Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

It is shown that both structural phase transitions in Cd 2 Re 2 O 7 , which occur at T s1 =200 K and T s2 =120 K, are due to an instability of the Re tetrahedral network with respect to the same doubly degenerate long-wavelength phonon mode. The primary structural order parameter transforms according to the irreducible representation E u of the point group O h . We argue that the transition at T s1 may be of the second order, in accordance with experimental data. We obtain the phase diagram in the space of phenomenological parameters and propose a thermodynamic path that Cd 2 Re 2 O 7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.