  High pressure x-ray diffraction up to 30 GPa and resonant emission x-ray
spectroscopy and partial fluorescence yield x-ray absorption spectroscopy up to
52 GPa were used to study how the structural and electronic properties of
UTe$_2$ evolve with pressure at room temperature. An orthorhombic to tetragonal
phase transition was observed to occur between 5 and 7 GPa, with a large volume
collapse of nearly 11% and a nearest U-U distance increase by about 4%. This
lower to higher symmetry transition suggests less 5f electron participation in
bonding when the weakly correlated superconducting phase in the tetragonal
structure of UTe$_2$ appears. Beyond 7 GPa, no new structural transitions were
found up to 30 GPa. The resonant x-ray emission spectra clearly demonstrate an
intermediate valence of U, nearly +3.74 at 1.8 GPa and room temperature, and
reveal that the U valence shifts towards 4+, passes through a peak at 2.8 GPa,
and then decreases towards 3+ and settles down to a nearly constant value above
15 GPa. These experiments reveal that some fundamental structural and valence
changes occur in UTe2 at relatively low pressures, which could be responsible
for the interplay between unconventional superconductivity, magnetic ordering,
and weakly correlated superconductivity that is manifested in the
temperature-pressure phase diagram of UTe2.
