Effect of lanthanum-doping on the dielectric and piezoelectric properties of PZN-based MPB composition

In this paper, effect of lanthanum doping on the dielectric and piezoelectric properties of the morphotropic phase boundary (MPB) composition in the Pb(Zn_1/3Nb_2/3)O₃-BaTiO₃-PbTiO₃ system is presented. Samples were prepared according to the formula Pb_0.85–x Ba_0.15La _x (Zn_1/3Nb_2/3)_0.7Ti_0.3]_1–x/4O₃ (0 < x < 0.3), where the compensation for La ions was achieved via the appearance of B-site vacancies. All the compositions were synthesized using columbite precursor method and sintered using inverted crucible approach. Results of x-ray diffraction demonstrate that the solid solubility limit of La₂O₃ in the PZN-based MPB composition is more than 30 mole%. Incorporation of La₂O₃ into A-site sublattice of perovskite structure stabilizes rhombohedral phase against tetragonal phase, as a consequence, the location of the MPB composition range is displaced toward PT-rich end. In addition, the lattice distortion degree, represented by either tetragonality for tetragonal phase or rhombohedrality for rhombohedral phase, is reduced. Increasing La₂O₃ content remarkably decreases both the dielectric permittivity maximum and the temperature of the dielectric permittivity maximum. However, addition of La₂O₃ obviously strengthens the degree of the frequency-dispersion, which is correlated with the weakened degree of ferroelectric couplings among oxygen octahedra. The degree of diffuse phase transition (DPT) is prominently enhanced with increasing La₂O₃, for which the underlying mechanism is explored. Moreover, the ever-increasing extent of departure from Curie-Weiss behaviour induced by the lanthanum doping is indicative of some changes with the respect to the scale of polar microregions inherent to relaxors. The longitudinal piezoelectric coefficient (d ₃₃) is maximized at 1 mole% La₂O₃, which is attributed to the combined effect of phase coexistence and enhanced contribution from non-180° domain wall process associated with rhombohedral phase.