An extremely pure powder of an oxide of two or more metals, homogeneous at the atomic level, is shaped and compressed (for example, compacted by cold isostatic pressing) to provide greenware which is sintered to provide finished ceramic articles such as laser cathodes and electrodes for chlorine/caustic electrolysis. For example, pure mixed oxide powder consisting of La 0.7 Sr 0.3 CoO₃ is shaped to a tube, which is cold-­isostatically pressed at 20,000 psi, then sintered to provide a cathode for a CO₂ laser. The starting oxides can be prepared by (1) reacting in water, soluble salts of two or more metals with quaternary ammonium carbonate; the metals being those that form water-insoluble carbonates, thereby to precipitate mixed carbonates of the metals; (2) recovering and drying the precipitate; and (3) calcining the precipitate to provide mixed metal oxides homogeneous at the atomic level and substantially free from extraneous contaminants.