*2.1. Sample Preparation*

A kind of Gd/Mn co-doped CaBi4Ti4O<sup>15</sup> piezoceramics, the formula of Ca1-*x*Gd*x*Bi4Ti4O<sup>15</sup> +0.2wt%MnCO<sup>3</sup> (abbreviated as CBT-*x*Gd/0.2Mn), were produced by the conventional solid-state reaction route. First of all, the CaCO<sup>3</sup> of 99% purity, Gd2O<sup>3</sup> of 99.99% purity, TiO<sup>2</sup> of 99% purity, Bi2O<sup>3</sup> of 99.99% purity, and MnCO<sup>3</sup> of 99% purity (as raw materials) were weighed according to the stoichiometric ratio (CaCO3, Gd2O3, Bi2O3, TiO2, and MnCO<sup>3</sup> produced in Chron Chemicals, Chengdu, China). These chemical compounds were balled for 6h with alcohol in planetary ball mill and calcined at 850 ◦C for 4 h in the muffle furnace. Then, the calcined powders, with 0.2 wt% MnCO3, were balled for another 12 h, pressed into discs, and sintered at 1050 ~ 1150 ◦C for 2h to obtain the ceramic chips. Lastly, Ag paste was painted on both sides of the CBT-*x*Gd/0.2Mn ceramics and fired at 700 ◦C for 10 min in air.
