Structural order parameter in the pyrochlore superconductor Cd2Re2O7

It is shown that both structural phase transitions in Cd2Re2O7, which occur at T-s1 = 200K 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 Cd2Re2O7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.