Reaction chemistry and subsolidus phase equilibria in lead-based relaxor systems Part II The ternary system PbO-MgO-Nb2O5

Abstract

Subsolidus compatibility relations in the lead-based relaxor system PbO-MgO-Nb2O5 were investigated by the solid-state reaction technique and the various phase assemblages that occur at 825°C in this system established. The existence of two previously reported ternary compounds, i.e. a cubic perovskite Pb3MgNb2O9 [Pb(Mg1/3Nb2/3)O3] and an oxygen- deficient cubic pyrochlore Pb6MgNb6O22 [Pb1.714(Mg0.286Nb1.714)O6.286] was confirmed. A minor amount of PbO enters into the pyrochlore lattice and forms a narrow homogeneity range within the ternary system PbO-MgO-Nb2O5 which corresponds to a general formula Pb2 − x (Mg0.286Nb1.714)O6.571 − x , where 0 > x > 0.286. At subsolidus temperatures (<825°C), the perovskite Pb(Mg1/3Nb2/3)O3 is compatible with the ternary pyrochlore solid solution and together they form a biphasic area within the system in which the perovskite and the pyrochlore phases coexist with one another. Both PbO and MgO are compatible with the perovskite Pb(Mg1/3Nb2/3)O3 and these phases constitute a compatibility triangle with one another in the ternary system. However, the perovskite Pb(Mg1/3Nb2/3)O3 is not compatible with Nb2O5 and these two phases react with one another to yield the pyrochlore Pb6MgNb6O22 and MgO. The ternary pyrochlore solid solution is compatible with several binary lead niobates, i.e. Pb3Nb2O8, Pb5Nb4O15, Pb2Nb2O7 and Pb3Nb4O13 and forms pseudobinary tie-lines with these compounds in the ternary system PbO-MgO-Nb2O5. The pyrochlores Pb3Nb4O13 [Pb1.5Nb2O6.5] and Pb6MgNb6O22 [Pb1.714(Mg0.286 Nb1.714)O6.286] are isostructural compounds and a series of continuous solid solution is formed between them.