Phase stability in pb(b½'3+b″5+½)o3and pb(b½'3+b″5+½)o3compositions

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Abstract
We have studied the phase stability of Pb(B'xB” 1−x)O3 compositions containing a combination of divalent or trivalent cations and pentavalent cation in the B’ and B” sites, respectively. The ceramic processing method of homogeneous mixing by ball-milling the starting oxide powders was used. It was found that Co2+ and Fe3+, in combination with Nb or Ta, favor the formation of perovskitc structure. With other divalent or trivalent cations, pyrochlore may form as a competing or dominant phase. Based on X-ray diffraction analysis, we concluded that both phases are made up of the same structural unit of (B', B”)O6 octahedra. The pyrochlore phase is cubic and probably deficient in oxygen. Compared to the pyrochlore structure of Pb2Nb2O7, a corresponding formula for the defective pyrochlore phase can be expressed as Pb(B'xB”1−x)O(3+y) where y = 0. compounds with Nb as one of the cations have larger dielectric constant than similar compounds containing Ta. The Pb(B', Nb)O3 perovskites also have a higher Curie temperature than the corresponding Pb(B', Ta)O3 perovskites. These results suggest that Nb has a larger ionic polarizability than Ta in the perovskite crystal structure.