CsFe₃(SeO₃)₂F₆ with <i>S</i> = 5/2 Cube Tile Lattice

A layered iron selenite fluoride CsFe₃(SeO₃)₂F₆ <b>1</b> was hydrothermally synthesized. Single-crystal X-ray diffraction studies show that <b>1</b> has a trigonal (<i>P</i>3̅<i>m</i>1) lattice, where [Fe₃(SeO₃)₂F₆]⁻ blocks of three iron sublayers are separated by Cs cations. Within the block, only Fe(2)­F₆ and Fe(1)­O₃F₃ octahedra are magnetically connected via superexchange Fe(1)<i>−</i>F<i>–</i>Fe­(2) pathways, giving an <i>S</i> = 5/2 cube tile (dice) lattice. At low magnetic field, <b>1</b> exhibits an antiferromagnetic transition at ∼130 K, where ferrimagnetic cube tile layers are arranged in a staggered manner. At low temperatures, we observed a field-induced transition to a ferrimagnetic state with a one-third magnetization plateau

Mixed-Spin Diamond Chain Cu₂FePO₄F₄(H₂O)₄ with a Noncollinear Spin Order and Possible Successive Phase Transitions

A diamond spin chain system, one of the one-dimensional frustrated lattices, is known to exhibit novel properties, but experimental studies have been exclusively confined to materials with a single spin component. Here, we report on the synthesis, structure, and magnetic properties of a new diamond chain compound Cu₂FePO₄F₄(H₂O)₄ <b>1</b> composed of mixed-spins of Cu²⁺ (<i>S</i> = 1/2 × 2) and Fe³⁺ (<i>S</i> = 5/2). Compound <b>1</b> crystallizes in the space group <i>C</i>2/<i>c</i> of the monoclinic crystal system with <i>a</i> = 7.7546(4) Å, <i>b</i> = 12.1290(6) Å, <i>c</i> = 9.9209(6) Å, β = 105.29(1)°, and <i>Z</i> = 4. DC magnetization, Mössbauer spectroscopy, and heat capacity measurements revealed an antiferromagnetic order at 11.3 K with a small ferromagnetic component. It is suggested that ferrimagnetic diamond chains are arranged in an antiferromagnetic fashion (i.e., [...Fe(↑)-2Cu(↓↓)-Fe(↑)...] and [...Fe(↓)-2Cu(↑↑)-Fe(↓)...]) within the <i>ab</i> plane to cancel net magnetization, and the spin orientation of the diamond chains changes alternately along the <i>c</i> axis due to the magnetic anisotropy, leading to a noncollinear spin order. Furthermore, another anomaly is observed in the heat capacity at around 3 K, suggesting a successive magnetic transition or crossover due to competing magnetic interactions

Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis

The problem of activating N₂ and its subsequent hydrogenation to form NH₃ has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N₂, but without catalytic formation of NH₃. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH₂ and BaTiO_2.5H_0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH₃ under H₂/N₂ flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·^–1·h^–1) almost comparable to conventional supported Ru catalysts such as Cs–Ru/MgO or Ru/BaTiO₃ that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activity for Ti, making this a previously overlooked element, but our results show that by introducing hydride, the repertoire of heterogeneous catalysts can be expanded to include formerly unexamined compositions without resorting to precious metals

Selective Preparation of Macroporous Monoliths of Conductive Titanium Oxides Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6)

Monolithic conductive titanium oxides Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6) with well-defined macropores have been successfully prepared as a single phase, via reduction of a macroporous TiO₂ precursor monolith using zirconium getter. Despite substantial removal of oxide ions, all the reduced monoliths retain the macropore properties of the precursor, i.e., uniform pore size distribution and pore volume. Furthermore, compared to commercial porous Ebonex (shaped conductive Ti_<i>n</i>O_{2<i>n</i>–1}), the bulk densities (1.8 g cm^–3) are half, and the porosities (60%) are about 3 times higher. The obtained Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6) macroporous monoliths could find applications as electrodes for many electrochemical reactions

Layered Perovskite Oxychloride Bi₄NbO₈Cl: A Stable Visible Light Responsive Photocatalyst for Water Splitting

Mixed anion compounds are expected to be a photocatalyst for visible light-induced water splitting, but the available materials have been almost limited to oxynitrides. Here, we show that an oxychrolide Bi₄NbO₈Cl, a single layer Sillen–Aurivillius perovskite, is a stable and efficient O₂-evolving photocatalyst under visible light, enabling a Z-scheme overall water splitting by coupling with a H₂-evolving photocatalyst (Rh-doped SrTiO₃). It is found that the valence band maximum of Bi₄NbO₈Cl is unusually high owing to highly dispersive O-2p orbitals (not Cl-3p orbitals), affording the narrow band gap and possibly the stability against water oxidation. This study suggests that a family of Sillen–Aurivillius perovskite oxyhalides is a promising system to allow a versatile band level tuning for establishing efficient and stable water-splitting under visible light

