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High-Pressure Synthesis (Chemistry) 377

pressure is still somewhat above the maximum reached in VI. JOURNEY TO THE CENTER
the diamond anvil to date (1985), about 270 GPa. Metallic OF THE EARTH
hydrogen has also been suggested as a room-temperature
superconductor, and undoubtedly would be a tremendous On the basis of seismological data from earthquakes,
propellant if it could be brought back alive, although Earth’s interior, in broad terms, consists of a crust, a man-
this is considered to be unlikely. The best evidence to tle, and a core. The crust and mantle are primarily oxides
date for the existence of metallic hydrogen is from shock and the core is primarily metallic. The details of this struc-
experiments. ture are continually being updated and redefined as tech-
Another candidate for a useful material from very niques become more sophisticated. Although Earth’s inte-
high pressure synthesis is the gem material, jadeite rior is essentially inaccessible by direct observations (the
(NaAlSi 2 O 6 ). The natural material of “Imperial” quality deepest well is only 12 km), there has been considerable
can cost as much as $2000 per carat. Jadeite can be syn- help in modeling the interior from high-pressure research,
thesized at about 30 kb and above in equipment similar principally by studying properties and phase transforma-
to that used for diamond growth, and it has been made tions of the known and surmised oxide minerals. Standard
into pieces of jewelry. Since jadeite is used as a poly- static apparatus plus the diamond anvil have been used
crystalline aggregate, synthesis is essentially hot pressing to establish the P and T conditions for stability of pos-
and sintering, much simpler than if single crystals were sible phases and to measure their densities and establish
needed. However, it does not appear to be a commercial crystal structures and seismic properties under pressure.
product in competition with the natural supply. Figure 12 shows the extent to which the interior of Earth
If one considers the “low” pressure range around 0.1– has been probed in the laboratory by high-pressure and
0.2 GPa, there are two economically viable syntheses: that high-temperature studies.
of the quartz form of SiO 2 and the magnetic tape material,
CrO 2 . The latter is made from the decomposition of CrO 3
in a steel vessel where the oxygen pressure is maintained at
about 0.03 GPa and the temperature is around 400 C. This
◦
pressure is necessary in order to maintain the Cr 4+ state.
The acicular grains that form are used for high-resolution
tape recording.
The quartz form of SiO 2 is a very important piezoelec-
tric material used for transducers and frequency control
devices. This industry used to be dependent on small,
often unreliable sources from sometimes unstable polit-
ical environments (e.g., Brazil) for optical-grade natural
quartz, and the useful yield from this material was vari-
able. Now many large crystals are grown in plants all
over the world on seeds in large pressure vessels from
the system Na 2 O–SiO 2 –H 2 O with a temperature gradi-
ent under much the same conditions nature is thought
to have used: about 1 kbar and 500 C. This combina-
◦
tion produces a well-controlled useful product compatible
with production methods and essentially frees the indus-
try from dependence on natural material. Natural mica,
topaz, asbestos, and other OH-containing minerals also
grew under similar hydrothermal conditions in nature’s
pressure vessels, such as pegmatite dikes. This part of the
universe, including the genesis of ore bodies in Earth’s
crust, is a continuing area of moderately high-pressure in-
vestigations. There has also been considerable success in
growth of a variety of crystals other than quartz using the
hydrothermal method, but in general a simpler alternative
FIGURE 12 Pressure versus depth in Earth. Crust, mantle, and
method is sought even in the 0.1- to 0.2-GPa range (e.g.,
core boundaries and densities are indicated along with pres-
emerald can be grown hydrothermally, but solution growth sures attainable with the diamond anvil and large-volume static
at high temperatures at 1 atm is preferred). apparatus.