Crystal field distortions

This expression is based on the approximation that the 15 normal modes of vibration of a FeN6 chromophore in each spin state may be reasonably well represented by one mean effective vibrational wave number VL and vH for the LS state and the HS state, respectively. The ej are the energy separations of the 15 electronic levels of the 5T2(Oh) term, split by crystal field distortion and eventually by spin-orbit coupling, from the Aj level. In a reasonable approximation the 15 Cj parameters may be reduced to only two, viz. the energy separation e between the baricenter of the ST2 term and the Ai level, and the trigonal distortion parameter St =... 

Because of the crystal field distortion, the hydrogen-bond lengths tend to be shorter in crystals than observed experimentally or predicted theoretically in hydrogen-bonded dimers. The information from gas-phase diffraction and micro-wave spectroscopy on hydrogen-bonded dimers is necessarily limited to the relatively few dimers which are stable in the gas phase. X-ray and neutron diffraction crystal structure analysis, in contrast, provides a plethora of structural data relating to hydrogen-bond formation, as is demonstrated in Part IB, Chapter 7 and subsequent chapters. 

Recently, the spectral study of DMTM(TCNQ)2 phase transition was performed [60]. The salt is a quarter-filled organic semiconductor containing segregated chains of TCNQ dimers and DMTM counterions. This material undergoes an inverted Peierls transition, which has tentatively been explained in terms of a crystal-field distortion. It was shown that the experimental values of unperturbed phonon frequencies and e-mv coupling constants are nearly independent of temperature. The dimer model fails to reproduce the phonon intensities and line shapes and underestimates the coupling constants, whereas the CDW model produces better results... 

The basis set consists of all combinations of the orbital kets L, ML) and the spin kets S, Ms), yielding the number N = (2L + 1)(2S + 1). The perturbation operator includes the crystal field distortion, and the spin-orbit coupling, Hs0, terms... 

Temperature dependence of p . The solid line is a theoretical curve for the thermal population model with 5 = 240cm" and A [(2Ai,)-t ( Ej,)] = 520cm" 5, low-symmetry crystal field distortion (5=[Pg.680]

The and 72 states are broadened as a result of slight variations in the crystal field. The 72 and E states are sharper but the E state is split into two components, 29 cm apart, because of the slight distortion of the octahedral field. Population inversion and... 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

Various theories have been postulated to explain the phenomenom that magnetic fields can change the way in which calcium carbonate precipitates and crystallizes, the most probable one being that the applied field distorts the electrical charge carried by small particles of calcium carbonate that have already formed in the water, thus affecting the way in which they adhere to form large particles. 

The series of deviation from a strictly octahedral crystal field follows the sequence ammonium tungstate > tungstic acid > WO3 [14]. The s->d transition becomes allowed as distortion of the octahedral environment increases. Viewed as a fingerprint technique. Figure 6 shows that the bulk of W present in both WZ (calcined) and PtWZ(acac, reduced) behaves as WO3, as also shown by our XRD data [10], sugesting that only a small extent of W reduction is expected to have taken place during catalyst conditioning prior to reaction. 

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

Foyt et al. [137] interpreted the quadrupole-splitting parameters of low-spin ruthenium(II) complexes in terms of a crystal field model in the strong-field approximation with the configuration treated as an equivalent one-electron problem. They have shown that, starting from pure octahedral symmetry with zero quadrupole splitting, A q increases as the ratio of the axial distortion to the spin-orbit coupling increases. 

High-valent iron can occur in a wide variety of electronic configurations. Figure 8.25 (a-c, e-i) presents a summary of the corresponding one-electron crystal-field states for the 3(/, 3J, and 3J electron configurations, allocated to HS and LS states in distorted octahedral and tetrahedral symmetry. Part d, in addition, depicts the case of low-low-spin iron(IV) found in some trigonal... 

FIGURE 5.16 Crystal-field pictures for the low-spin d5 system in deformed octahedra. (a) relative energy of d-orbitals for an octahedron with strong crystal field (b) d-orbitals of the T2s set in a rhombically distorted octahedron (c) three possible electron configurations of increasing total energy (d) the term scheme for the configurations in (c). 

I shall take the simple view that most metal oxide structures are derivatives of a closest packed 02 lattice with the metal ions occupying tetrahedral or octahedral holes in a manner which is principally determined by size, charge (and hence stoichiometry) and d configuration (Jj). The presence of d electrons can lead to pronounced crystal field effects or metal-metal bonding. The latter can lead to clustering of metal atoms within the lattice with large distortions from idealized (ionic) geometries. 

Wood B. I and Strens R. G. J. (1972). Calculation of crystal field splittings in distorted coordination polyhedra Spectra and thermodynamic properties of minerals. Min. Mag, 38 909-917. 

---