**2. Materials and Methods**

Bi1-xSmxFe0.98Mn0.02O3 thin films (x = 0, 0.02, 0.04, 0.06) were prepared by the sol-gel method on ITO/glass substrate. Fe(NO3)3·9H2O (purity of 98.5%), Bi(NO3)3·5H2O (purity of 98.5%), Sm(NO3)3·6H2O (purity of 98.5%), and MnC4H6O4·4H2O (purity of 98.5%) were taken as solutes, according to stoichiometric ratio. Bi excess of 5% compensated for bismuth volatilization during high-temperature annealing. The solutes were successively added to a solvent mixture of CH3COOH and C2H6O, with a volume ratio of 3:1, and stirred at room temperature at a uniform speed until completely dissolved. Then, C5H8O2 was added to the solution as a chelating agent and stirred at room temperature for 12 h at a constant speed to obtain a red-brown and transparent precursor solution. Finally, the stable precursor solution of 0.3 mol/L was obtained by allowing the precursor solution to rest for 24 h. Then, the BSFM precursor solution was rotated onto ITO/glass substrate, and the film was coated at 3500 r/min. The wet film was dried on an electric heating plate at 250 ◦C to remove excess organic solvents and water. It was then placed in an annealing furnace and annealed at 550 ◦C. The coating was repeated, and the film was dried and annealed 10 times to obtain the desired samples.

Before testing the electrical properties of the sample, Au was sputtered on the surface of the sample to achieve the effect of conduction. We used a small-ion sputtering instrument (JS-1600, Beijing Hetong Venture Technology Co., Ltd., Beijing, China) to complete this process. The samples were characterized by an X-ray diffractometer (D8-Advance, Drudy, Germany) recorded in the 2-theta range of 20–60◦ with a step of 0.02◦, and by a microconfocal Raman spectrometer (HR800, LabRAM, Horiba Co., Palaiseau, France) to measure in the shift range of 50–650 cm−1. The Fe and O elements in the samples were analyzed by a Wscabb X-ray photoelectron spectrometer. A dielectric tester (TH2828, Xintonghui Electronics Co., Ltd., Suzhou, China) was used to test the dielectric properties of the samples in the range of 1 kHz–1 MHz with an oscillation voltage of 1 V. The ferroelectric properties at 1 kHz and leakage properties of the samples were measured via a multiferroic tester (Radiant Co., Albuquerque, NM, USA).
