Crystal field splitting parameter

Crystal field splitting parameter, 2, 309 Crystal field theory, 1, 215-221 angular overlap model, 1, 228 calculations, 1, 220 generality, 1,219 low symmetry, 1,220 /-orbital, 1, 231 Crystal hydrates, 2, 305,306 bond distances, 2, 307 Crystals... 

A prediction of crystal field theory as outlined in the preceding subsections is that the crystal field splitting parameter, A, should be rather critically dependent upon the details of the crystal lattice in which the transition metal ion is found, and that the splittings of the /-orbital energies should become larger and quite complicated in lattices of symmetry lower than cubic. The theory could not be expected to apply, for example, to the spectra of transition metal ions in solution. 

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 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... 

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

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... 

Chapter 5 summarizes the crystal field spectra of transition metal ions in common rock-forming minerals and important structure-types that may occur in the Earth s interior. Peak positions and crystal field parameters for the cations in several mineral groups are tabulated. The spectra of ferromagnesian silicates are described in detail and correlated with the symmetries and distortions of the Fe2+ coordination environments in the crystal structures. Estimates are made of the CFSE s provided by each coordination site accommodating the Fe2+ ions. Crystal field splitting parameters and stabilization energies for each of the transition metal ions, which are derived from visible to near-infrared spectra of oxides and silicates, are also tabulated. The CFSE data are used in later chapters to explain the crystal chemistry, thermodynamic properties and geochemical distributions of the first-series transition elements. 

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. 

Figure 9.4 Effect of pressure on crystal field splitting parameters for transition metal-bearing periclase and corundum (from Drickamer Frank, 1973 Bums, 1985a). (a) Change of A with pressure for four cations in MgO (b) and (c) (on facing page) pressure variations of A with changes of the unit cell a0 dimension of MgO and A1203.

Such an order is intuitively one of increasing covalent bonding characteristics of the ligands based on their polarizabilities. The nephelauxetic series departs significantly from the spectrochemical series described in 2.9.2, which is based on relative values of the crystal field splitting parameter, A [see eq. (2.19)]. 

Faye, G. H. (1972) Relationship between crystal-field splitting parameter, Ay, and Mhos,-0 bond distance as an aid in the interpretation of absorption spectra of Fe2+ minerals. Canad. Mineral., 11,473-87. 

FIGURE 4 1.7 K reflectance spectra for GaN epilayers grown on various C-plane AI2O3 and 6H-SiC substrates. The strains are those measured from the radius of curvature at 293 K and corrected to 1.7 K. The data are taken from Skromme [1], who can also fit the evolution of the oscillator strengths with strain using his zero strain value for the A]s exciton line and a crystal field splitting parameter of 3.7 1.4 meV. 

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