NOVELTY - An oxygen separator has a chamber comprising a gas impermeable barrier (10) with a mixed oxide of fluorite-type structure.    USE - For separating oxygen from a gaseous mixture and for producing hydrogen.    ADVANTAGE - The invention includes higher order multi-functional rare earth oxide systems that is tailored for specific uses. It also produces 99 mole percent of oxygen. The process is environment friendly because the primary chemical processes utilizing the oxides is carried out at temperatures accessible to solar energy (e.g. without the use of fossil fuels), and the oxides are renewable.    DETAILED DESCRIPTION - The apparatus for separating oxygen from a gaseous mixture comprises a chamber with an inlet for the mixture that is maintained at a higher pressure (P1), and an outlet for a gas comprising a higher concentration of oxygen that is maintained at a lower pressure (P2). A gas impermeable barrier with two sides is fixedly attached within the chamber to form a high pressure portion of the chamber towards the inlet, and a low pressure portion towards the outlet. A heater (50) e.g. furnace, mirrors, or lenses is positioned proximate to the exterior of the chamber to maintain a temperature suitable for oxidation and reduction of the fluorite-type oxide. The barrier comprises a mixed oxide of fluorite-type structure.    INDEPENDENT CLAIMS are also included for:    (i) a process of separating oxygen from the gaseous mixture comprises heating a gas impermeable barrier to 400-800 degreesC, contacting the mixture with a side of the barrier, and maintaining partial pressure differential of oxygen to 10 torr between the first and the second side of the barrier; and    (ii) a mixed oxide of fluorite-type structure Ce1-x-y-zPrxTbyMzO2-approximatelyd, where delta is greater than 0 but less than 0.5; x, y, and z at least0 but less than 1; and M = scandium, yttrium, lanthanum, neodymium, promethium, samarium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, zirconium, bismuth, cobalt, manganese, copper, palladium, silver, titanium, vanadium, niobium, molybdenum, tantalum, and/or iron.    DESCRIPTION OF DRAWING(S) - The figure shows an apparatus for separating oxygen from a gaseous mixture.    Gas impermeable barrier (10)    Heater (50).