Structural Order Parameter in the Pyrochlore Superconductor Cd₂Re₂O₇

Abstract

It is shown that both structural phase transitions in Cd₂Re₂O₇, 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 Cd₂Re₂O₇ follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.