Page 19 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19

Actinide Elements 227

successful for the separation of trivalent lanthanide and ac- In condensed phases, spectra are commonly measured
tinide ions. A group separation of trivalent actinides and in absorption. Three main types of transitions are observed
lanthanides may be accomplished also by anion exchange. in the absorption spectra of the actinide ions: (1) Laparte-
The trivalent actinide ions form much more stable chlo- forbidden f to f transitions, (2) orbitally allowed 5 f to
ride complexes than do the trivalent lanthanide ions. They 6d transitions, and (3) metal to ligand charge transfer.
are therefore sorbed on anion-exchange resins from con- Of these, study of internal f to f transitions has found
centrated hydrochloric acid, while the lanthanides are not. wide use in the investigation of actinide chemistry. These
band usually in the visible and ultraviolet regions, can
be easily identified because of their sharpness, and are
C. Magnetic Properties sensitive to the metal environment. As discussed earlier,
The actinides exhibit nearly all of the types of magnetism the 5 f orbitals of the actinide elements are more exposed
found in transition and lanthanide metals. Thorium be- than the lanthanide 4 f orbitals, and therefore, crystal field
haves like a 6d transition metal. The magnetic suscepti- effects are larger in the 5 f series. The f to f transitions
bility is large, and the temperature dependence is low. The for actinide elements may be up to 10 times more intense
actinide metals protactinium to plutonium do not have or- and twice as broad as those observed for the lanthanides,
dered ground state moments. Hybridization of 5 f and 6d due to the action of crystal fields. In addition, extra lines
levels broadens the f levels and suppresses the formation resulting from vibronic states coupled to f → f states
of localized moments. The temperature-independent para- have been observed.
magnetic susceptibilities indicate an itinerant character of The 5 f to 6d bands are orbitally allowed and therefore
the 5 f electrons. From americium on the 5 f electrons be- more intense than those of the f to f transitions. They
come localized and the heavy metals are localized mag- are also usually broader and often observed in the ultra-
nets, similar to the lanthanide metals. For americium, the violet region. The metal to ligand charge-transfer bands
are also fully allowed transitions that are broad and oc-
susceptibility is large with little temperature dependence.
cur commonly in the ultraviolet region. When these bands
Curium has an antiferromagnetic transition at 65 K, but the
trail into the visible region, they produce the intense colors
face-centeredcubicphaseshowsaferrimagnetictransition
associated with many of the actinide compounds. Metal-
near 200 K. Berkelium metal exhibits high-temperature
ligand frequencies are also observed in the infrared and
magnetic behavior like its lanthanide homolog terbium.
Raman spectra of actinide compounds.
Californium metal exhibits either ferro- or ferrimagnetic
behavior below 51 K and paramagnetic behavior above Actinide spectra are used in different ways. First, be-
160 K. cause of their characteristic properties, actinide spectra
Actinide compounds and ions exhibit very different can be used for the direct speciation of (complexed) ac-
magnetic behavior arising from the spin and orbital an- tinide ions, the observation and quantification of reac-
gular moments of the unpaired electrons. Spin-orbit cou- tions taking place in solution, or the identification of com-
pling is about twice that for the lanthanides, and the crystal pounds. On the other hand, actinide spectra can be used
field strengths for the actinides are an order of magnitude to study electronic and physicochemical properties, in-
greater. There is a wealth of information about the mag- cluding information on symmetry, coordination number,
netic properties of various actinide materials which has or stability constants.
been reviewed elsewhere. Conventional optical absorption spectrometry has de-
tection limits of between 0.01 and 1 mM for the actinides.
Highly sensitive spectroscopic methods have been devel-
oped, based on powerful laser light sources. Time resolved
D. Spectroscopic Properties
laser fluorescence spectroscopy (TRLFS), based on the
Actinide spectra reflect the characteristic features of the combined measurement of relaxation time and fluores-
5 f orbitals which can be considered as both containing the cence wavelength, is capable of speciating Cm(III) down
opticallyactiveelectronsandbelongingtothecoreoffilled to 10 −12 mol/L but is restricted to fluorescent species like
shells. The electronic transition spectra of actinide ions in U(VI) and Cm(III). Spectroscopic methods based on the
solution are dominated by the structure of the f levels and detection of nonradiative relaxation are the laser-induced
transitions within the f shell. Free-atom spectra provide photoacoustic spectroscopy (LPAS) and the laser-induced
more information about the interactions between the 5 f thermal lensing spectroscopy (LTLS). Like conventional
and the valence electrons. The emission spectra of the free absorption spectroscopic methods, these newly devel-
actinide atoms have an enormous number of lines. In the oped methods are capable of characterizing oxidation
uranium spectrum, about 100,000 lines have been mea- and complexation states of actinide ions but with higher
sured, from which about 2500 lines have been assigned. sensitivity.