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 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.Comment: 5 pages. Submitted to J. Phys. Soc. Jpn. Best quality figures are
available at http://www.physics.mun.ca/~isergien/pubs.htm