Structural and Elastic Aspects in Phase Transitions of Superconducting Pyrochlore Oxide Cd2Re2O7

The Landau theory of the successive structural phase transitions in the pyrochlore oxide Cd2Re2O7 is presented. The primary transition parameters are the basis of the two-dimensional irreducible representation E-u of the m (3) over barm point group; they quadratically couple with the tensile strain components e(1), e(2), and e(3). In addition to the expected phase diagram, expressions of the elastic stiffness constants in the tetragonal phases are presented. It is conjectured that the elastic stiffness constant c(11)-c(12) should vanish at the cubic-tetragonal transition temperature.