Mixed ionic conductor and device using the same

A mixed ionic conductor is disclosed comprising an ion conductive oxide having a perovskite structure of the formula BaeZr1-zM2zO3-β, wherein 0.9 ≤ e ≤ 1; M2 is at least one element selected from the group consisting of trivalent rare earth elements, Bi, Ga, Sn, Sb and In;with 0.01 ≤ z ≤ 0.3;and (2 + z - 2e)/2 ≤ β < 1.5. The mixed ionic conductor may be used in a fuel cell or gas sensor.