Abnormal Curie temperature behavior and enhanced strain property by controlling substitution site of Ce ions in BaTiO₃ ceramics

Dopants have a great effect on the phase transition behavior and the properties of the ferroelectrics. Here we report an abnormal Curie temperature (T_c) behavior and enhanced strain property by controlling the doping site of Ce ions in the BaTiO₃ ceramics. The Ce doped A-site and B-site BaTiO₃ ceramics (BT-xCe-A and BT-xCe-B, x=2, 4, 6, 8 mol%) were prepared by a conventional solid state reaction method through a different sintering temperature. The Raman test and the XPS results give evidence that Ce is successfully incorporated into Ba-site as Ce³⁺ in the BT-xCe-A samples, and into Ti-site as Ce⁴⁺ in the BT-xCe-B samples. Different doping sites have distinct phase transition behavior. Compared with the BT-xCe-A ceramics, the BT-xCe-B ceramics show higher T_c, and the T_c show abnormal increasing behavior with the increase of the Ce content. In the Ce doped BaTiO₃ system, this phenomenon has not been reported before. The origin of the higher T_c and its increasing behavior is discussed from the viewpoint of the larger local strain field generated by the Ce⁴⁺ ions entering into B-site. Besides, the BT-xCe-B ceramics show a stronger diffuse phase transition behavior. The reason is considered that the Ce substituting B-site leads to a multiphase coexistence, which induces more frustration states for the polarization according to the random defect field theory. Due to such distinct phase transition behavior, the BT-xCe-B ceramics show the enhanced maximum polarization (P_max) and enhanced strain properties compared with the BT-xCe-A ceramics. This work may provide a promising way to design high performance materials by controlling the substituting site of the dopant in other lead-free systems.