RESUMO
Mixed anion oxyhalides with the formula Ca2MnO3X (X = Cl, Br) are synthesized using solid-state reaction methods. These two materials crystallize in a novel structure type due to the small ionic radius of Ca and the strong Jahn-Teller effect of Mn3+. The resulting structure (space group Cmcm) contains one-dimensional chains of MnO4 square planes, with an angle of â¼120° between neighboring planes. At low temperatures, the two materials adopt magnetic arrangements, with ferromagnetic chains coupled antiferromagnetically. On applying a magnetic field, both materials experience spin-flop transitions.
RESUMO
We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.
RESUMO
For the composition (Sr(0.61)Pb(0.18))(Fe(0.75)Mn(0.25))O(2.29), a new modulated crystallographic shear structure, related to perovskite, has been synthesized and structurally characterized by transmission electron microscopy. The structure can be described using a monoclinic supercell with cell parameters a(m) = 27.595(2) A, b(m) = 3.8786(2) A, c(m) = 13.3453(9) A, and beta(m) = 100.126(5) degrees, refined from powder X-ray diffraction data. The incommensurate crystallographic shear phases require an alternative approach using the superspace formalism. This allows a unified description of the incommensurate phases from a monoclinically distorted perovskite unit cell and a modulation wave vector. The structure deduced from the high-resolution transmission electron microscopy and high-angle annular dark-field-scanning transmission electron microscopy images is that of a 1/2[110](p)(203)(p) crystallographic shear structure. The structure follows the concept of a phasoid, with two coexisting variants with the same unit cell. The difference is situated at the translational interface, with the local formation of double (phase 2) or single (phase 1) tunnels, where the Pb cations are likely located.
