Charge Disproportionation and Magnetoresistivity in a Double Perovskite with Alternate Fe4+(d4) and Mn4+(d3) Layers

Exploring the Limits of Fe-Rich Chemistries in Na-Based CaFe2O4-Type Postspinel Oxides

Structural trends, physical properties, and electrochemical performances of the NaFexRu2-xO4 system have been investigated. Synthesis attempts using both conventional solid-state routes and high-pressure methods were explored for the compositional range 1.0 ≤ x ≤ 1.67. Based on Rietveld refinements against powder X-ray diffraction data, analyses of 57Fe Mössbauer spectroscopy data, and elemental analysis by electron microprobe, the existence of a confined compositional solid solution (1 ≤ x ≤ 1.3) adopting the CaFe2O4-type postspinel structure is demonstrated. This is contrasted with the NaFexTi2-xO4 system, for which no evidence of a solid solution is observed. However, for all explored synthetic routes of NaFexRu2-xO4 compositions, a trivalent iron oxidation state is maintained. Structural analysis and qualitative bond valence energy landscape models reveal that the progressive integration of iron into the postspinel framework results in narrowed sodium ion diffusion channels, restricting electrochemical deintercalation of sodium. Consequently, the CaFe2O4-type iron-rich compounds explored in this study demonstrate limited potential as positive electrode materials for sodium batteries. It is expected that this fundamental insight will help guide the exploration of alternative NaM2O4-based positive electrode materials with similar structure types.

Spin reorientation in antiferromagnetic Dy2FeCoO6 double perovskite

We explored the electronic and magnetic properties of the lanthanide double perovskite Dy₂FeCoO₆ by combining magnetization, Raman and Mössbauer spectroscopy and neutron diffraction along with density functional theory (DFT) calculations. Our magnetization measurements revealed two magnetic phase transitions in Dy₂FeCoO₆. First, a paramagnetic to antiferromagnetic transition at T _N = 248 K and subsequently, a spin reorientation transition at T _SR = 86 K. In addition, a field-induced magnetic phase transition with a critical field of H _c ≈ 20 kOe is seen at 2 K. Neutron diffraction data suggested cation-disordered orthorhombic structure for Dy₂FeCoO₆ in Pnma space group which is supported by Raman scattering results. The magnetic structures ascertained through representational analysis indicate that at T _N, a paramagnetic state is transformed to Γ₅(Cx, Fy, Az) antiferromagnetic structure while, at T _SR, Fe/Co moments undergo a spin reorientation to Γ₃(Gx, Ay, Fz). The refined magnetic moment of (Fe/Co) is 1.47(4) μ _B at 7 K. The antiferromagnetic structure found experimentally is supported through the DFT calculations which predict an insulating electronic state in Dy₂FeCoO₆.

Ba3(Cr0.97(1)Te0.03(1))2TeO9: in Search of Jahn-Teller Distorted Cr(II) Oxide

A novel 6H-type hexagonal perovskite Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ was prepared at high pressure (6 GPa) and temperature (1773 K). Both transmission electron microscopy and synchrotron powder X-ray diffraction data demonstrate that Ba₃(Cr_0.97(1)Te_0.03(1))₂TeO₉ crystallizes in P6₃/mmc with face-shared (Cr_0.97(1)Te_0.03(1))O₆ octahedral pairs interconnected with TeO₆ octahedra via corner-sharing. Structure analysis shows a mixed Cr²⁺/Cr³⁺ valence state with ∼10% Cr²⁺. The existence of Cr²⁺ in Ba₃(Cr²⁺_0.10(1)Cr³⁺_0.87(1)Te⁶⁺_0.03)₂TeO₉ is further evidenced by X-ray absorption near-edge spectroscopy. Magnetic properties measurements show a paramagnetic response down to 4 K and a small glassy-state curvature at low temperature. In this work, the octahedral Cr²⁺O₆ component is stabilized in an oxide material for the first time; the expected Jahn-Teller distortion of high-spin (d⁴) Cr²⁺ is not found, which is attributed to the small proportion of Cr²⁺ (∼10%) and the face-sharing arrangement of CrO₆ octahedral pairs, which structurally disfavor axial distortion.