Russell J. Hemley and Ho-kwang Mao

2005 Balzan Prize
been shown to exhibit unexpected properties at very high pressures, such as an
unusual infrared response indicative of the evolution of a van der Waals to a
charge transfer (or ionic) state.
The discovery of dense clathrates or filled ices, first observed in hydrogen-water
mixtures [Phys. Rev. Lett. 71, 3150 (1993)] has opened up new lines of research.
These novel structures may host trapped gases within the moons of the solar system
(e.g., in the CH 4-H 2O system) [Properties of Earth and Planetary Materials at
High Pressure and Temperature, 1998, p. 173]. 58 Subsequent studies have shown
that hydrogen remains trapped in ice below ambient pressure [Science 297, 2247
(2002)]. Moreover, the infrared spectra matches what is found in certain interstellar
clouds, suggesting a mode of hydrogen incorporation in growing planetary
bodies. Together with high-pressure van der Waals compounds, these dense molecular
materials may be abundant in the molecular layers in extraterrestrial bodies,
including surfaces and interiors of moons in the outer solar system – pointing
to a new type of extraterrestrial mineralogy. New high-pressure phases in gas hydrates
discovered in the methane-water system at even modest pressures of a few
kilobars suggest that much of the methane clathrate buried deep in sediments in
the Earth may exist in a different crystal structure than what has been observed
on recovery [Proc. Nat. Acad. Sci. 97, 13484 (2000)]. Studies of these systems provide
an opportunity to investigate the evolution of bonding from purely van der
Waals to ionic-covalent on compression and provide an entirely new window on
the nature of the chemical bond [Ann. Rev. Phys. Chem. 51, 763 (2000)].
Very recently, these materials have become extremely interesting hydrogen storage
materials because of their high hydrogen content and “green” chemistry (simply
hydrogen gas and water). The low-pressure H 2-H 2O clathrate contains 5.4%
hydrogen and therefore meets the U.S. Department of Energy 2005 milestone
for a useful hydrogen storage material. The material has been patented and is being
pursued by industrial and other research groups. 59 Moreover, the (H 2) 4(CH 4)
phase has an astonishingly high hydrogen content of 33.4%, the highest of any
known hydrogen containing material, and it can be stabilized near ambient pressure
at low temperature [Chem. Phys. Lett. 402, 66 (2005)]. Like the polymerization
studies of molecular materials, these discoveries are examples of the societal
benefits of fundamental research in high-pressure mineral physics [Rev.
Mineral. Geochem. 41, 335 (2001)].
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