High-Level Doping of Nitrogen, Phosphorus, and Sulfur into Activated Carbon Monoliths and Their Electrochemical Capacitances

The present report demonstrates a new technique for doping heteroatoms (nitrogen, phosphorus, and sulfur) into carbon materials via a versatile post-treatment. The heat-treatment of carbon materials with a reagent, which is stable at ambient temperatures and evolves reactive gases on heating, in a vacuum-closed tube allows the introduction of various heteroatom-containing functional groups into a carbon matrix. In addition, the sequential doping reactions give rise to dual- and triple-heteroatom-doped carbons. The pore properties of the precursor carbon materials are preserved through each heteroatom doping process, which indicates that independent tuning of heteroatom doping and nanostructural morphology can be achieved in various carbon materials. The electrochemical investigation on the undoped and doped carbon monolithic electrodes applied to supercapacitors provides insights into the effects of heteroatom doping on electrochemical capacitance

A Nearly Ideal One-Dimensional <i>S</i> = 5/2 Antiferromagnet FeF₃(4,4′-bpy) (4,4′-bpy =4,4′-bipyridyl) with Strong Intrachain Interactions

An ideal one-dimensional (1D) magnet is expected to show exotic quantum phenomena. For compounds with larger <i>S</i> (<i>S</i> = 3/2, 2, 5/2, ...), however, a small interchain interaction <i>J</i>′ tends to drive a conventional long-range ordered (LRO) state. Here, a new layered structure of FeF₃(4,4′-bpy) (4,4′-bpy = 4,4′-bipyridyl) with novel <i>S</i> = 5/2 (Fe³⁺) chains has been hydrothermally synthesized by using 4,4′-bpy to separate chains. The temperature-dependent susceptibility exhibits a broad maximum at high as 164 K, suggesting a fairly strong Fe–F–Fe intrachain interaction <i>J</i>. However, no anomaly associated with a LRO is seen in both magnetic susceptibility and specific heat even down to 2 K. This indicates an extremely small <i>J</i>′ with <i>J</i>′/<i>J</i> < 3.2 × 10^–5, making this new material a nearly ideal 1D antiferromagnet. Mössbauer spectroscopy at 2.7 K reveals a critical slowing down of the 1D fluctuations toward a possible LRO at lower temperatures

Sr₂FeO₃ with Stacked Infinite Chains of FeO₄ Square Planes

The synthesis of Sr₂FeO₃ through a hydride reduction of the Ruddlesden–Popper layered perovskite Sr₂FeO₄ is reported. Rietveld refinements using synchrotron and neutron powder diffraction data revealed that the structure contains corner-shared FeO₄ square-planar chains running along the [010] axis, being isostructural with Sr₂CuO₃ (<i>Immm</i> space group). Fairly strong Fe–O–Fe and Fe–Fe interactions along [010] and [100], respectively, make it an <i>S</i> = 2 quasi two-dimensional (2D) rectangular lattice antiferromagnet. This compound represents the end-member (<i>n</i> = 1) of the serial system Sr_<i>n</i>+1Fe_<i>n</i>O_2<i>n</i>+1, together with previously reported Sr₃Fe₂O₅ (<i>n</i> = 2) and SrFeO₂ (<i>n</i> = ∞), thus giving an opportunity to study the 2D-to-3D dimensional crossover. Neutron diffraction and Mössbauer spectroscopy show the occurrence of <i>G</i>-type antiferromagnetic order below 179 K, which is, because of dimensional reduction, significantly lower than those of the other members, 296 K in Sr₃Fe₂O₅ and 468 K in SrFeO₂. However, the temperature dependence of magnetic moment shows a universal behavior

From Tetrahedral to Octahedral Iron Coordination: Layer Compression in Topochemically Prepared FeLa₂Ti₃O₁₀

Synthesis, characterization, and thermal modification of the new layered perovskite FeLa₂Ti₃O₁₀ have been studied. FeLa₂Ti₃O₁₀ was prepared by ion exchange of the triple-layered Ruddlesden–Popper phase Li₂La₂Ti₃O₁₀ with FeCl₂ at 350 °C under static vacuum. Rietveld refinement on synchrotron X-ray diffraction data indicates that the new phase is isostructural with CoLa₂Ti₃O₁₀, where Fe^II cations occupy slightly compressed/flattened interlayer tetrahedral sites. Magnetic measurements on FeLa₂Ti₃O₁₀ display Curie–Weiss behavior at high temperatures and a spin-glass transition at lower temperatures (<30 K). Thermal treatment in oxygen shows that FeLa₂Ti₃O₁₀ undergoes a significant cell contraction (Δ<i>c</i> ≈ −2.7 Å) with a change in the oxidation state of iron (Fe²⁺ to Fe³⁺); structural analysis and Mössbauer studies indicate that upon oxidation the local iron environment goes from tetrahedral to octahedral coordination with some deintercalation of iron as Fe₂O₃ to produce Fe_0.67La₂Ti₃O₁₀