Superspace description, crystal structures, and electric conductivity of the Ba4in6-xMgxO13-x/2 solid solutions

Ba4In6-xMgxO13-x/2 solid solutions have been prepared for 0 e x e 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 ≈ 20.5A, c0 ≈ ap� 2( ap - parameter of the perovskite subcell) and modulation vector q )R a0*, which is compositionally dependent according to the Ba4In4+4RMg2-4RO12+2R formula. The superspace group Xmm2(R00)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+R 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 ionicelectronic 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.