ИСТИНА

Recently we have discovered the new homologous series of anion-deficient perovskites AnBnO3n-2, the anion deficiency presents due to formation of crystallographic shear (CS) planes. CS planes periodically slice the parent perovskite structure on quazi two-dimensional blocks with thickness n delimited between each other by anion-deficient slabs. In the present work we demonstrate two new members with n = 5 (Pb2Ba2BiFe4ScO13 and Pb2Ba2BiFe4O13) of this homologous series and structural and magnetic characterization of them. Single phase samples with compositions Pb2Ba2BiFe4ScO13 and Pb2Ba2BiFe5O13 were prepared by conventional solid state synthesis. Crystal structures of obtained compounds were determined using synchrotron X-ray powder diffraction, electron diffraction (ED), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique and Mössbauer spectroscopy. ED of these compounds at room temperature (RT) shows A-centered unit-cell and may be indexed in the orthorhombic crystal system with unit-cell parameters a ≈ 5.5Å, b ≈ 4.0Å, c ≈ 27.0 Å that are related to the parameter of the basic perovskite subcell a ≈ 2ap, b ≈ bp, c ≈ 9.7 Å + (n -2 )2ap and space group Ammm as the most symmetric. Such reflections on ED images are slightly split that indicates small monoclinic distortion of orthorhombic structure. Synchrotron experiment for Pb2Ba2BiFe5O13 at different temperatures shows strong anisotropic broadening of such group reflections like 115, 213 and 2010, the same data for Pb2Ba2BiFe4ScO13 were not collected, but we expect the same behavior for it. This broadening don’t allow to obtain adequate profile fitting for RT experiment, so crystal structure of these compounds were determined from high-temperature (900 K) synchrotron experiment, where orthorhombic form is stable. Crystal structure of these compounds is shown on fig. 1, it consists of anion-deficient slab with composition Pb2Fe2O42+ and three octahedral layers with general composition n ABO3 (n = 3, A = Ba, Bi; B = Fe (Fe, Sc)). Mössbauer spectroscopy data were obtained on samples enriched by 57Fe (20%) and shows that all Fe-cations in both substances are presented in the oxidation number +3 in two crystallographic positions in ratio close to 50:50 and 60:40 for Pb2Ba2BiFe4ScO13 and Pb2Ba2BiFe5O13, respectively. Crystal structure of Pb2Ba2BiFe4ScO13 can be refined by two ways with close values of factors Rp, Rwp, RI. In the first one, refinement of occupancies between Sc- and Fe-cations in the perovskite slab leads to strong ordering between them (central octahedral layer is almost fully occupied by Sc3+), but the second refinement shows mixed site occupancies between these cations. Mössbauer spectra for these compounds were measured in wide temperature ranges. Below 370 K for Pb2Ba2BiFe4ScO13 and 555 K for Pb2Ba2BiFe5O13 Mössbauer spectra are magnetically split. The transition from paramagnetic to antiferromagnetic states for both the samples is expanded over the wide temperature range where magnetic and paramagnetic components coexist. Neel temperatures observed from magnetic susceptibility measurements are 350 K for Pb2Ba2BiFe4ScO13 and 623 K for Pb2Ba2BiFe5O13.