Ferroelectric transition in the ternary system of Pb(Mg1/3 Nb2/3)O3-La(Mg2/3Nb1/3 )O3-PbTiO3

The ferroelectric transition of ceramics in the ternary system of (1-x-y)Pb(Mg_1/3Nb_2/3)O₃-xLa(Mg_2/3 Nb_1/3)O₃-yPbTiO₃ with x=0.05~0.13 and y=0.275~0.575 were investigated by the measurements of X-ray diffraction, dielectric properties and P-E hysteresis. Three morphotropic phases of perovskite structure, i.e. cubic, tetragonal, and rhombohedral, were involved in the study. An increase of PbTiO₃ content would cause the structure to evolve to the tetragonal phase; Pb(Mg_1/3Nb_2/3)O₃, to the rhombohedral phase; and, La(Mg_2/3Nb_2/3)O₃ , to the cubic phase. A triple point was found at the composition of x=0.11 and y=0.425. For the composition in the tetragonal phase field, a normal ferroelectric transition and hysteresis characteristic of high coercive field, Ec, over remanent polarization, Pr, were observed. However, for that in the rhombohedral or cubic phase field, a diffuse phase transition and hysteresis characteristic of low Ec over Pr were found. Moreover, the composition at the triple point has shown the a significant diffuse phase transition, and excellent dielectric properties of k≈16000 with high frequency and temperature stability