Development of a new state-of-the-art beamline optimised for single crystal and powder X-ray diffraction under extreme conditions at the ESRF

Neutron diffraction provides key information onmagnetic structures and position of light elements in solids. While usually neutron experiments are limited to relatively low pressures (2-3 GPa), we present neutron diffraction studies in the 40 GPa pressure range. The techniques developed in the Laboratoire Leon Brillouin allowed us to combine very low temperatures (down to 0.1 K), magnetic fields (up to 8 Tesla) with singlecrystal and powder neutron diffraction [1]. The extended range of thermodynamical parameters revealed new phenomena in ’’exotic’’ magnetic material, such as molecular magnets (O2) [2-3], topologically frustrated systems (Laves phases or pyrochlores) or systems close to the instability limit between the localized and itinerant magnetic states [4-7]. Pressure modifies magnetic interactions, changes the balance between different magnetic sublattices and can inducemagnetic collapse when exceeds some critical value. The studied compounds exhibit many unusual pressure-induced magnetic and structural phenomena, such as pressure-induced spin crystallization in magnetically disordered pyrochlores, first-order magnetostructural transitions in Laves phases, and magnetic collapse in high-pressure oxygen. Complementarities between neutron and synchrotron techniques are emphasized. New pressure techniques allow to carry out neutron and synchrotronmeasurements on the same sample in the same pressure cell. The combination of the two diffraction probes was recently successfully used to study the crystal structure of the broken-symmetry phase in solid deuterium at the pressure of 38 GPa [8].