TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The heat-treating is performed at 600-900 degrees C. The method further comprises irradiating microwaves on the precursor solution before heat-treating of the precursor solution. Preferred Components: The chloride salt comprises at least one of sodium, lithium, potassium and/or magnesium. The amount of chloride salt is 5-10 moles, based on 1 mole of the dopant metal precursor and the zirconium precursor or the cerium precursor. The zirconium precursor comprises at least one zirconium acetate and/or zirconium nitrate, and the cerium precursor comprises at least one cerium acetate, cerium nitrate and/or cerium chloride. The dopant metal precursor comprises at least one yttrium, scandium, ytterbium, gadolinium, samarium, indium, lutetium, dysprosium, lanthanum, bismuth, praseodymium, actinium, aluminum, gallium and/or boron. The chlorine is 0.5-2 wt.% based on a total weight of the oxide and total weight of the solid oxide electrolyte. In the oxide, the metal-doped zirconia or the metal-doped ceria is in the form of particles, and an average particle diameter of the metal-doped zirconia or the metal-doped ceria particles is 20-60 nm. In the solid oxide electrolyte, the relative density of the solid oxide electrolyte is greater than or equal to 90 %.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The precursor solution further comprises a precipitant. The precipitant comprises at least one urea, citric acid, tartaric acid, oxalic acid, hexadecanediol, ammonia aqueous solution, hydrazine aqueous solution, acetic acid, sodium acetate, ammonium acetate, oleic acid, sodium oleate, ammonium oleate, ammonium succinate, polyacrylate, glycine and/or acyl glutamate. EXAMPLE - Zirconium acetate and yttrium chloride were added to diethylene glycol (100 ml) to a concentration of 0.01 M to have a molar ratio of (84:16 mol). Citric acid was added so that the molar ratio between the mixture of zirconium acetate and yttrium chloride and citric acid satisfies the ratio 1:3, and sodium chloride was added so that the molar ratio between the mixture of zirconium acetate and yttrium chloride and sodium chloride satisfies the ratio 1:6. Then, the mixture was sufficiently stirred at 100 degrees C to prepare a precursor solution. Microwaves having a power of 600 W were irradiated onto the precursor solution for 3-4 minutes, and the precursor solution was centrifuged. The centrifuged solution was heat treated at 700 degrees C for 2 hours, cleaned with distilled water, and dried to obtain an oxide powder.