Low-symmetry sites

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. 

When high-spin Fe2+ is present in a distorted (low-symmetry) site, however,... 

Further adsorption of Na on Al(llO) leads to the formation of a poorly-ordered (3 X 1)—2Na phase at a coverage of 2/3 ML. This phase co-exists with the c(2 X 2)—Na phase. However a well-ordered (4 x 1)—3Na phase is formed after adsorption of 3/4 ML. A LEED analysis indicates that the unit cell of this phase contains one substitutionally-adsorbed Na atom and two Na atoms adsorbed in low-symmetry sites, displaced by 0.33 A from two four-fold coordinated nearest-neighbour sites [75]. A hard-sphere model of the structure is shown in Fig. 23. It is interesting to note that the c(2 x 2)—Na phase, as shown... 

Experiments on these synthetic tri-hydroxamic acids have shown the rhombic symmetry effects in the Mossbauer data (Fig. 12). Other experiments using Mossbauer spectroscopy on low symmetry sites have been published by Lang et al. (33) in EDTA and by Zabinski et al. 

It is clear that further work is necessary in the development of a reliable general method of deriving good first approximations to crystal-field parameters for lanthanides in the low-symmetry sites. 

The crystal field interaction is probably relatively high in RNiCj compounds because the rare earth ions occupy a low-symmetry site (C2v) (Gignoux et al. 1982). This may cause the deviations from Curie-Weiss behavior observed for this series of compounds. 

For a system with octahedral or tetrahedral symmetry, saturation data collected at different temperatures superimpose for all spin systems. For low-symmetry sites (e.g., protein-active sites) only isotherms for 5=1/2 systems superimpose in general. The spread or nesting behavior observed in VTVH MCD for systems with Sf j2 is ascribed to the zero-field splitting (ZFS) of the Ms sublevels of the ground state, and is discussed in the next sections, first for Kramers-type systems (half-integer spin) and then for non-Kramers ions (integer spin), as exemplified by Fe . 

If the symmetry is high, more transitions are forbidden by symmetry restrictions than for systems with a low site symmetry. Thus, in a spectrum of a trivalent lanthanide ion in a low symmetry site more peaks within a spin-orbit coupling band will be found than in the spectrum of the same lanthanide ion occupying a site of high symmetry. For the symmetries Cs, Ca and C] no transitions are forbidden and 27+1 peaks are found in an even electron system for transitions between a crystal-field level of the ground state and the crystal-field levels of an excited multiplet. 

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