Tetragonal (I4/mmm) BagIn6O17 is an oxygen-deficient intergrowth of BaO rocksalt (001) planes and oxygen-deficient 3 BaInO2.5 perovskite-type layers with a = b= 4.201(6) A ̊ and c = 30.48(4) A ̊. An oxygen concentration cell and a.c.-impedance data over the temperature range 200 < T < 950°C give a predominantly oxide-ion conductivity with an activation energy of 1.06 eV, which is comparable to that found in commercial stabilized zirconias. © 1990.