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Pyrochlore Formation, Phase Relations, and Properties in the CaO-TiO2-(Nb,Ta)2O5 Systems

Published

Author(s)

Terrell A. Vanderah, Robert S. Roth, I E. Grey, W G. Mumme, P Bordet, Juan C. Nino

Abstract

Ceramic dielectric materials based on complex titanates, niobates, and tantalates are important for a variety of components in communications systems. As part of ongoing studies of phase equilibria in this class of electroceramics, a determination of the CaO:TiO2:Nb2O5 system was carried out. Formation of six ternary phases was confirmed, i.e., pyrochlore (A2B2O6O ) and five members of the (110) perovskite-slab series Can(Ti,Nb)nO3n+2, with n = 4.5, 5, 6, 7, and 8. Relations in the quasibinary Ca2Nb2O7?CaTiO3 system, which contains the Can(Ti,Nb)nO3n+2 phases, were determined in detail. CaTiO3 forms solid solutions with Ca2Nb2O7 as well as CaNb2O6, resulting in a triangular single-phase perovskite region with corners CaTiO3 70Ca2Ti2O6:30Ca2Nb2O7 80CaTiO3:20CaNb2O6. A pyrochlore solid solution forms from 42.68:42.68:14.64 to 42.15:40.85:17.00 CaO:TiO2:Nb2O5. Several compositions in the CaO:TiO2:Ta2O5 system were equilibrated to check its similarity to the niobia system in the pyrochlore region, which was confirmed. Structural refinements of the pyrochlores Ca1.5Ti1.5NbO7 and Ca1.5Ti1.5TaO7 using single-crystal X-ray diffraction data are reported, with Ti mixing on the A-type Ca sites as well as the octahedral B-type sites.Identical displacive disorder was found for the niobate and tantalate pyrochlores. The Ca-Ti-(Nb,Ta)-O pyrochlores both exhibited dielectric relaxation similar to that observed for some Bi-containing pyrochlores, which also exhibit displacively disordered crystal structures. Observation of dielectric relaxation in the Ca-Ti-(Nb,Ta)-O pyrochlores suggests that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi3+.
Citation
Journal of Solid State Chemistry
Volume
181
Issue
3

Keywords

Ca-Ti-Nb-O, CaO, crystal structure, dielectric relaxation, phase diagram, pyrochlore, TiO2:Nb2O5
Created April 23, 2008, Updated February 19, 2017