Information regarding the structure of new complex oxide based on Ca, Cd, Ri and Nb from dielectric measurements. Aurivillius or pyrochlore?

Abstract

The synthesis of mixed oxides based on Ca, Cd, Bi and Nb made the object of different studies, the obtained materials having, according to the authors, pyrochlore or perovskite structures. The pyrochlore type compounds usually crystallize in the cubic system (space group Fd3m and a=10 A), containing eight molecules per unit cell, while the real perovskite type compounds present distortions to lower symmnetry classes. On the other hand, Rao and Medvedeva pointed out the presence of Aurivillius type structures for the mixed oxides containing Bi, Ca and Nb, The Aurivillius type oxides could be represented as a combination of (Bi/sub 2/O/sub 2/) fluorite type and (A/sub n-1/B/sub n/O/sub 3n-1/ perovskite-like layers, where n is an integer. As we already know, some members of the perovskite and Aurivillius type structures show superconductivity, but not even one compound with pyrochlore structure have been reported to present this phenomenon. On the other hand, due to their structural compatibility, the Aurivillius and perovskite type oxides are suitable to be used as supports for the thin film HTS. Our attempts to synthesise mixed oxides in the pseudo-quaternary system CaO-CdO-Bi/sub 2/O/sub 3/-Nb/sub 2/O/sub 5/ led to a compound having the chemical composition close both to a substituted pyrochlore and to a superior member of an Aurivillius series. In the present work, we will try to elucidate, by means of physicochemical characterization methods, including the dielectric measurements, the membership of the synthesised oxide to one of the above mentioned structures.