1：Experimental Design and Method Selection:

The samples were prepared by a conventional solid-state reaction method. The raw materials were powders of K2CO3, Na2CO3, Li2CO3, Nb2O5, CaCO3, TiO2, BaCO3, ZrO2, Co3O4, Fe2O3, and ZnO with a purity of more than 99%. The materials were calcined at 930°C in air for 4 h.

2：9%. The materials were calcined at 930°C in air for 4 h. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: The calcined powders were weighed and mixed with a ball mill for 15 h, then re-calcined at 930°C in air for 4 h. The dispersant and binder were added to the calcined powder, and the mixture was ball-milled for 15 h. The slurry was filtered through a 25 μm mesh sieve and dried, and the dried powder was classified with a 250 μm mesh sieve.

3：List of Experimental Equipment and Materials:

The samples were pressed into discs under a uniaxial pressure of 200 kg/cm2 and sintered in air at 1150°C for 4 h. The samples used for electrical properties measurements were 35 mm in diameter and 2 mm in thick for x = 0.51, and 15 mm in diameter and 1 mm in thick for x = 0.33, 0.42, 0.50, 0.56, 0.58, 0.61, 0.67, 0.71, and 0.

4：51, and 15 mm in diameter and 1 mm in thick for x = 33, 42, 50, 56, 58, 61, 67, 71, and Experimental Procedures and Operational Workflow:

75. 4. Experimental Procedures and Operational Workflow: The samples were polarized for 30 min in silicone oil under an electric field of 6 kV/mm at 80°C. The piezoelectric properties of 1-day-old samples were measured using the resonance–antiresonance method with Hewlett-Packard 4194A impedance analyzer.

5：Data Analysis Methods:

The crystal structure was analyzed by Rietveld refinement with the help of the RIETAN-2000 code. The profile parameters were refined using the split-type Pearson VII function, and partial profile relaxation was applied to the diffraction peaks from the domain-wall planes.