Superspace description, crystal structures, and electric conductivity of the Ba4In6-xMgxO13-x/2 solid solutions

Ba3In6-xMgxO13-x/2 solid solutions have been prepar 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 a(0)approximate to a(p) root 2, b(0)approximate to 20.5 angstrom, c(0)approximate to a(p) root 2 (a(p) - parameter of the perovskite subcell) and modulation vector.q=alpha a(0)* , which is compositionally dependent according to the Ba4In4+4 alpha Mg2-4 alpha O12+2 alpha formula.The -superspace group Xmms(alpha 00)0s0 Xrnrn2(a00)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 propose ed from electron diffraction and structural considerations. A unified suoersoace model is" construtted'to describe the atomic arrangements and the variable oxygen content in the modulated (In, Mg)2Q2+a layers with rock-salt type structure. This model was used to refine the crystai structures of two x= 0 (R-I 0.040, R-p = 0.042) and x = 0.4 (R-I = 0.062, R-p = 0.040) limiting points of the solid solu tions. 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 ionicelectronic type of conductivity. Electronic conductivity at different P(O-2) was compared for Ba4In6-xMgxO13-x/2 and Ba2In2O5 and discussed taking into account the peculiarities of their crystal structures.