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 T_C′∼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 T_C′, 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 T_C′. 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 T_C′.