Inelastic light scattering investigation of the pyrochlore superconductor Cd2Re2O7

The Structural phase transitions of the pyrochlore superconductor Cd$_{2}$Re$_{2}$O$_{7}$, T$_{c} \quad \approx $ 1.5 K, are investigated by Raman light scattering. The cubic to tetragonal transition at 200 K is characterized by the gradual appearance of a broad phonon mode originating from motion of the oxygen ions that form the apices of the ReO$_{6}$ octahedra. In contrast, the rapid growth of well-defined low frequency modes below the 120 K transition indicates that it is driven by ordering of the Cd ions within the channel voids of the distorted pyrochlore. A physical model describing the consecutive phase transitions in terms of the interplay between the Re-O and Cd-O networks will be presented.