Structural ordering and symmetry breaking in Cd 2 Re 2 O 7

Single-crystal x-ray-diffraction measurements have been carried out on ${\mathrm{Cd}}_{2}{\mathrm{Re}}_{2}{\mathrm{O}}_{7}$near and below the phase transition it exhibits at ${T}_{{C}^{\ensuremath{'}}}\ensuremath{\sim}195\mathrm{K}.$ ${\mathrm{Cd}}_{2}{\mathrm{Re}}_{2}{\mathrm{O}}_{7}$ 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 ${T}_{{C}^{\ensuremath{'}}},$ 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 ${I}_{0}(1\ensuremath{-}{\mathrm{BT}}^{2}),$ and resolution limited critical-like scattering is seen above ${T}_{{C}^{\ensuremath{'}}}.$ 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 ${T}_{{C}^{\ensuremath{'}}}.$