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

2 Rare Earth Elements and Materials

all of the rare earths, their separation and recovery in physical properties follow more or less directly from the
pure form have been extremely difficult. It was not un- details of the atomic structure, also called their electronic
til the 1940s that many of the properties of the individ- configuration, which will be discussed in Section II.
ual rare earths and the materials containing these ele-
ments were investigated. The basic studies have greatly A. Origin of the Names of the Elements
accelerated the pace at which rare earth materials are uti-
As will be clear from Section I.B, the general appellation
lized in both fundamental scientific research and modern
fortheseelementsas“rareearths”isinappropriate.Several
technology.
decades of usage, however, make it unlikely that this term
will soon disappear from the vocabulary of the working
I. THE RARE EARTH ELEMENTS scientist or engineer. It arose, presumably, because many
of these elements were first discovered as components of
Rare earth elements are a series of chemical ele- certain minerals that are indeed rare.
ments of the periodic table, including the elements with Table I gives the names of the rare earth elements to-
atomic numbers 57 through 71, and, named in order, gether with other pertinent information regarding their ori-
are lanthanum (La), cerium (Ce), praseodymium (Pr), gins. The identity of the discoverer is included as it was
neodymium (Nd), promethium (Pm), samarium (Sm), eu- he, often, who enjoyed the privilege of naming the new
ropium (Eu), gadolinium (Gd), terbium (Tb), dysprosium element.
(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytter- Note that many of the rare earths were first discovered in
bium (Yb), and lutetium (Lu). Yttrium (Y, atomic no. 39) Sweden. In fact most were isolated from mineral samples
and scandium (Sc, atomic no. 21) are sometimes included mined from a quarry near Ytterby, a small village that has
in the group of rare earth elements. The elements cerium given various parts of its name to no fewer than four ele-
(Ce, atomic no. 58) through lutetium (Lu, atomic num- ments: yttrium, ytterbium, terbium, and erbium. Although
ber 71) are commonly known as the lanthanide series. As their initial discovery was a Scandanavian event, the truly
will be clearer later, there are ample reasons to include Y formidable task of separating, isolating, and identifying
as a rare earth, but the inclusion of Sc is less common and this group of 16 incredibly similar elements was an inter-
more controversial. national affair drawing equally on the talents of the French
In the usual short form of the periodic table, the lan- spectroscopists, Demarcay, Lecoq de Boisboudran, and
thanides are segregated (along with the actinides, num- Urbain, the Austrian, von Welsbach, and many others.
bers 89 to 103) from the other elements giving the cor- In one sense Table I, while containing interesting his-
rect impression that they comprise a very closely related torical facts, is somewhat misleading in an important way.
group with unique characteristics, but also the false im- It implies that most of the rare earths have been known
pression that their properties are somehow unrelated or since the late 19th and early 20th centuries (except for the
unconnected to the remaining elements. In the long form “artificial” element prometheum). While this is true in a
of the periodic table (shown in Fig. 1) it is clearly seen that limited sense, samples of the rare earths of sufficient purity
the rare earths form a bridge between the very reactive for modern chemical and physical studies were only pre-
metallic elements Cs and Ba and the transition element pared in the 1950s. Widespread commercial availability
series beginning with hafnium (Hf ), tantalum (Ta), and of these elements has come about even more recently. In a
tungsten (W). Of course, the position of the rare earths in true sense, we are yet in an early stage of basic study and
the periodic table of the elements and their chemical and technological exploitation of this group of elements.

FIGURE 1 The “long form” of the periodic table of the elements. The rare earths are elements numbers 21 (Sc), 39
(Y), and 57 (La) to 71 (Lu) inclusive. This version emphasizes that the rare earths are a bridge between the reactive
“alkaline-earth” elements Ca (20), Sr (38), and Ba (56) and the “transition metals” Ti (22), Zr (40), and Hf (72).