Magnetic Structure and Properties of the Rechargeable Battery Insertion Compound Na₂FePO₄F

The magnetic structure and properties of sodium iron fluorophosphate Na₂FePO₄F (space group <i>Pbcn</i>), a cathode material for rechargeable batteries, were studied using magnetometry and neutron powder diffraction. The material, which can be described as a quasi-layered structure with zigzag Fe-octahedral chains, develops a long-range antiferromagnetic order below ∼3.4 K. The magnetic structure is rationalized as a super-exchange-driven ferromagnetic ordering of chains running along the <i>a</i>-axis, coupled antiferromagnetically by super-super-exchange via phosphate groups along the <i>c</i>-axis, with ordering along the <i>b</i>-axis likely due to the contribution of dipole–dipole interactions

Synthesis and Characterization of the Crystal and Magnetic Structures and Properties of the Hydroxyfluorides Fe(OH)F and Co(OH)F

The title compounds were synthesized by a hydrothermal route from a 1:1 molar ratio of lithium fluoride and transition-metal acetate in an excess of water. The crystal structures were determined using a combination of powder and/or single-crystal X-ray and neutron powder diffraction (NPD) measurements. The magnetic structure and properties of Co­(OH)F were characterized by magnetic susceptibility and low-temperature NPD measurements. M­(OH)F (M = Fe and Co) crystallizes with structures related to diaspore-type α-AlOOH, with the <i>Pnma</i> space group, <i>Z</i> = 4, <i>a</i> = 10.471(3) Å, <i>b</i> = 3.2059(10) Å, and <i>c</i> = 4.6977(14) Å and <i>a</i> = 10.2753(3) Å, <i>b</i> = 3.11813(7) Å, and <i>c</i> = 4.68437(14) Å for the iron and cobalt phases, respectively. The structures consist of double chains of edge-sharing M­(F,O)₆ octahedra running along the <i>b</i> axis. These infinite chains share corners and give rise to channels. The protons are located in the channels and form O–H···F <i>bent</i> hydrogen bonds. The magnetic susceptibility indicates an antiferromagnetic ordering at ∼40 K, and the NPD measurements at 3 K show that the ferromagnetic rutile-type chains with spins parallel to the short <i>b</i> axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of goethite α-FeOOH

Synthesis and Characterization of the Crystal and Magnetic Structures and Properties of the Hydroxyfluorides Fe(OH)F and Co(OH)F

The title compounds were synthesized by a hydrothermal route from a 1:1 molar ratio of lithium fluoride and transition-metal acetate in an excess of water. The crystal structures were determined using a combination of powder and/or single-crystal X-ray and neutron powder diffraction (NPD) measurements. The magnetic structure and properties of Co­(OH)F were characterized by magnetic susceptibility and low-temperature NPD measurements. M­(OH)F (M = Fe and Co) crystallizes with structures related to diaspore-type α-AlOOH, with the <i>Pnma</i> space group, <i>Z</i> = 4, <i>a</i> = 10.471(3) Å, <i>b</i> = 3.2059(10) Å, and <i>c</i> = 4.6977(14) Å and <i>a</i> = 10.2753(3) Å, <i>b</i> = 3.11813(7) Å, and <i>c</i> = 4.68437(14) Å for the iron and cobalt phases, respectively. The structures consist of double chains of edge-sharing M­(F,O)₆ octahedra running along the <i>b</i> axis. These infinite chains share corners and give rise to channels. The protons are located in the channels and form O–H···F <i>bent</i> hydrogen bonds. The magnetic susceptibility indicates an antiferromagnetic ordering at ∼40 K, and the NPD measurements at 3 K show that the ferromagnetic rutile-type chains with spins parallel to the short <i>b</i> axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of goethite α-FeOOH

Largely Enhanced Mobility in Trilayered LaAlO₃/SrTiO₃/LaAlO₃ Heterostructures

LaAlO₃ (LAO)/SrTiO₃ (STO)/LaAlO₃ (LAO) heterostructures were epitaxially deposited on TiO₂-terminated (100) SrTiO₃ single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 10⁴ cm²/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10^–5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 10³ cm²/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO₂ interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the first-principle calculation. Our results will pave the way for designing high-mobility oxide nanoelectronic devices based on LAO/STO heterostructures

Key Role of Bismuth in the Magnetoelastic Transitions of Ba₃BiIr₂O₉ and Ba₃BiRu₂O₉ As Revealed by Chemical Doping

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉ has been confirmed by systematically replacing bismuth with La³⁺ and Ce⁴⁺. Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉ have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L₃-edge spectrum of Ba₃BiRu₂O₉). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba₃BiRu₂O₉ and Ba₃BiIr₂O₉, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials