Successive phase transitions of the spin-orbit-coupled metal CdReOprobed by high-resolution synchrotron x-ray diffraction.

The 5pyrochlore oxide superconductor CdReO(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-3), has been clearly observed and reveals the following significant facts: inversion symmetry breaking and tetragonal distortion occur simultaneously at= 201.5(1) K; the previously believed first-order transition between phase II (-42) and phase III (422) at∼120 K consists of two close second-order transitions at= 115.4(1) K and∼ 100 K; there is a new orthorhombic phase XI (222) in between. The order parameters (OPs) of these continuous transitions are uniquely represented by a two-dimensional irreducible representationof thepoint 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.