Dielectric properties of 0.68Pb(Mg_1/3Nb_2/3) O₃-0.32PbTiO₃ ceramics with lithium salt sintering aids

Relaxor ferroelectric 0.68Pb(Mg_1/3Nb_2/3) O₃-0.32PbTiO₃ [PMN-PT] ceramics with lithium salts (Li₂CO₃, LiNO₃, LiF) sintering aids were prepared by the columbite precursor method. The phase composition of the ceramic samples was investigated by X-ray diffraction (XRD), and the dielectric properties of these ceramics were measured. The results showed that pure perovskite phase is obtained for all the specimens. The dielectric loss decreases and other dielectric properties have not distinctly changed for the PMN-PT ceramics with Li₂CO₃ or LiNO₃ addition, but the dielectric loss increases, the peak of dielectric constants is broaden, the dielectric relaxor characteristic is obvious and the Curie temperature descends for the PMN-PT ceramics with LiF addition. Moreover, the peak of dielectric constants and dielectric loss for polarized PMN-PT ceramics with lithium salts sintering aids shows double peaks. The first peak is the transition between rhombohedra ferroelectric phase and tetragonal ferroelectric phase, which is about 80°C; and the second peak is the transition between tetragonal ferroelectric phase and paraelectric phase, which is about 130°C for the sample with LiF addition and about 150°C for the others. However, the sample with Li₂CO₃ or LiNO₃ addition may only have cationic (Li⁺) substitution at B site of PMNT with oxygen vacancies formation, but the sample with LiF addition may have anionic (F^-) substitution with lead vacancies formation, which may be compensated with oxygen vacancies formed by cationic (Li⁺) substitution.