*2.1. Sample Preparation*

A kind of Gd/Cr codoped Bi3TiNbO<sup>9</sup> Aurivillius phase ceramics, with the formula of Bi2.8Gd0.2TiNbO<sup>9</sup> + 0.2 wt% Cr2O<sup>3</sup> (abbreviated as the BGTN−0.2Cr ceramic hereafter), was prepared by using the conventional solid-state reaction route in two steps. First, the metal oxides Bi2O<sup>3</sup> of 99% purity, Gd2O<sup>3</sup> of 99.99% purity, TiO<sup>2</sup> of 99% purity, and Nb2O<sup>5</sup> of 99.99% purity (as raw materials) were weighed according to the stoichiometric ratio (Bi2O3, Gd2O3, TiO<sup>2</sup> and Nb2O<sup>5</sup> produced in Chron Chemicals, Chengdu, China; Cr2O<sup>3</sup> produced in Aladdin, Shanghai, China). These raw materials were mixed evenly by ball milling for 6 h, using alcohol as solvent and zirconia balls as grinding media. The dried mixture was calcined at 850 ◦C for 4 h to synthesize the components of Bi2.8Gd0.2TiNbO9. Second, the calcined powders, with *x* wt% Cr2O<sup>3</sup> of 99.95% purity added, were ground for 12 h under the same grinding conditions and granulated with polyvinyl alcohol (abbreviated as PVA, produced in Chron Chemicals, Chengdu, China) as a binder. The powders were pressed into discs with a diameter of 10 mm and a thickness of 0.8 mm under an isostatic pressure of 10 MPa. After firing at 650 ◦C for 2 h to burn out the PVA, these discs were sintered in a sealed alumina crucible at 1100 ◦C for 2 h to obtain the dense ceramics. Finally, Ag electrodes were screen-printed on both surfaces of the polished ceramics and fired at 700 ◦C for 10 min. For comparison, the pure Bi3TiNbO<sup>9</sup> ceramic was prepared by the same process.
