Effects of lanthanum dopants on the Curie-Weiss and the local order behaviors for Pb_1-xLa_x(Fe_2/3W _1/3)_0.7Ti_0.3O₃ relaxor ferroelectrics

The effects of lanthanum dopants are investigated on the dielectric responses for Pb_1-xLa_x(Fe_2/3W _1/3)_0.7Ti_0.3O₃ ceramics. According to the experimental data and fitting results, it is concluded that the dielectric characteristics are changed from a long-range-ordered (LRO) ferroelectric state to a short-range-ordered (SRO) relaxor by increasing the amounts of lanthanum dopants. Furthermore, the spin glassy behaviors and the freezing process of local polarizations are not only affected by the ordering degree of B-site cations but also affected by the defect polar pairs. Their response mechanisms are proposed that the growth of the 1:1 ordered domain Pb_1-xLa_x(Fe_1/2W_1/2)O₃ is impeded since its charge imbalance is enhanced by increasing the amounts of lanthanum dopants, and the defect polar pairs of lead, tungsten and oxygen vacancy are induced by the pyrochlore phase, PbWO₄ or Pb _0.99La_0.01WO₄, when the amount of lanthanum dopants exceed 5 mol.%.