A new structure family of oxide-ion conductors based on BaGdInO4

Abstract

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 (GdBaInO₄) and BaGd_0.9A_0.1InO_3.95 (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 BaGd_0.9A_0.1InO_3.95 (A = Mg, Ca, and Sr) are quite different from those of the monoclinic BaRInO₄-based materials. We found that BaGdInO₄ and BaGd_0.9A_0.1InO_3.95 (A = Mg, Ca, and Sr) have an orthorhombic Pnma Ba₂Y₂CuPtO₈-type structure consisting of InO₅ square pyramids, InO₆ octahedra, (Gd,A)O₇ polyhedra (monocapped trigonal prisms) and Ba cations where (Gd,A) denotes Gd or Gd_0.9A_0.1. Four edge-sharing (Gd,A)O₇ polyhedra build an infinite one-dimensional ((Gd,A)₄O₂₀)_∞ chain. Oxide-ion conduction was confirmed for BaGdInO₄ and BaGd_0.9Ca_0.1InO_3.95 by the electrical conductivity measurements at different oxygen partial pressures P(O₂). The oxide-ion conductivity of BaGd_0.9Ca_0.1InO_3.95 was 381 times higher than that of BaGdInO₄ at 400 °C, which was attributed to the higher oxygen vacancy concentration in BaGd_0.9Ca_0.1InO_3.95. The oxide-ion conductivity of BaGd_0.9Ca_0.1InO_3.95 was comparable to those of monoclinic BaNdInO₄-type BaRInO₄-based materials. The bulk conductivity of BaGd_0.9Ca_0.1InO_3.95 was 1.3 × 10⁻³ S cm⁻¹ at 700 °C and P(O₂) = 10⁻⁴ atm in the electrolyte domain. The bond valence-based energy landscapes of BaGd_0.9Ca_0.1InO_3.95 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 the materials chemistry of oxide-ion conductors.