Page 200 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 9

high temperatures (1400 C), resistance to corrosion ex- tahedrally coordinated by hydrogen. In the gas phase it
◦
cept by a very strong acid or base, a high tensile strength, apparently has some tendency to form a dimer analo-
and a remarkably silky texture. Composites made with gous to B 2 H 6 . The compound is a strong reducing agent,
metals reinforced by fibers gain in thoughness and benefit but it has a reactivity pattern quite different from that of
from the low density of the fibers. LiAlH 4 . It is, however, thermally unstable, is difficult to
A related compound, often called β-alumina, is actually prepare and maintain in pure form, and has not, in con-
sodium β-alumina (idealized as NaAl 11 O 17 ). The com- sequence, assumed a very large place in synthetic chem-
pound can be prepared by heating Na 2 CO 3 with any form istry. The important LiAlH 4 is prepared by direct reac-
of Al 2 O 3 . The structure consists of layers of tightly bound tion of sodium, hydrogen gas, and aluminum at a high
aluminum and oxygen in a spinel-like array alternating pressure and temperature and subsequent displacement of
+
+
+
with loosely packed layers containing the Na ions. This the Na by Li . The resulting ether-soluble compound
highly “defect” structure, has many more available sites has become a standard reagent in the organic chemist’s
than Na ions, with the consequence that the Na ions can laboratory for accomplishing many otherwise tedious or
+
+
be exchanged for many other univalent cations in suitable dangerous reduction reactions. There is substantial co-
+
molten salt media and, more important for the present, that valent interaction between the Li cation and the AlH 4 −
+
the Na ions are highly mobile under the influence of an anion in the crystal and in solution. The extent of aggrega-
electric field. This property has proved to be important in tion, and consequently the reactivity, varies with the con-
the continuing effort to develop high-energy density and centration and solvent. The reactions of organoaluminum
practical storage batteries. There remains substantial inter- compounds with amines give rise to a remarkably com-
est in the sodium sulfur cell in which sodium β-alumina plex series of cluster compounds containing Al N bonds
serves as the membrane to separate molten sodium and in the framework. There is as yet no clearly defined pat-
molten sulfur. The Na ions produced on discharge of tern to these structures, and this remains an active area of
+
the cell migrate through the solid electrolyte membrane to investigation.
neutralize the sulfide ions produced simultaneously and
complete the electrical circuit. The migration is reversed
E. Gallium
on recharging of the cell. The relatively low operating tem-
perature (300–350 C) and the very high energy storage per Gallium is far less abundant than its predecessor and for
◦
unit mass make this cell attractive for many applications. many years was notable mainly for having been predicted
A second crystalline modification of similar composi- byMendeleev.Asindicatedearlier,itissimilarinitschem-

tion, called sodium β -alumina, permits exchange for Na + istry to aluminum but is physically quite different. Its very
◦
by metal ions and supports the rapid migration of most low melting point (29.78 C) ensures that it is not of struc-
cations in the periodic table, including multivalent ones. tural value but that it is of value where low-melting metal
Because it can serve as a single crystal or powder host or alloys are desired. Remarkably, the liquid metal does
for multivalent cations, sodium β -alumina has a variety not boil until 2403 C, somewhat higher than the boiling
◦

of potential uses as a solid electrolyte and in optical and point aluminum, giving it the largest liquid range of any
electrical devices and has excited much current interest. substance known. Its major economic value at present de-
Related to the aluminas, especially the β-aluminas, are rives from its role in semiconductor technology. Several
tricalcium aluminate and its hydrated derivatives. These of its binary compounds with nitrogen family elements
are major components of Portland cement and of high- [the Group 13–15 (IIIA–VA) semiconductors] are of sig-
alumina cement. A structural modification and the heat nificant commercial value by virtue of their possessing
released on hydration are factors in the “setting” process appropriate band gaps for various applications. For ex-
of these cements. ample, GaAs x P 1−x is used in light-emitting diode display
devices. The Group 13–15 (IIIA–VA) compounds can be
prepared directly from the elements at high temperatures
D. Aluminum Hydrides
or alternatively by means of the reactions of the hydrides
Aluminum hydride, AlH 3 , seems to be the unique bi- of the elements, for example, that of gallium with NH 3 or
nary hydride of aluminum. This is indeed a dramatic con- Me 3 GawithEH 3 (E = As,P,Sb).MostoftheGroup13–15
trast to the extensive hydride chemistry of boron. In fact, (IIIA–VA) compounds are subject to chemical attack,
this example is remarkably recent as a well-defined com- which limits their utility to encapsulated forms. Methods
pound because solvents useful in its preparation tend to for producing crystalline samples of semiconductor grade
remain bound in the crystals. The hydride can form at are still under development and are largely proprietary.
least six crystalline modifications. The most stable form, In general, gallium forms compounds analogous
the α form, has a saltlike structure, with aluminum oc- to those of aluminum, though there are significant