NOVELTY - Rare-earth manganese oxide (I) comprising metal, manganese and oxygen, is new.    USE - (I), (II) or (III) is useful: in an oxygen conducting membrane; in preparing oxygen, which is used in generating electricity; for catalyzing a reaction with oxygen (all claimed); and for elevated temperature gas separation and oxygen storage methods, including nitrogen production and components for oxy-fuel (clean coal) power plants, automotive exhaust catalysts, hydrogen-oxygen fuel cells, solar water splitting methods and steel, copper and plastic production and any other various industrial production processes which require high-purity oxygen and have large amounts of waste heat.    ADVANTAGE - (I), (II) or (III): selectively absorbs and releases oxygen with near 100% selectivity and not absorb other gases; can replace cryogenic distillation or pressure swing absorption for commercial air separation; has comparable reversible oxygen storage/release capacities with current commercial ceramic materials while operating at lower temperatures and has a smaller necessary temperature gradient for oxygen absorption/desorption; has faster oxygen absorption/desorption rates; and can be prepared using inexpensive and abundant elements.    DETAILED DESCRIPTION - Rare-earth manganese oxide (I) comprising metal (M1, optionally M2), manganese and oxygen, is new, where manganese and oxygen are present at an atomic ratio of 1:z.    M1 = Sc, Y, Dy, Ho, Er, Tm, Yb or Lu;    M2 = Bi, In, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu; and    z = 3.15.    INDEPENDENT CLAIMS are also included for:    (1) a rare-earth manganese oxide (II) comprising M1, M2, manganese and oxygen, where M1 and M2 are present at an atomic ratio of x1:1-x1, and manganese and oxygen are present at an atomic ratio of 1:z1;    (2) a rare-earth manganese oxide (III), comprising manganese having a formal oxidation state between 3 and 4, oxygen and at least one element comprising bismuth, indium, scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium or lutetium;    (3) an oxygen conducting membrane, comprising (I), (II) or (III) and a support material, where the membrane has first and second opposing surfaces, the membrane is not permeable to nitrogen gas, (I), (II) or (III) forms a contiguous structure exposed on both the first and second opposing surfaces;    (4) preparing oxygen, comprising separating oxygen from a mixture of gases containing the oxygen, by conducting the oxygen through a manganese oxide, or absorbing and releasing the oxygen from the manganese oxide, where the manganese oxide is (I), (II) or (III);    (5) catalyzing a reaction with oxygen, comprising catalyzing the reaction with (I), (II) or (III); and    (6) generating electricity, comprising burning a carbon-containing fuel with oxygen, in a generator or power plant, where the oxygen is prepared by the above process.    x1 = 0.1-0.9; and    z1 = greater than 3.