Structural ordering and symmetry breaking in Cd2Re2O7

Single-crystal x-ray-diffraction measurements have been carried out on Cd2Re2O7near and below the phase transition it exhibits at TC′∼195 K. Cd2Re2O7 was recently discovered as the first, and to date only, superconductor that displays the cubic pyrochlore structure at room temperature. Superlattice Bragg peaks show an apparently continuous structural transition at TC′, however, their behavior is unconventional. The evolution with temperature of these Bragg intensities show anomalously strong temperature dependence at low temperatures, where it falls off as I0(1-BT²), and resolution limited critical-like scattering is seen above TC′. High-resolution measurements show that the high-temperature cubic Bragg peaks split on entering the low-temperature phase, indicating a (likely tetragonal) lowering of symmetry below TC′.