  The 5d pyrochlore oxide superconductor Cd2Re2O7 (CRO) has attracted
significant interest as a spin-orbit-coupled metal (SOCM) that spontaneously
undergoes a phase transition to an odd-parity multipole phase by breaking the
spatial inversion symmetry due to the Fermi liquid instability caused by strong
spin-orbit coupling. Despite the significance of structural information during
the transition, previous experimental results regarding lattice deformation
have been elusive. We have conducted ultra-high resolution synchrotron
radiation x-ray diffraction experiments on a high-quality CRO single crystal.
The temperature-dependent splitting of the 0 0 16 and 0 0 14 reflections, which
are allowed and forbidden, respectively, in the high-temperature cubic phase I
(space group Fd-3m), has been clearly observed and reveals the following
significant facts: inversion symmetry breaking and tetragonal distortion occur
simultaneously at Ts1 = 201.5(1) K; the previously believed first-order
transition between phase II (I-4m2) and phase III (I4122) at Ts2 ~ 120 K
consists of two close second-order transitions at Ts2 = 115.4(1) K and Ts3 ~
100 K; there is a new orthorhombic phase XI (F222) in between. The order
parameters (OPs) of these continuous transitions are uniquely represented by a
two-dimensional irreducible representation Eu of the Oh point group, and the
OPs of phase XI are a linear combination of those of phases II and III. Each
phase is believed to correspond to a distinct odd-parity multipole order, and
the complex successive transitions observed may be the result of an electronic
phase transition that resolves the Fermi liquid instability in the SOCM.
