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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

12 Rare Earth Elements and Materials

3+
FIGURE 5 (a) Schematic representation of the mechanism of RE luminescence (ISC, intersystem crossing).
(b) Matrix-assisted luminescence of RE ions (S, sensitizer; A, activator).
angular momentum give rise to magnetic “moments,” very large number of ways to realize antiferromagnetism
all rare earths except those mentioned have magnetic exist. Now the rare earths exhibit both kinds of basic mag-
properties. In fact, for the rare earths, the magnetic netic order and, in addition, several very complex forms
moment is proportional to the total angular momentum in which the moments assume a spiral, helical, or even a
J = |L ± S |, where the minus sign is taken for n = 1 to sine-wave pattern. Critical temperatures range from about
6 and the plus sign for n = 8 to 13. These values are also room temperature for Gd to near absolute zero.
listed above. A magnetic moment may be visualized as a Furthermore, a single element may show more than
vector of magnitude proportional to J. one type of order as a function of temperature. An at-
Recall our picture of the rare earth metals—a close tempt to summarize this very complex behavior is given in
packed array of trivalent ions (divalent for Eu), each with Table VIII.
a magnetic moment proportional to J, imbedded in a sea Some of these structures for the rare earths Gd, Tb,
of conduction electrons. As the 4 f electrons have only a Dy, Ho, Er, and Tm are pictured in Fig. 7. The origin
small radial extent, these moments are well localized on of these very complex structures is thought to arise from
the ions. It is the interaction between these moments that the combined effects of the RKKY mechanism and giant
gives rise to an ordered arrangement called “magnetic or- magnetic anisotropy. The latter is the tendency for the
dering.” For the rare earths these interactions occur via the moments to align along a particular direction in the crystal.
conduction electrons, the so-called RKKY (Ruderman– Magnetic anisotropy is largest for ions with high values of
Kittel–Kasuya–Yosida) interaction. Magnetic ordering L, the total orbital angular momentum, and the rare earths
occurs only below a certain temperature called a critical together with the actinides possess the largest L values of
temperature, T c . Basically, there are two simple types of any groups of elements.
magnetic order—ferromagnetism in which all of the mo-
ments align parallel to each other along some direction III. RARE EARTH MATERIALS
in the crystal and antiferromagnetism in which for each
magnetic moment pointing in some direction there is an-
The rare earths are very reactive metals as expected from
other moment exactly antiparallel to it. While there is only
their position in the periodic table. They combine readily
one way to achieve a truly ferromagnetic arrangement, a
with nearly all of the other elements to form a wide variety
of materials in both solid and solution states. In addition,
the synthesis of rare earth-containing organometallic com-
pounds has received considerable attention in the past 2
decades. The preparation and properties of these materials
are briefly described here.

A. Solid-State Materials

1. Oxides
FIGURE 6 Sensitization of rare earth luminescence by energy
transfer from an excited organic chromophore to a nearby rare The rare earth oxides of the general composition of RE 2 O 3
earth ion. are well-defined and stable solids usually obtained as the