  The 5d-transition metal pyrochlore oxide Cd2Re2O7, which was recently
suggested to be a prototype of the spin-orbit-coupled metal [Phys. Rev. Lett.
115, 026401 (2015)], exhibits an inversion-symmetry breaking (ISB) transition
at 200 K and a subsequent superconductivity below 1 K at ambient pressure. We
study the crystal structure at high pressures up to 5 GPa by means of
synchrotron X-ray powder diffraction. A rich structural phase diagram is
obtained, which contains at least seven phases and is almost consistent with
the electronic phase diagram determined by previous resistivity measurements.
Interestingly, the ISB transition vanishes at ~4 GPa, where the enhancement of
the upper critical field was observed in resistivity. Moreover, it is shown
that the point groups at 8 K, probably kept in the superconducting phases,
sequentially transform into piezoelectric, ferroelectric, and centrosymmetric
structures on the application of pressure.
