Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd2Nb2O7 Single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T-c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T-c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F-1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter.