A structure, conductivity and dielectric properties investigation of A3CoNb2O9 (A = Ca2+, Sr2+, Ba2+) triple perovskites

The room temperature structures as well as the temperature-dependent conductivity and dielectric properties of the A(3)CoNb(2)O(9) (A = Ca2+, Sr2+ and Ba2+) triple perovskites have been carefully investigated. A constrained modulation wave approach-to Rietveld structure refinement is used to determine their room temperature crystal structures. Correlations between these crystal structures and their physical properties are found. All three compounds undergo insulator to semiconductor phase transitions as a function of increasing temperature. The hexagonal Ba3CoNb2O9 compound acts as an insulator at room temperature, while the monoclinic Ca3CoNb2O9 compound is already a semiconductor at room temperature. The measured dielectric frequency characteristics of the A = Ba compound are excellent. (C) 2004 Elsevier Inc. All rights reserved.