## 2 Experimental Procedure  
Dielectric powders of \((1-x)\)BaTiO\({}_{3}\)-\(x\)(Bi\({}_{0.5}\)Na\({}_{0.5}\))TiO\({}_{3}\) ceramics with various amounts of Ta\({}_{2}\)O\({}_{5}\) and Nb\({}_{2}\)O\({}_{5}\) were prepared using the solid-state reaction technique. Highly pure (\(>\)99.9% purity) BaTiO\({}_{3}\) (Ferro, reagent grade), Bi\({}_{2}\)O\({}_{3}\) (SHOWA, reagent grade), Na\({}_{2}\)CO\({}_{3}\) (SHOWA, reagent grade), TiO\({}_{2}\) (SHOWA, reagent grade), Ta\({}_{2}\)O\({}_{5}\) (Alfa, reagent grade), and Nb\({}_{2}\)O\({}_{5}\) (Alfa, reagent grade) were used as raw materials. The host material (Bi\({}_{0.5}\)Na\({}_{0.5}\))TiO\({}_{3}\) was prepared by mixing appropriate amounts of Bi\({}_{2}\)O\({}_{3}\), Na\({}_{2}\)CO\({}_{3}\), and TiO\({}_{2}\) and calcining them at 800 \({}^{\circ}\)C for 12 h, and the phase was identified using X-ray diffraction (XRD, Rigaku DMX-2200, Rigaku, Tokyo, Japan). Formulations based on compositions of \((1-x)\)BaTiO\({}_{3}\)-\(x\)(Bi\({}_{0.5}\)Na\({}_{0.5}\))TiO\({}_{3}\) (\(x\) = 0.05, 0.1, and 0.2) with various amounts of Ta\({}_{2}\)O\({}_{5}\) and Nb\({}_{2}\)O\({}_{5}\) (0, 1.0, 1.5, and 2.0 mol%) were mixed and milled in methyl alcohol solution using polyethylene jars and zirconia balls for 24 h and then dried at 80 \({}^{\circ}\)C in an oven overnight. The mixed powders were added to 4 wt% of a 15 wt% Polyvinyl Alcohol (PVA) solution and were pressed into disc-shaped compacts under a uniaxial pressure of 120 MPa. The samples were then heat treated at 550 \({}^{\circ}\)C for 4 h to eliminate the PVA, followed by sintering at 1200 \({}^{\circ}\)C to 1245 \({}^{\circ}\)C. The heating and cooling rates were controlled at 5 \({}^{\circ}\)C/min for all samples, and the dwell time was set to 2 h.  
The apparent density of the sintered ceramics was measured on five samples for each composition using the Archimedes method with de-ionized water. Phase identification on the polished surfaces of the sintered bulk ceramics was performed using X-ray diffraction (XRD, Siemens D5000, Karlsruhe, Germany). The microstructures of the sintered samples were examined with scanning electron microscopy (SEM, JEOL 6500F, Tokyo, Japan) and energy-dispersive spectroscopy (EDS). The average grain sizes of the sintered samples were determined using the linear intercept, which was calculated as the mean value of the measurement diagonals of about 200 grains. The dielectric properties of the sintered samples were measured as a function of temperature using an automatic measurement system incorporating a Delta Design Environmental Chamber with an LCR meter (HP 4284A, Santa Clara, CA, USA) at a frequency of 1 kHz. Before measurement, electrodes were fabricated on the disc samples using silver paste applied on opposing surfaces and fired at 750 \({}^{\circ}\)C. The measurements were performed during heating cycles from \(-\)55 \({}^{\circ}\)C to 130 \({}^{\circ}\)C at a rate of 2 \({}^{\circ}\)C/min with an accuracy of 0.1 \({}^{\circ}\)C. An alternating voltage of 1 V was applied. The electrical resistivity of the bulk ceramics was determined by a high resistance meter (HP 4339B, Santa Clara, CA, USA) at 25 and 200 \({}^{\circ}\)C at a DC voltage of 500 V. The measurements of dielectric properties and electrical resistivity were performed on at least three samples in each case, with an accuracy of \(\pm\)3%.