Elimination of pyrochlore phase in high-concentration Sm³⁺-doped PMN-PT piezoelectric ceramics by excessive MgO

Undesirable pyrochlore phase often appears in Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT)-based ceramics with high rare-earth ion (RE³⁺) doping concentration, which greatly limits their development. In this study, 0–5 mol% Sm³⁺-doped Pb(Mg_1/3Nb_2/3)O₃-29PbTiO₃ (PMN-29PT:0-5Sm) ceramics were first synthesized using traditional precursor method. In the X-ray diffraction spectra and scanning electron microscope images of PMN-29PT:3-5Sm ceramics, the diffraction peaks of pyrochlore phase and pyrochlore grains with octahedral morphology were observed, respectively. The reason for the appearance of the pyrochlore phase is that Sm³⁺ doping causes the Nb-rich regions. To eliminate the pyrochlore phase, PMN-29PT:3-5Sm ceramics were resynthesized by an improved precursor method in which an excess of 4 mol% MgO was added to the reactants before pre-sintering. After adding an excess of 4 mol% MgO, the concentration ratio of Nb⁵⁺ and Mg²⁺ in the pyrochlore grains returned to the value in the perovskite grains, and the pyrochlore phase was transformed into the perovskite phase PMN. The dielectric, ferroelectric, and electromechanical properties were compared before and after eliminating the pyrochlore phase. The results show that the comprehensive performance of the ceramics is improved after eliminating the pyrochlore phase.