Page 390 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GQT/GRI P2: GTV/FFV P3: GTV/FFV QC: GSS Final pages
Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

14 Rare Earth Elements and Materials

−
−
3+
formulated as RE (H ) 2 (e ) with electrons delocalized exist have RE = Sm, Eu, and Yb. The most extensive se-
in a metallic conduction band, accounting for their high ries of REX 2 compounds occurs for X = I which numbers
conductivity for electricity. The trihydrides may be 11 at present. Only Y, Tb, Ho, and Er do not exist. The
prepared at higher H 2 pressure, but these species are poor compounds formed when RE = La, Ce, Pr, and Gd all
conductors for electricity, favoring the formation of RE 3+ have high electrical conductivities and must be regarded
−
and 3H . as trivalent as with the hydrides.
In addition to the above “normal” valence materi-
als, recent research has uncovered a number of mixed
3. Intermetallics
valence and apparent subvalent halides. For example, dis-
The rare earths form a large number of intermetallic com- creet compounds with halogen to rare earth ratios of 2:200,
pounds with transition elements. Of these intermetallic 2:167,and2:140havebeenobservedforchloridesandbro-
compounds, some most intensively studied are those with mides of Dy, Yb, Ho, and Sm. These are really mixed va-
the elements Fe, Co, and Ni. A list of the typical RE–Ni lence 3+, 2+ compounds with formulas such as Dy 5 Cl 11 ,
materials gives an idea of the variety involved: RENi, Yb 6 Cl 13 , and Sm 11 Br 24 . The complex structures of these
RE 2 Ni 3 , RENi 2 , RENi 3 , RE 2 Ni 7 , RENi 5 , and RE 2 Ni 17 . materials have been the subject of considerable research.
The rare earth–transition metal intermetallics are to be Perhaps even more remarkable are the so-called subva-
distinguished from alloys that normally form between two lent materials Gd 2 Cl 3 and GdCl. Formally, the rare earth
or more metallic elements of similar size and crystal struc- valences in these compounds are less than 2+. In fact both
3+
ture and can be regarded as “solid solutions.” The rare compounds probably contain Gd . In the Gd 2 Cl 3 mate-
earths are generally larger than most of the metallic ele- rial the structural unit is a six-atom cluster of Gd atoms
ments so they tend to form compounds rather than al- in the shape of a regular octahedron which share edges
loys in combination. Many of these species form ternary to form chains. The GdCl structure can be regarded as a
hydrides: condensation of the Gd 2 Cl 3 octahedra to form layers of
these octahedral units. Both are shown in Figs. 8 and 9.
RE–M + Hydrogen ⇔ RE–M–Hydride.
The GdCl compound has the same structure as ZrCl, indi-
The crystalline structures containing atoms of disparate cating the kinship of the rare earths with their neighbors
sizes offer a wide variety of interstitial sites, all of which to the right in the periodic table.
can accommodate the small H atom. This inherent variety,
along with a range of interatomic metal–metal contacts, al- 5. Chalcogenides (Sulfides, Selenides,
lowsforaninherentflexibilitywithinthecrystallatticeand and Tellurides)
a low activation energy for hydrogen absorption. Of the
many RE–transition metal intermetallics, LaNi 5 has been These are the elements related to oxygen in the periodic
found to absorb and desorb large amounts of hydrogen gas table and the rare earths form compounds of composition
more or less reversibly under very mild conditions. The similar to those of the oxides which can be illustrated
hydrogen content corresponds to LaNi 5 H 6 and there has by examples from the sulfides, RES 2 ,RE 2 S 3 ,RE 3 S 4 , and
been intensive research into this compound as a medium RES. Most similarities end with composition as these ma-
for hydrogen storage. terials are very different from the oxides. RES 2 , for ex-
ample, does not involve the 4+ valence as part of the
sulfur is present as S 2− (disulfide) and part as S . One
2−
4. Halides (Fluorides, Chlorides, 2
Bromides, and Iodides)
The behavior of the rare earths with the halogen elements
fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) is
somewhat similar to that with hydrogen. The rare earths
lose their valence electrons, donating them to the halo-
− −
gens to form halide ions F (fluoride), Cl (chloride),
−
−
Br (bromide), and I (iodide). If we represent a “halide”
ion as X , compounds of composition REX 4 , REX 3 , and
−
REX 2 form readily and have been known for some time.
The REX 4 compounds are least common, existing only FIGURE 8 A view of the atomic arrangement (crystal structure) of
GdCl, ScCl, etc. The large circles are Gd and these atoms form a
for X = F and RE = Ce, Pr, and Tb, those commonly ex-
metal–metal bonded double layer sandwiched between Cl atoms
hibiting a 4+ valence. Not surprisingly, REX 3 compounds (smaller circles). [From Corbett, J. D. (1981). Acct. Chem. Res.
exist for all RE and all X. For X = F, the only REX 2 that 14, 244.]