Structural and uranium valence change in UTe₂ and UFe₂Si₂ detected by high pressure x-ray diffraction and resonant x-ray emission spectroscopy

High pressure x-ray diffraction studies in UTe₂ up to 30 GPa at room temperature revealed an orthorhombic to tetragonal phase transition between 5 and 7 GPa [1,2,3], with a large volume collapse of nearly 10% and a nearest U-U distance increase by about 4% [1]. In conjunction, resonant x-ray emission spectroscopy and partial fluorescence yield x-ray absorption spectra clearly demonstrate a U intermediate valence of nearly +3.74 at 1.8 GPa and room temperature. With increasing pressure, the U valence shifts towards 4+, passes through a peak at 2.8 GPa, and then decreases towards 3+ and settles to nearly a constant value above 15 GPa and up to 52 GPa [1]. This lower to higher symmetry transition suggests less 5f electron participation in bonding when the weakly correlated superconducting phase in the tetragonal structure of UTe₂ appears. Additionally, high pressure x-ray diffraction spectra in UFe₂Si₂ exhibit new peaks at P > 20 GPa – signaling a possible phase transition. UFe₂Si₂ resonant x-ray emission spectra are consistent with a valence of 3.7 at 1.5 GPa which steadily decreases to 3.6 up to 7 GPa, while data collected at 32 and 54 GPa indicates an increase in valence back to 3.7.