Superspace Description, Crystal Structures, and Electric Conductivity of the Ba4In6−xMgxO13−x/2 Solid Solutions

Abstract

Ba4In6−xMgxO13−x/2 solid solutions have been prepared for 0 ≤ x ≤ 0.4 and their crystal structures have been studied by transmission electron microscopy and X-ray powder diffraction. The compounds adopt a modulated structure with a face-centered orthorhombic average unit cell with a0 ≈ ap√2, b0 ≈ 20.5Å, c0 ≈ ap√2 (ap − parameter of the perovskite subcell) and modulation vector q = αa0*, which is compositionally dependent according to the Ba4In4+4αMg2−4αO12+2α formula. The superspace group Xmm2(α00)0s0 with centering vectors (0,1/2,1/2,1/2), (1/2,0,1/2,0), (1/2,1/2,0,1/2) was proposed from electron diffraction and structural considerations. A unified superspace model is constructed to describe the atomic arrangements and the variable oxygen content in the modulated (In, Mg)2O2+α layers with rock-salt type structure. This model was used to refine the crystal structures of two x = 0 (RI = 0.040, RP = 0.042) and x = 0.4 (RI = 0.062, RP = 0.040) limiting points of the solid solutions. The cations in these layers adopt a five-fold coordination that can be virtually classified as strongly distorted trigonal bipyramids and tetragonal pyramids. The close similarity of the structural organization of the Ba4In6−xMgxO13−x/2 solid solutions, Sr4Fe6O13 and its anion-deficient derivatives is discussed. Electric conductivity of the Ba4In6−xMgxO13−x/2 was studied by impedance spectroscopy indicating mixed ionic-electronic type of conductivity. Electronic conductivity at different P(O2) was compared for Ba4In6O13 and Ba2In2O5 and discussed taking into account the peculiarities of their crystal structures.