Significantly enhanced hybrid improper ferroelectricity of Ca₃Ti₂O₇ ceramics by the oxygen vacancy engineering

Ca₃Ti₂O₇ materials with hybrid improper ferroelectricity call tremendous interest due to their great potential in designing novel room-temperature single-phase multiferroics with a strong magnetoelectric coupling effect. However, achieving Ca₃Ti₂O₇-based ceramics with superior room-temperature ferroelectricity remains a crucial challenge. Herein, Ca₃Ti₂O₇ ceramics were prepared by a tartaric acid sol–gel method and sintered in air and oxygen atmospheres, respectively. It was found that the long-time high-temperature sintering in air could lead to the formation of CaTiO₃/Ca₃Ti₂O₇/CaTiO₃ sandwich structure, and oxygen-enriched sintering could effectively avoid the formation of CaTiO₃ phase on the surface. The superior hybrid improper ferroelectricity is attained in Ca₃Ti₂O₇ ceramics sintered in oxygen atmosphere with a higher remnant polarization (3.63 μC/cm²) and lower coercive electric field (72.6 kV/cm) obtained by positive-up and negative-down method, which is better than the ceramic sample sintered in air. The significantly better ferroelectricity is benefited from the synergistic effects of its larger distortion of oxygen octahedron, larger grain size, and lower oxygen vacancy concentration. Due to its low leakage current density, Ca₃Ti₂O₇ ceramics sintered in oxygen atmosphere exhibit the remanent polarization as high as 7.40 μC/cm² under the conditions of 1 Hz and 200 kV/cm obtained by dynamic hysteresis measurement mode. Furthermore, there are obvious stripe ferroelectric domains, which provide direct evidence for the excellent hybrid improper ferroelectricity of Ca₃Ti₂O₇ ceramics. These findings demonstrate that the oxygen vacancy engineering is an effective strategy to improve hybrid improper ferroelectricity of Ca₃Ti₂O₇ materials.