Anionic Ordering in Ba₁₅V₁₂S₃₄O₃, Affording Three Oxidation States of Vanadium and a Quasi-One-Dimensional Magnetic Lattice

Anionic Ordering in Ba₁₅V₁₂S₃₄O₃, Affording Three Oxidation States of Vanadium and a Quasi-One-Dimensional Magnetic Lattic

Successive Phase Transitions in Fe²⁺ Ladder Compounds Sr₂Fe₃Ch₂O₃ (Ch = S, Se)

Small single crystals of Sr₂Fe₃Ch₂O₃ (Ch = S, Se) have been synthesized by flux methods, and bulk materials have been obtained by solid state reactions. Both compounds are isostructural to the compound Sr₂Co₃S₂O₃ (space group <i>Pbam</i>), which contains a novel hybrid spin ladder: a combination of a 2-leg rectangular ladder and a necklace ladder. The 2-leg ladder acts as a well-defined magnetic entity, while intimate magnetic coupling to the necklace ladder induces three successive phase transitions in the range of 40–120 K in each composition (Ch = S or Se), as revealed by Mössbauer spectroscopy, thermodynamics, and magnetometry. The complex magnetic behaviors can be explained by the unique spin–lattice topology

Three Oxidation States of Manganese in the Barium Hexaferrite BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉

The coexistence of three valence states of Mn ions, namely, +2, +3, and +4, in substituted magnetoplumbite-type BaFe_{12–<i>x</i>}Mn_<i>x</i>O₁₉ was observed by soft X-ray absorption spectroscopy at the Mn-L_2,3 edge. We infer that the occurrence of multiple valence states of Mn situated in the pristine purely iron­(III) compound BaFe₁₂O₁₉ is made possible by the fact that the charge disproportionation of Mn³⁺ into Mn²⁺ and Mn⁴⁺ requires less energy than that of Fe³⁺ into Fe²⁺ and Fe⁴⁺, related to the smaller effective Coulomb interaction of Mn³⁺ (d⁴) compared to Fe³⁺ (d⁵). The different chemical environments determine the location of the differently charged ions: with Mn³⁺ occupying positions with (distorted) octahedral local symmetry, Mn⁴⁺ ions prefer octahedrally coordinated sites in order to optimize their covalent bonding. Larger and more ionic bonded Mn²⁺ ions with a spherical charge distribution accumulate at tetrahedrally coordinated sites. Simulations of the experimental Mn-L_2,3 XAS spectra of two different samples with <i>x</i> = 1.5 and <i>x</i> = 1.7 led to Mn²⁺:Mn³⁺:Mn⁴⁺ atomic ratios of 0.16:0.51:0.33 and 0.19:0.57:0.24

Canted Antiferromagnetism on Rectangular Layers of Fe²⁺ in Polymorphic CaFeSeO

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca[FeSe2/2O2/2]∞2 can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [FeSe2/2O2/2]2−∞2 layers. The two polymorphs have similar unit cells but different crystal symmetries (<i>Cmc</i>2₁ and <i>Pnma</i>), of which the former is non-centrosymmetric. Fe is divalent (d⁶) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (<i>T</i>_N = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth