  We report a structural phase transition found at Ts = 200 K in a pyrochlore
oxide Cd2Re2O7 which shows superconductivity at Tc = 1.0 K. X-ray
diffractionexperiments indicate that the phase transition is of the second
order, from a high-temperature phase with the ideal cubic pyrochlore structure
(space group Fd-3m) to a low-temperature phase with another cubic structure
(space group F-43m). 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.
