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

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(Mg_1/3Nb_2/3)O₃] and an oxygen- deficient cubic pyrochlore Pb₆MgNb₆O₂₂ Pb_1.714(Mg_0.286Nb_1.714)O_6.286] 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 Pb_{2 – x} (Mg_0.286Nb_1.714)O_{6.571 – x} , where 0 > x > 0.286. At subsolidus temperatures (<825°C), the perovskite Pb(Mg_1/3Nb_2/3)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(Mg_1/3Nb_2/3)O₃ and these phases constitute a compatibility triangle with one another in the ternary system. However, the perovskite Pb(Mg_1/3Nb_2/3)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 Pb_1.5Nb₂O_6.5] and Pb₆MgNb₆O₂₂ Pb_1.714(Mg_0.286 Nb_1.714)O_6.286] are isostructural compounds and a series of continuous solid solution is formed between them.