Eur. Phys. J. B 19, 9-16 (2001)
https://doi.org/10.1007/s100510170344

Phase transitions sequence in pyrochlore Cd₂ Nb₂O₇ studied by IR reflectivity

¹ Institute of Physics, ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic
² A.F. Ioffe Physico-Technical Institute, RAS, Politechnicheskaya 26, 194021 St. Petersburg, Russia

Corresponding author: ^a buixader@fzu.cz

Received: 17 July 2000
Published online: 15 January 2001

Abstract

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 symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter.