Achieving ultralow coercive electric field in (Ca1-1.5Gd )3Ti2O7 ceramics via dual inhibitory effects on oxygen vacancies

(Ca1-1.5xGdx)3Ti2O7 ceramics with ultralow coercive field were obtained by the flowing oxygen sintering combined with donor doping, and their microstructure, dielectric properties ferroelectric properties, and negative piezoelectric properties were studied. The Gd3+ was introduced into A sites of Ca3Ti2O7 ceramics when x is less than 0.75%, but the Gd3+ occupied B sites of Ca3Ti2O7 ceramics when x is above 0.75%. The introduction of Gd3+ suppresses grain growth and leads to a transition from rod-like grain to irregularly shaped grain. The variation of oxygen vacancy induced by the doping of Gd3+ leads to the change of dielectric and ferroelectric properties of (Ca1-1.5xGdx)3Ti2O7 ceramics. The ultralow coercive field of (Ca1-1.5xGdx)3Ti2O7 (x = 0.25%) sample obtained at PUND and DHM modes reached 58.80 kV/cm and 45.76 kV/cm, respectively, and the higher room-temperature remnant polarization (1.797 μC/cm2/PUND and 2.055 μC/cm2/DHM) was achieved simultaneously. These results indicate that the inhibition of oxygen vacancies contributes to realizing the ultralow coercive field in (Ca1-1.5xGdx)3Ti2O7 ceramics. These findings demonstrate that introducing rare-earth ions as donor dopants into the A site of Ca3Ti2O7 ceramics combined with the flowing oxygen sintering is an essential strategy to achieve ultralow coercive field for Ca3Ti2O7-based materials.