Piezoelectric properties of lead magnesium niobate-lead titanate ceramics with lithium salts sintering aids

Relaxor ferroelectric 0.68 Pb(Mg_1/3Nb_2/3)O₃-0.32 PbTiO₃ (PMN-PT68/32) ceramics with lithium salts (lithium carbonate (Li₂CO₃), lithium nitrate (LiNO₃), lithium fluoride (LiF)) as 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 piezoelectric properties of these ceramics were measured. The results show that pure perovskite phase is obtained for all the specimens and no other phase is detected. Moreover, the piezoelectric properties of PMN-PT68/32 ceramics with LiF sintering aids were improved more than those of PMN-PT ceramics with Li₂CO₃ or LiNO₃ sintering aids, and the sample with LiF addition sintered at 1150°C had good piezoelectric properties: a piezoelectric planar coupling coefficient (k_p) of 64%, a piezoelectric thick coupling coefficient (k_t) of 54%, a piezoelectric constant (d₃₃) of 623 pC/N, and a mechanical quality factor (Q_m) of 96. The sample with Li₂CO₃ or LiNO₃ addition may only have cationic (Li⁺) substitution at the B-site of PMN-PT68/32 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. So the good piezoelectric properties of PMN-PT68/32 ceramics with LiF sintering addition may be attributed to not only low temperature liquid phase sintering and cationic (Li⁺) substitution but also mainly anionic (F^--O^2-) substitution.