The exploration of iron phase diagram at high-pressure using serial femto-second hard X-rays pulses at European XFEL

Abstract:

Iron is considered to be the main constituent of many planetary cores. Numerous studies have thus addressed the iron structure, elastic or transport properties, at extreme pressure and temperature, so as to yield for instance a precise model of the structure, composition and dynamics of the Earth's core. The melting line of iron is still the focus of many studies, as this phase transition is a unique reference to experimentally constrain the temperature at the boundary between the solid inner core and the liquid outer core of the Earth. For such complex measurements, it has been shown that it was mandatory to preserve the chemical integrity of liquid iron during experiments, in particular those conducted in diamond-anvil cells. Moreover, as far as the structure is concerned, the relative phase stability may also be greatly affected by the presence of light elements in the system, or subtle changes of experimental conditions or temperature gradients.

In this report, we propose to use the unique hard-x-ray capabilities of European XFEL to explore the phase diagram of pure iron, kept under pressure within a diamond-anvil cell. Coupling the traditional technique of x-ray diffraction on samples contained at high static pressures with fast probing and heating using brilliant X-ray irradiation from femtosecond X-ray pulse trains at MHz repetition rates, we prepare the ground for a new generation of high pressure experiments where phase transitions can be observed while desired chemical compositions can be preserved through use of the very short timescale. In this report, we use the experimental platform for high pressure research provided by the HIBEF user consortium to the HED instrument at the European XFEL to revisit part of the iron phase diagram and provide constraints on the melting temperature and stable phases for Fe at high-pressure and high-temperature. We demonstrate measurements of structure and phase transformation using a wholly new methodology of pulsed X-ray laser heating in the diamond anvil cell, which truly opens a new era for high-pressure experiments.

Acknowledgments: The presenter acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation Programme (grant agreement No. 670787).