Low temperature symmetry of pyrochlore oxide Cd2Re2O7

We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two symmetry-lowering structural transitions were observed at T-s1 = 200 K and T-s2 = 120 K. The former is of the second order from the ideal cubic pyrochlore structure with space group Fd (3) over barm to a tetragonally distorted structure with i (4) over bar m2, while the latter is of the first order likely changing to another tetragonal space group l4(1)22. We discuss the feature of the lattice deformation.