The crystal structure of Cd2Nb2O7: symmetry mode analysis of the ferroelectric phase

The crystal structure of the ferroelectric Cd2Nb2O7 (CNO) has been determined down to T = 98 K using synchrotron radiation. Low temperature structure refinement is conducted in space group Ima2, starting from the crystal structure previously determined by ab initio methods. Symmetry mode analysis indicates that the primary order parameter is of T-2u symmetry with the largest displacement amplitude at the Nb(2) position. The associated phase transition temperature is obtained as T-c = 194 K by extrapolation. Long exposure x-ray diffraction scans confirm the presence of anisotropic diffuse scattering intensity in layers normal to < 110 >(cub) over the entire temperature range. The diffuse scattering intensity significantly decays below T = 198 K. The refined thermal parameters indicate that the disorder responsible for the diffuse scattering is primarily associated with Nb.