New Structure Family of Oxide-ion Conductors Based on BaGdInO4
BaRInO4-based materials (R = Nd, Sm, Ho, Y, Er and Yb) have attracted much attention due to their oxide-ion conduction and unique (110) layered perovskite structures (monoclinic P21/c BaNdInO4-type structure). Here, we report new compounds BaGdInO4 and BaGd0.9A0.1InO3.95 (A = Mg, Ca and Sr). It was found that these compounds belong to a new structure family of oxide-ion conductors. Surprisingly, the single-crystal and powder X-ray diffraction analyses have shown that the crystal structures of BaGdInO4 and BaGd0.9A0.1InO3.95 (A = Mg, Ca, Sr) are quite different from those of the monoclinic BaRInO4-based materials. We found that BaGdInO4 and BaGd0.9A0.1InO3.95 (A = Mg, Ca, Sr) have an orthorhombic Pnma Ba2Y2CuPtO8-type structure consisting of InO5 square pyramid, InO6 octahedron, (Gd,A)O7 polyhedron (monocapped trigonal prism) and Ba cation where Gd,A denotes Gd or Gd0.9A0.1. Four edge-sharing (Gd,A)O7 polyhedra build up an infinite one-dimensional ((Gd,A)4O20)∞ chain. Oxide-ion conduction was confirmed for BaGdInO4 and BaGd0.9Ca0.1InO3.95 by the electrical conductivity measurements at different oxygen partial pressures P(O2). The oxide-ion conductivity of BaGd0.9Ca0.1InO3.95 was 381 times higher than that of BaGdInO4 at 400 oC, which was attributed to the higher oxygen vacancy concentration in BaGd0.9Ca0.1InO3.95. The oxide-ion conductivity of BaGd0.9Ca0.1InO3.95 was comparable to those of monoclinic BaNdInO4-type BaRInO4-based materials. The bulk conductivity of BaGd0.9Ca0.1InO3.95 was 1.3 × 10−3 S cm−1 at 700 oC and P(O2) = 10−4 atm in the electrolyte domain. The bond valence-based energy landscapes of BaGd0.9Ca0.1InO3.95 and BaGdInO4 indicated one-dimensional oxide-ion migration pathways on the faces of (Gd,Ca)O7 polyhedra along the ((Gd,Ca) 4O20)∞ chain in the b direction. The discovery and unique features of the orthorhombic BaGdInO4-based materials might develop materials chemistry of oxide-ion conductors.