Intermediate crystal field

Free- ion terms Weak crystal field Intermediate crystal field Strong- field terms Strong-field configurations... 

The magnetic properties of Pu compounds in different oxidation states are reviewed. New measurements on Pu(C8H8)2, PuFi, [(C2Hs)itN]2PuCl6, and [ (C2H5)itN]itPu(NCS)s are presented. The interpretation of the data is based on intermediate, j-j mixed crystal field states and orbital reduction due to covalency. Especially in the case of the organometallic compounds a large orbital reduction is found. 

Mixing of LS-states by spin orbit coupling will be stronger with an increasing number of f-electrons. As a consequence, intermediate values of Lande g factor and reduced crystal field matrix elements must be used. 

Due to the intermediate coupling the sign of the crystal field matrix element 6 is reversed compared to the pure Russell-Saunders state. Thus for 8-fold cubic coordination a F7 ground state was found. From EPR measurements on Pu3"1" diluted in fluorite host lattices, a magnetic moment at T=0 K can be calculated, ranging from li ff = 1.333 (in Ce02) to y ff = 0.942 (in SrCl2) (24,... 

As seen in Fig. 8 the experimentally determined magnetic moments at room temperature are in general much lower than the free ion values. To extract the contribution of orbital reduction, the influence of intermediate coupling, crystal field effects and j-j mixing must be considered. 

The electrostatic and spin-orbit parameters for Pu + which we have deduced are similar to those proposed by Conway some years ago (32). However, inclusion of the crystal-field interaction in the computation of the energy level structure, which was not done earlier, significantly modifies previous predictions. As an approximation, we have chosen to use the crystal-field parameters derived for CS2UCI6 (33), Table VII, which together with the free-ion parameters lead to the prediction of a distinct group of levels near 1100 cm-. Of course a weaker field would lead to crystal-field levels intermediate between 0 and 1000 cm-1. Similar model calculations have been indicated in Fig. 8 for Nplt+, Pu1 "1 and Amlt+ compared to the solution spectra of the ions. For Am t+ the reference is Am4" in 15 M NHhF solution (34). 

The method mentioned has been appUed to ClFe(Et2dfc)2 first, because for this compound, and some related ones, an abnormally large QS has been observed, while the crystal field theory predicts QS = 0 for this intermediate spin (S = 3/2) compound. 

Another typical example for anisotropic covalency is found in five-coordinate ferric compounds with intermediate spin S = 3/2 (also discussed in Sect. 8.2). Crystal field theory predicts a vanishing valence contribution to the EFG, whereas large quadrupole splittings up to more than 4 mm s are experimentally found. 

In a crystal-field picture, the electronic structure of iron in the five-coordinate compounds is usually best represented by a (d yf idyz, 4cz) ( zO configuration [66, 70], as convincingly borne out by spin-unrestricted DFT calculations on the Jager compound 20 [68]. The intermediate spin configuration with an empty d 2 yi orbital in the CF model, however, has a vanishing valence contribution to the... 

Figure 9.6 Electron configurations possible for Co3+ (3d6) cations in an octahedral crystal field (a) low spin (LS), (b) intermediate spin (IS), and (c) high spin (HS).

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

If the atom or ion is situated in an environment of different atoms or ions, as, for instance, ions in a soUd, the surrounding hgands exert on it a further interaction, which is called the crystal field interaction Hcp and enters Eq. (9). One has to compare Hcf with Hi and H2, in order to decide whether it is or not a small perturbative term. In actinide solids, it is usually found that Hcf is of the same order of magnitude as Hi and H2, so that intermediate coupling schemes are necessary which include Hcf as well. (For a more exhaustive treatment of couplings in actinides, see Chap. D.)... 

Also, rearrangement in many cases would be difficult because of the crystal field stabilization effects. But it could be possible that somewhere, when this bond had lengthened sufficiently, this group X could slip in and as Dr. Langford indicated, we would then have an intermediate, or transition state, or something which would have these two groups partially bonded. 

Table 7.1. Splitting of the states of angular momentum L in an intermediate crystal field.

Example 8.2-1 Examine the effect of spin-orbit coupling on the states that result from an intermediate field of O symmetry on the Russell-Saunders term 4F. Correlate these states with those produced by the effect of a weak crystal field of the same symmetry on the components produced by spin-orbit coupling on the 4F multiplet. 

The correlation diagram that correlates the intermediate- (or medium-) field states with the weak-field states is shown in Figure 8.1. The same states must arise independently of the order in which the crystal-field and spin-orbit coupling perturbations are applied. The numbers in parentheses are the degeneracies of the states they provide a useftd check on the accuracy of numerical calculations. 

The splitting of atomic energy levels in a crystal field (CF) with the symmetry of one of the magnetic point groups has been considered in detail by Cracknell (1968). Consider an atomic 2P level (L = 1) in an intermediate field of 2mm or C2v symmetry and assume that Hs. L < Hcf. The degenerate 2P level is split into three components, T i T3 T4. But in a field of 4mm symmetry the two levels T3 and F4 stick together, that is, are degenerate, since 14 = r3 r4 (case (b)), while 14 is re-labeled as 14 (case (a)). (See Table 14.4 and... 

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