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 piezoelectr ic properties of the morphotropic phase boundary (MPB) composition in the P b(Zn1/3Nb2/3)O-3-BaTiO3-PbTiO3 system is presented. Samples were prepared a ccording to the formula Pb-0.85-xBa(0.15)La(x)[(Zn1/3Nb2/3)(0).7Ti(0.3)](1- x/4)O-3 (0 < x < 0.3), where the compensation for La ions was achieved via the appearance of B-site vacancies. All the compositions were synthesized u sing columbite precursor method and sintered using inverted crucible approa ch. Results of x-ray diffraction demonstrate that the solid solubility limi t of La2O3 in the PZN-based MPB composition is more than 30 mole%. Incorpor ation of La2O3 into A-site sublattice of perovskite structure stabilizes rh ombohedral phase against tetragonal phase, as a consequence, the location o f the MPB composition range is displaced toward PT-rich end. In addition, t he lattice distortion degree, represented by either tetragonality for tetra gonal phase or rhombohedrality for rhombohedral phase, is reduced. Increasi ng La2O3 content remarkably decreases both the dielectric permittivity maxi mum and the temperature of the dielectric permittivity maximum. However, ad dition of La2O3 obviously strengthens the degree of the frequency-dispersio n, which is correlated with the weakened degree of ferroelectric couplings among oxygen octahedra. The degree of diffuse phase transition (DPT) is pro minently 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 chang es with the respect to the scale of polar microregions inherent to relaxors . The longitudinal piezoelectric coefficient (d(3)3) is maximized at 1 mole % La2O3, which is attributed to the combined effect of phase coexistence an d enhanced contribution from non-180 degrees domain wall process associated with rhombohedral phase. (C) 2001 Kluwer Academic Publishers.