Membrane and use thereof

This invention relates to a dense single phase membrane having both high ionic and electronic conductivity and capable of separating oxygen from an oxygen containing gaseous mixture and further use of the membrane. The present invention relates to a dense single-phase membrane having both high ionic and electronic conductivity and capable of separating oxygen from an oxygen containing gaseous mixture, where the membrane comprises a mixed metal oxide material with interstitial oxygen excess represented by the formula AA′A″BB′B″B′″O, where A, A′ and A″ are chosen from group 1, 2 and 3 and the lanthanides; and B, B′, B″ and B′″ are chosen from the transition metals according to the periodic table of the elements adopted by IUPAC wherein 0 y 2, 0 y′ 2, 0 y″ 2, 0 x 1, 0 x′ 1, 0 x″ 1, 0 x′″ 1, and x and y each represents a number such that y+y′+y″=2, x+x′+x″+x′″=1, and is a number where 0