Successive phase transitions of the spin-orbit-coupled metal Cd 2 Re 2 O 7 probed by high-resolution synchrotron x-ray diffraction.

The 5 d pyrochlore oxide superconductor Cd 2 Re 2 O 7 (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 -3 m ), has been clearly observed and reveals the following significant facts: inversion symmetry breaking and tetragonal distortion occur simultaneously at T s1 = 201.5(1) K; the previously believed first-order transition between phase II ( I -4 m 2) and phase III ( I 4 1 22) at T s2 ∼120 K consists of two close second-order transitions at T s2 = 115.4(1) K and T s3 ∼ 100 K; there is a new orthorhombic phase XI ( F 222) in between. The order parameters (OPs) of these continuous transitions are uniquely represented by a two-dimensional irreducible representation E u of the O h 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.