Systematics lanthanide contraction

Crystal structure data obtained by X-ray diffraction methods for the actinide element halides are collected in Table IV. Crystal structure determinations have been most important in identifying new compounds of the actinide elements the data are sufficiently extensive now for use in drawing conclusions regarding systematic trends and relations among the actinide elements. The tetrafluorides, for instance, supply one of the best illustrations of an actinide contraction that is entirely similar to the well-known lanthanide contraction (Table V). 

The tripositive lanthanide ions constitute the longest range of chemically similar metal ions and in consequence provide an opportunity to study mechanistic variations as their ionic radii systematically decrease with the lanthanide contraction. However, in water in particular, the absence of direct determinations of the number of solvent molecules in the first coordination sphere of these ions has... 

Today the lanthanide eontraction is still one of the most important tools available to the scientist in applying systematics to the behavior of lanthanide materials. Deviations from the lanthanide contraction established for a given compound series gives a measure of anomalous valences for cerium, samarium, europiun, thulium and ytterbium (see section 3.2) which are important in evaluating the nature of these elements in valence fluctuation, heavy fermion, and spin fluctuation behaviors (see section 4.4.4). 

The lanthanide contraction allows for a controlled, systematic study of the inclusion of lanthanide cations... 

Lanthanide contraction Systematic decrease in atomic and ionic size that occurs from lanthanum (La) to lutetium (Lu). The difference in size between adjacent elements is only about 1 % but the cumulative effect is about a 20% reduction. 

Thus the rather easily obtained atomic sizes are the best indicator of what the f-electrons are doing. It has been noted that for all metallic compounds in the literature where an f-band is believed not to occur, that the lanthanide and actinide lattice parameters appear to be identical within experimental error (12). This actually raises the question as to why the lanthanide and actinide contractions (no f-bands) for the pure elements are different. Analogies to the compounds and to the identical sizes of the 4d- and 5d- electron metals would suggest otherwise. The useful point here is that since the 4f- and 5f-compounds have the same lattice parameters when f-bands are not present, it simplifies following the systematics and clearly demonstrates that actinides are worthy of that name. 

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