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NEWS
December 2024 ESRFnews
EBS brilliance reveals fate of
‘over-compressed’ water
cubic-bonded ice VII, which is stable
over a vast pressure range from 2 to
80 gigapascals, equivalent to those
present on icy planets and moons.
The gateway to ice VII may be higher
pressures, but the speed of compression
is critical. Take it slowly, and normal
water freezes at about one gigapascal
into ice VI, a tetrahedral phase, before
forming ice VII at about 2 gigapascals.
Go faster, though, and the freezing is
waylaid, occurring at higher and higher
pressures. Until now, no-one has been
sure what water ultimately freezes into
when it is compressed very quickly.
The answer is important, because the
freezing of water on other planets and
moons could have taken place when it
was over-compressed during planetary
impact.
Charles Pépin, Paul Loubeyre and
colleagues at the CEA Laboratory for
Materials at Extreme Conditions at
the Université ParisSaclay in France
together with scientists at the ESRF
and the Paul Scherrer Institute PSI
in Switzerland have finally solved the
mystery using a range of cuttingedge
instrumentation for timeresolved
Xray diffraction One part of the
toolkit was a special dynamicpiezo
diamond anvil cell dDAC designed
by the CEA team to compress water
in a wellcontrolled manner Another
was the latest Jungfrau detector
the result of a joint PSIESRF
collaboration – which can record an
X-ray image every few microseconds.
Most important, however, was
the extremely high brilliance of
X-rays streaming through the ID09
beamline, provided by the EBS.
“It was the coupling of our d-DAC
with the ESRF instrumentation at
ID09, plus the use of the Jungfrau
detector, that made this experiment
possible and successful,” says Loubeyre.
The results were clear: over-
compressed water freezes into ice VII
– not amorphous ice or even a novel
solid phase, as other theorists had
speculated. Loubeyre and colleagues
could confirm this outcome for
compression rates spanning over six
orders of magnitude, by including
their own data with the results of
previous nanosecond dynamic-
compression studies. They could also
verify a theoretical model for the onset
of freezing which will help scientists
to understand the composition of icy
planets and moons Nat Commun
15 8239
Our next steps are to investigate
the mechanisms of solid phase
transitions and to further explore
microsecond chemistry under high
pressures which could be a path
to synthesise novel materials says
Loubeyre We want to perform a
similar freezing measurement on the
liquid metal gallium
ESRF users have exploited the highly
brilliant X-rays of the EBS to confirm
that water freezes into a particular
“cubic” form of ice when it is
compressed very quickly. The results
clear up a long-standing mystery
in high-pressure physics, and will
provide insights into the composition
of the solar system’s icy moons.
Water is so familiar to us that the
ancients considered it one of the four
basic elements. To modern physicists,
however, it is a marvel – a liquid that,
unlike almost all others, becomes not
easier but harder to solidify at high
pressures and, when it does solidify,
expands rather than contracts. The
behaviour results from the way the
constituent hydrogen atoms bond
with one another, and is vital for
life. Without it, lakes and seas would
freeze from the bottom up, killing
everything inside
In fact the freezing of water is even
more complicated than this Under
various pressures and temperatures
water is known to form at least 19
distinct phases of ice The one we
know well on Earth has its oxygen and
hydrogen atoms in hexagonal rings
On the other hand the most common
phase in the universe is likely to be
a type of lowdensity amorphous
ice without any longrange crystal
structure at all Another very common
phase with big scientific interest is the
“It was the
coupling of our
dDAC with
the ESRF
instrumentation
at ID09 plus
the use of the
Jungfrau
detector that
made this
experiment
possible and
successful
E S R F/ S T E F C A N D É