Structural Ordering and Symmetry Breaking in Cd_2Re_2O_7

Single crystal X-ray diffraction measurements have been carried out on Cd_2Re_2O_7 near and below the phase transition it exhibits at Tc' ~195 K. Cd_2Re_2O_7 was recently discovered as the first, and to date only, superconductor with the cubic pyrochlore structure. Superlattice Bragg peaks show an apparently continuous structural transition at Tc', however the order parameter displays anomalously slow growth to ~Tc'/10, and resolution limited critical-like scattering is seen above Tc'. High resolution measurements show the high temperature cubic Bragg peaks to split on entering the low temperature phase, indicating a (likely tetragonal) lowering of symmetry below Tc'.