{110}-Layered B-cation ordering in the anion-deficient perovskite Pb 2.4 Ba 2.6 Fe 2 Sc 2 TiO 13 with the crystallographic shear structure

A novel anion-deficient perovskite-based compound, Pb 2.4 Ba 2.6 Fe 2 Sc 2 TiO 13 , was synthesized via the citrate-based route. This compound is an n = 5 member of the A n B n O 3n−2 homologous series with unit-cell parameters related to the perovskite subcell a p ≈ 4.0 Å as a p √2 × a p × 5 a p √2. The crystal structure of Pb 2.4 Ba 2.6 Fe 2 Sc 2 TiO 13 consists of quasi-2D perovskite blocks with a thickness of three octahedral layers separated by the 1/2[110](1̄01) p crystallographic shear (CS) planes, which are parallel to the {110} plane of the perovskite subcell. The CS planes transform the corner-sharing octahedra into chains of edge-sharing distorted tetragonal pyramids. Using a combination of neutron powder diffraction, 57 Fe Mössbauer spectroscopy and atomic resolution electron energy-loss spectroscopy we demonstrate that the B-cations in Pb 2.4 Ba 2.6 Fe 2 Sc 2 TiO 13 are ordered along the {110} perovskite layers with Fe 3+ in distorted tetragonal pyramids along the CS planes, Ti 4+ preferentially in the central octahedra of the perovskite blocks and Sc 3+ in the outer octahedra of the perovskite blocks. Magnetic susceptibility and Mössbauer spectroscopy indicate a broadened magnetic transition around T N ∼ 45 K and the onset of local magnetic fields at low temperatures. The magnetic order is probably reminiscent of that in other A n B n O 3n−2 homologues, where G-type AFM order within the perovskite blocks has been observed.