Crystal field effect parameters

As was the case with lanthanide crystal spectra (25), we found that a systematic analysis could be developed by examining differences, AP, between experimentally-established actinide parameter values and those computed using Hartree-Fock methods with the inclusion of relativistic corrections (24), as illustrated in Table IV for An3+. Crystal-field effects were approximated based on selected published results. By forming tabulations similar to Table IV for 2+, 4+, 5+ and 6+ spectra, to the extent that any experimental data were available to test the predictions, we found that the AP-values for Pu3+ provided a good starting point for approximating the structure of plutonium spectra in other valence states. However,... 

A general model for electronic relaxation of the Gd3+ S = 7/2 ion in various complexes in solution was presented by Rast el al. [86]. Contrary to the usual assumption, the electronic relaxation in their model is not only due to the effects of the transient zero field splitting, but is also strongly influenced by the static crystal field effect which is modulated by the random Brownian rotation of the complex. Experimental peak-to-peak widths of three gadolinium complexes could be well interpreted as a function of temperature and frequency using three static and one transient crystal field parameters. Moreover, their interpretation of experimental data did not require the addition of any field independent contribution to the line width like the spin-rotation mechanism. 

The terms in the Hamiltonian that represent the non-spherical part of the interaction with the crystal are modeled using the so-called crystal-field Hamiltonian. It is important to recognize that this Hamiltonian is not restricted to electrostatic effects, which form a minor part of the total crystal-field effect (Ng and Newman, 1987). When the parameters are fitted to experimental energies, their values reflect all one-electron non-spherical interactions. The crystal-field Hamiltonian is expressed in Wyboume (1965) notation as,... 

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.

Also, external parameters influencing the crystal field effects (besides the temperature-dependent dynamical behaviour) such as pressure or external electric fields have to be taken into account. 

Within this framework we consider the bonding electrons as the main source of the EFG. For the crystal field, effects A) and B) are most important. Lattice defects seem to play a minor role in molecular crystals. The influence of the external parameters on NQR is quite important since, with external fields, the crystal field may be influenced experimentally. However, very little work has been done in this field. 

The determination of the asymmetry parameters and the direction cosines, together with x-ray investigations of the crystal structure, will shed some light on the crystal field effect in solids. NQR powder spectra can only permit very rough and qualitative conclusions on the intermolecular forces. The results on the Mentschukin complexes with AsCl3 show that the metal-chlorine bond is virtually unaffected by the formation of molecular compounds. From singlecrystal NQR spectroscopy, particularly on the As nucleus, some geometrical information about the molecular compounds can be expected. 

VI. Study of the Crystal Field Effect by Continuous Variation of Crystal Field Parameters... 

At first sight, experiments in which the influence of external parameters on the NQR spectrum is continuously varied seem very promising in elucidating the character of the crystal field effect. 

The application of the plane wave basis set implies that the periodicity is taken into account and the method is therefore well suited for the simulation of the crystal field effects. For a recent review of Car-Parrinello methodology see Ref. [31]. With Car-Parrinello method the proton motion in PANO including the effects of crystal environment was simulated. The molecular dynamics simulation was carried out at a constant volume, i.e. the unit cell parameters were fixed during the simulation. Fictitious orbital mass was set to 150 a.u. (note that 1 a.u. corresponds to the electron mass) and the propagation time step was set to 2 a.u. 

K. Klier No time dependence of spectra was observed, the complexes formed being stable for weeks. Concerning the small shifts in frequencies of fully hydrated sieves, I refer to our work in /. Phys. Chem. Solids 1968, 29, 951, where similar shifts were observed with hydrated NiA and NiX sieves. From these results, it would seem that the hexa-hydrate Ni2+ ion in Type A sieve is in a positive pressure field whereas that in Type X is in a negative pressure field, expanding the complex and lowering effectively the crystal field splitting parameters. I should be interested to see your [Co2+ (HoO)c] spectra, which I have not followed closely, in order to learn how pronounced the differences are. 

The essential features of the ESR of Mn in RbN3 are similar to those discussed above under potassium azide. In addition, Owens [46] studied the effect of temperature and uniaxial stress on the crystal-field splitting parameter D for... 

In the following sections, we consider theoretical and experimental aspects of the influence of pressure on electronic energies. We emphasize the two major effects, covalency and crystal field strength, that are responsible for determining electronic energies in transition metal and lanthanide systems. For convenience of discussion, we choose to treat covalency and crystal field effects separately and to describe each effect with an independent set of theoretical parameters. We note, however, that the separation of covalency and crystal field effects is only approximate and that each effect influences the parameter values of the other effect to some extent in most systems. Our discussion will focus on experimental variations of covalency (B, C and F ) and crystal field strength (Dq, B q) parameters with pressure and the predictions of the theoretical models that have been proposed to explain the variations. 

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