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

Ba 4 In 6− x Mg x O 13− 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 a 0 ≈ a p √2, b 0 ≈ 20.5Å, c 0 ≈ a p √2 ( a p − parameter of the perovskite subcell) and modulation vector q = α a 0 *, which is compositionally dependent according to the Ba 4 In 4+4α Mg 2−4α O 12+2α formula. The superspace group Xmm 2(α00)0 s 0 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) 2 O 2+α layers with rock-salt type structure. This model was used to refine the crystal 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 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 Ba 4 In 6− x Mg x O 13− x /2 solid solutions, Sr 4 Fe 6 O 13 and its anion-deficient derivatives is discussed. Electric conductivity of the Ba 4 In 6− x Mg x O 13− x /2 was studied by impedance spectroscopy indicating mixed ionic-electronic type of conductivity. Electronic conductivity at different P(O 2 ) was compared for Ba 4 In 6 O 13 and Ba 2 In 2 O 5 and discussed taking into account the peculiarities of their crystal structures.