Phase stability in pb(b_½^'3+b^″5+_½)o₃ and pb(b_½^'3+b^″5+_½)o₃ compositions

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.