The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8)

A correlated ferromagnetic polar metal by design

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature-magnetic field phase space, arising from dipole-induced Rashba spin-orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

3D to 2D Magnetic Ordering of Fe3+ Oxides Induced by Their Layered Perovskite Structure

The antiferromagnetic behavior of Fe³⁺ oxides of composition RE_1.2Ba_1.2Ca_0.6Fe₃O₈, RE_2.2Ba_3.2Ca_2.6Fe₈O₂₁, and REBa₂Ca₂Fe₅O₁₃ (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe³⁺ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe³⁺ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Néel temperature (T_N) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above T_N. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers.