|
Using
computer simulations, a RUB researcher has shown that the oxygen
molecule (O2) is stable up to pressures of 1.9 terapascal, which is
about nineteen million times higher than atmosphere pressure. Above
that, it polymerizes, i.e. builds larger molecules or structures. “This
is very surprising” says Dr. Jian Sun from the Department of Theoretical
Chemistry. “Other simple molecules like nitrogen or hydrogen do not
survive such high pressures.” In cooperation with colleagues from
University College London, the University of Cambridge, and the National
Research Council of Canada, the researcher also reports that the
behaviour of oxygen with increasing pressure is very complicated. It’s
electrical conductivity first increases, then decreases, and finally
increases again. The results are published in Physical Review Letters.
Weaker bonds, greater stability
The
oxygen atoms in the O2 molecule are held together by a double covalent
bond. Nitrogen (N2), on the other hand, possesses a triple bond. “You
would think that the weaker double bond is easier to break than the
triple bond and that oxygen would therefore polymerize at lower
pressures than nitrogen” says Sun. “We found the opposite, which is
astonishing at first sight.”
Coming together when pressure increases
However,
in the condensed phase when pressure increases, the molecules become
closer to each other. The research team suggests that, under these
conditions, the electron lone pairs on different molecules repel one
another strongly, thus hindering the molecules from approaching each
other. Since oxygen has more lone pairs than nitrogen, the repulsive
force between these molecules is stronger, which makes polymerization
more difficult. However, the number of lone pairs cannot be the only
determinant of the polymerization pressure. “We believe that it is a
combination of the number of lone pairs and the strength of the bonds
between the atoms”, says Sun.
The many structures of oxygen
At
high pressures, gaseous molecules such as hydrogen, carbon monoxide, or
nitrogen polymerize into chains, layers, or framework structures. At
the same time they usually change from insulators to metals, i.e. they
become more conductive with increasing pressure. The research team,
however, showed that things are more complicated with oxygen. Under
standard conditions, the molecule has insulating properties. If the
pressure increases, oxygen metallises and becomes a superconductor. With
further pressure increase, its structure changes into a polymer and it
becomes semi-conducting. If the pressure rises even more, oxygen once
more assumes metallic properties, meaning that the conductivity goes up
again. The metallic polymer structure finally changes into a metallic
layered structure.
Inside planets
“The
polymerization of small molecules under high pressure has attracted
much attention because it helps to understand the fundamental physics
and chemistry of geological and planetary processes” explains Sun. “For
instance, the pressure at the centre of Jupiter is estimated to be about
seven terapascal. It was also found that polymerized molecules, like N2
and CO2, have intriguing properties, such as high energy densities and
super-hardness.” Dr. Jian Sun joined the RUB-research group of Prof. Dr.
Dominik Marx as a Humboldt Research Fellow in 2008 to work on
vibrational spectroscopy of aqueous solutions. In parallel to this joint
work in “Solvation Science” he developed independent research interests
into high pressure chemical physics as an Early Career Researcher.
Persistence and eventual demise of oxygen molecules at terapascal pressures
