X-ray free-electron laser heating and structural studies in the diamond anvil cell

Abstract

X-ray free-electron lasers (XFELs) constitute the world’s brightest light sources, capable of producing focused, short wavelength, coherent X-ray pulses for atomic scale investigation of matter under extreme conditions. Sources such as the European XFEL (EuXFEL) can produce serial hard X-ray pulses at a rate of several million pulses per second, each carrying sufficient energy to heat and probe targets up to thousands of degrees Kelvin and beyond. By pairing XFEL heating and probing capabilities with the well-established compression methodology of the diamond anvil cell (DAC), investigation of matter under extreme pressure and temperature (P − T) conditions, such as those found within planetary interiors, can be performed with unprecedented temporal resolution.

This thesis presents the results of single-pulse, hard X-ray heating of metals under vacuum, including aluminium (Al), titanium (Ti), vanadium (V), copper (Cu), iron (Fe), stainless steel (SS), zirconium (Zr), molybdenum (Mo), rhodium (Rh), tantalum (Ta), tungsten (W), gold (Au), and bismuth (Bi). Foil targets of varying thicknesses, 2 - 500 μm, were irradiated with 15 or 18 keV XFEL pulses and recovered samples were examined using focused ion beam milling and scanning electron microscopy. Damage morphology constrained the spatial profile of the heating source, the mechanisms for damage formation, and the thresholds for damage onset. The spatial characterisation results displayed excellent agreement across materials of varying atomic number. Only in the case of an Al target was a notable difference recorded in the effective beam size which was related to the reduced electron binding energies and stopping power of the target. Systematic trends were also found in the onset damage thresholds for different materials which correlated well with a simple model accounting for X-ray absorption properties, isochoric melting temperatures, and specific heat capacities.

Serial X-ray heating and diffraction experiments were also performed on a freestanding Ti foil, in vacuum, at megahertz (MHz) repetition rate. By varying the energy of pulses irradiating the target, the rate and extent of heating was controlled. Sample temperature was inferred from the X-ray diffraction (XRD) data collected every 443 ns and a discontinuous jump in the temperature was observed at the α-β Ti phase transition. The influence of temperature gradients and spatial distribution of the probe on the diffraction patterns was considered and forward modelling of the heating dynamics was performed using the finite volume code COMSOL while carefully accounting for the pulse-to-pulse X-ray energetics.

Low atomic number materials of interest to planetary science, such as H₂O, can also be indirectly X-ray heated by a coupler material in close proximity. Pure Rh metal was identified as a prospective X-ray coupler due to its X-ray absorption and thermal properties. However, published experimental data on the volumetric behaviour of Rh at high P − T was limited, restricting its use as an internal standard. Therefore, high-resolution, synchrotron angle-dispersive powder diffraction studies were conducted on Rh up to ∼191 GPa and 2700 K which retained a face-centered cubic (fcc) structure for all conditions studied. A thermal equation of state (EoS) was refined to the data enabling Rh to be used as an internal coupler and standard in future X-ray heating experiments.

Above ∼18 GPa and between ∼800 - 2000 K, H₂O ice VII transitions from the body-centered cubic (bcc) structure to a “superionic” bcc phase exhibiting highly-mobile protons. Further elevation in pressure and temperature yields a second superionic phase with a fcc structure. This thesis demonstrates serial X-ray heating and XRD on high-pressure ice VII between ∼36 to 58 GPa using a Rh X-ray coupler and the thermal EoS developed in the present work. Diffraction patterns collected every 443 ns displayed migration of Bragg peak positions to lower 2θ angles for both the coupler and ice VII as they underwent thermal expansion. Also observed were discontinuous Bragg peak shifts representative of the bcc-bcc superionic H₂O phase transition.