shows the manner by which the 2D spectroscopic term is resolved into two different levels, or crystal field states, when the cation is situated in an octahedral crystal field produced by surrounding ligands. In a similar manner the spectroscopic terms for each 3d" configuration become separated into one or more crystal field states when the transition metal ion is located in a coordination site in a crystal structure. The extent to which each spectroscopic term is split into crystal field states can be obtained by semi-empirical calculations based on the interelectronic repulsion Racah B and C parameters derived from atomic spectra (Lever, 1984, p. 126). 

For the pseudo-tetragonally elongated Ml octahedral site in the olivine structure, an alternative method for obtaining the CFSE of Fe2+ ions in this site is to evaluate the ligand field splitting parameters in the equatorial plane and along the axial direction using eqs (3.14) to (3.16). The crystal field spectral data for the fayalite Ml site (see fig. 5.11a) yield values for 10 Dq(m) and 10 Dq(eq) of 8,172 cm-1 and 9,327 cm-1, respectively. The mean 10 Dq value of 8,942 cm-1 and the CFSE of 4,133 cm-1 compare favourably with A0 = 8,830 cm-1 and CFSE = 4,250 cm-1 for Fe2+ in the fayalite Ml sites determined by the baricentric method (fig. 5.11a). 

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,... 

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