Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd2Nb2O7 Single crystal was studied in the tem perature interval of 10-540 K, together with complementary dielectric measu rements. A ferroelectric soft mode was revealed above the ferroelectric pha se 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-diso rder nature of the transition. Below T-c many new modes were detected due t o lowering of the symmetry, especially below the previously suggested incom mensurate 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 para meter, whereas the intermediate ferroelastic transition is improper and tri ggered by the coupling with the ferroelectric order parameter.