Effect of the element ratio in the doping component on the properties of 0.975(0.8Bi<sub>1/2</sub>Na<sub>1/2</sub>TiO<sub>3</sub>-0.2Bi<sub>1/2</sub>K<sub>1/2</sub>TiO<sub>3</sub>)-0.025Bi <sub>x</sub><sub>/3</sub>Mg <sub>y</sub><sub>/3</sub>Nb <sub>z</sub><sub>/3</sub>O<sub>3</sub>ceramics