|dc.description.abstract||The ability to diagnose the structure of a material at extreme conditions
of high-pressure and high-temperature is fundamental to understanding its
behaviour, especially since it was found that materials will adopt complex crystal
structures at pressures in the Terapascal regime (1TPa). Static compression,
using the diamond anvil cell coupled with synchrotron radiation has to date
been the primary method for structural studies of materials at high pressure.
However, dynamic compression is the only method capable of reaching pressures
comparable to the conditions found in the interior of newly discovered exo-planets
and gas giants where such exotic high-pressure behaviour is predicted to be
commonplace among materials.
While generating extreme conditions using shock compression has become a
mature science, it has proved a considerable experimental challenge to directly
observe and study such phase transformations that have been observed using
static studies due to the lack of sufficiently bright X-ray sources.
However, the commissioning of new 4th generation light sources known as free
electron lasers now provide stable, ultrafast pulses of X-rays of unprecedented
brightness allowing in situ structural studies of shock compressed materials and
their phase transformation kinetics in unprecedented detail.
Bismuth, with its highly complex phase diagram at modest pressures and
temperatures, has been one of the most studied systems using both static and
dynamic compression. Despite this, there has been no structural characterisation
of the phases observed on shock compression and it is therefore the ideal candidate
for the first structural studies using X-ray radiation from a free electron laser.
Here, bismuth was shock compressed with an optical laser and probed in situ with
X-ray radiation from a free electron laser. The evolution of the crystal structure
(or lack there of) during compression and shock release are documented by taking
snapshots of successive experiments, delayed in time.
The melting of Bi on release from Bi-V was studied, with precise time scans
showing the pressure releasing from high-pressure Bi-V phase until the melt curve
is reached off-Hugoniot. Remarkable agreement with the equilibrium melt curve
is found and the promise of this technique has for future off-Hugoniot melt curve
studies at extreme conditions is discussed.
In addition, shock melting studies of Bi were performed. The high-pressure Bi -
V phase is observed to melt along the Hugoniot where melting is unambiguously
identified with the emergence of a broad liquid-scattering signature. These
measurements definitively pin down where the Hugoniot intersects the melt curve
- a source of some disagreement in recent years. Evidence is also presented for a
change in the local structure of the liquid on shock release. The impact of these
results are discussed.
Finally, a sequence of solid-solid phase transformations is observed on shock
compression as well as shock release and is detected by distinct changes in
the obtained diffraction patterns. The well established sequence of solid-solid
phase transformations observed in previous static studies is not observed in our
experiments. Rather, Bi is found to exist in some metastable structures instead
of forming equilibrium phases. The implications these results have for observing
reconstructive phase transformations in other materials on shock timescales are