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 BaZrCeMy Owherein [0002] Mis at least one element selected from the group consisting of trivalent rare earth elements, Bi and In; with 0.98 ≤ d ≤ 1; 0.01 ≤ x ≤ 0.5; 0.01 ≤ y ≤ 0.3; (2+y-2d)/2 ≤ Υ < 1.5. The mixed ionic conductor may be used in a fuel cell or gas sensor.