Structural phase transition in the superconducting pyrochlore oxide Cd₂Re₂O₇

We report a structural phase transition found at T_s = 200 K in a pyrochlore oxide Cd₂Re₂O₇ which shows superconductivity at T_c = 1.0 K. X-ray diffraction experiments indicate that the phase transition is of the second-order, from a high-temperature phase with the ideal cubic pyrochlore structure (space group Fd3̄m) to a low-temperature phase with another cubic structure (space group F4̄3m). It is accompanied by a dramatic change in the resistivity and magnetic susceptibility and thus must induce a significant change in the electronic structure of Cd₂Re₂O₇.