On the nature of the phase transitions in cadmium pyroniobate

The existing, doubtful interpretations of the numerous phase transitions in cadmium pyroniobate Cd₂Nb₂O₇, which is a ferroelectric with a pyrochlore-type structure and a Curie temperature lying near 200 K, are critically analyzed. The manifestations of all phase transitions in cadmium pyroniobate, the mechanisms proposed for these transitions, and doubts about their validity are discussed. Various dielectric anomalies observed near 200 K are related to three phase transitions in this temperature range. The causes of the large number of phase transitions observed in this material, the dislocation structure of the crystal, the diffuseness of the ferroelectric phase transition, and the relaxor properties of the crystal are also explained.