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Finally, the major refurbishment of high-pressure XRD beamline ID27 is now finalised. This instrument resumed user operation in November 2021, as initially planned. High-quality powder and single-crystal XRD data under extreme pressure-temperature conditions have already been collected. The new instrument will provide unique nano-focusing monochromatic and pink beam capabilities and a gain in intensity of up to three orders of magnitude.

The selection of works presented in this chapter is a good illustration of the multidisciplinary nature of the Matter at Extremes group. It includes high-quality, in-house and user research examples from a diversity of research fields. It features four contributions from the Earth and environmental sciences. Koemets et al. have combined in-situ single-crystal X-ray diffraction, XANES and Mössbauer spectroscopy (page 14) using four Matter at Extremes group beamlines: ID15B, ID18, ID24 and ID27 to observe an unexpected feature in the Fe-O bonding at extreme conditions that is important for understanding the oxygen cycle in the deep Earth interior. Pokrovski et al. have employed in-situ XAS measurements at BM30 (French CRG beamline) and molecular simulations to study a particular chemical form of sulfur, the trisulfur radical ion [S3 ]− that forms highly stable soluble complexes with platinum (page 15). These molecular vehicles are capable of massively transporting the metal by hydrothermal fluids in the Earth s crust. At ID27, Dziubek et al. have explored the phase diagram of hot dense carbon dioxide using in- situ high-temperature high-pressure XRD (page 15). At the same beamline, but using a different X-ray technique (i.e., XES), Farsang et al. have shown that the long-term storage of atmospheric CO2 in the deep Earth is facilitated by poorly soluble carbonates at the extreme pressure-temperature conditions of the deep Earth interior (page 18).

In the field of chemistry, Pinilla-Herrero et al. have investigated industrial-type catalysts at BM23 (page 20)

to reveal how the binder material, zeolite component and metal additives function together when the active sites responsible for the catalytic performance are formed. Friedrich et al. give new insights in the pressure-induced formation of columnar hydrofluorocarbons from polycyclic arene-perfluoroarene co-crystals using in-situ single crystal XRD at ID15B (page 27). At ID06-LVP, Serghiou et al. have determined unconventional routes to synthesis and unexpected new cubic symmetry in group IV alloys (page 25).

Finally, four examples conducted in the fields of physics and materials science are highlighted. At nuclear resonance beamline ID18, Heeg et al. were able to control nuclear excitations using X-ray light for the first time, achieving a temporal control stability of a few zeptosecond (page 27). Also at ID18, Caporaletti et al. have investigated the microscopic origin of glass formation near the glass- transition temperature (page 28). The results provide strong experimental support for theories of an intermittent mosaic structure in the deeply supercooled liquid phase.

At ID15B, Semenok et al. have determined the structures of ternary lanthanum yttrium superhydrides using in-situ powder XRD (page 30). The results show that alloying is an effective way of stabilising near-room-temperature superconductors YH10 and LaH6. Purans et al. combined XAS, XRD at BM23 and Raman measurements at very high pressure (P>180 GPa) to elucidate the nature of pressure-induced rearrangements in the electronic and atomic structure in YH3, a material that exhibits a very high superconducting critical temperature (page 32).

This selection of highlights gives a flavour of what can be achieved in the future by the Matter at Extremes group beamlines in a diversity of research fields

M. MEZOUAR