Crystal-Like Atomic Arrangement and Optical Properties of 25La2O3-75MoO3 Binary Glasses Composed of Isolated MoO42.

Transparent and brown La2O3-MoO3 binary glasses were prepared in bulk form using a levitation technique. The glass-forming range was limited, with the primary composition being approximately 25 mol % La2O3. The 25La2O3-75MoO3 glass exhibited a clear crystallization at 546 °C, while determining its glass transition temperature was difficult. Notably, despite its amorphous nature, the glass possessed a density and packing density comparable to those of crystalline La2Mo3O12. X-ray absorption fine structure and Raman scattering analyses revealed that the glass structure closely resembles La2Mo3O12 due to the presence of isolated MoO42- units, whereas disordered atomic arrangement around La atoms was confirmed. The glass demonstrated transparency ranging from 378 to 5500 nm, and the refractive index at 1.0 µm was estimated to be 2.0. The optical bandgap energy was 3.46 eV, which was slightly smaller than that of La2Mo3O12. Additionally, the glass displayed a transparent region ranging from 6.5 to 8.0 µm. This occurrence results from the decreased diversity of MoOn units and connectivity of Mo-O-Mo, which resulted in the reduced overlap of multiphonon absorption. This glass formation, with its departure from conventional glass-forming rules, resulted in distinctive glasses with crystal-like atomic arrangements.

Cascade Charge Transitions of Unusually High and Mixed Valence Fe3.5+ in the A-Site Layer-Ordered Double Perovskite SmBaFe2O6.

The A-site layer-ordered double perovskite SmBaFe2O6 was obtained by topochemical of oxidizing A-site layer-ordered SmBaFe2O5 in ozone at a low temperature. The compound contained unusually high and mixed valence Fe3.5+ and was found to show cascade charge transitions, described as SmBaFe3.5+2O6 → SmBa(Fe3+Fe4+)O6 → SmBa(Fe3+Fe(4-δ)+0.5Fe(4+δ)+0.5)O6 → SmBa(Fe3+1.5Fe5+0.5)O6, to relieve its electronic instability. The first Verwey-like charge-order transition occurred at 340 K and was accompanied by a significant structural change and a sudden increase in magnetic susceptibility. The following transition was the charge disproportionation of metastable Fe4+ to Fe3+ and Fe5+, and each of the spins resulted in the antiferromagnetic ground state. The most plausible charge-ordered patterns are proposed based on the electrostatic lattice energy calculations.

Ultralong Distance Hydrogen Spillover Enabled by Valence Changes in a Metal Oxide Surface.

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeOx (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeOx support extends over long distances (∼600 µm). Furthermore, the hydrogen-spillover-induced reduction of Fe4+ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports' surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties.

Tripodal Triazatruxene Derivative as a Face-On Oriented Hole-Collecting Monolayer for Efficient and Stable Inverted Perovskite Solar Cells.

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

Magnetic Control of Cells by Chemical Fabrication of Melanin.

Melanin is an organic material biosynthesized from tyrosine in pigment-producing cells. The present study reports a simple method to generate tailored functional materials in mammalian cells by chemically fabricating intracellular melanin. Our approach exploits synthetic tyrosine derivatives to hijack the melanin biosynthesis pathway in pigment-producing cells. Its application was exemplified by synthesizing and using a paramagnetic tyrosine derivative, m-YR, which endowed melanoma cells with responsiveness to external magnetic fields. The mechanical force generated by the magnet-responsive melanin forced the cells to elongate and align parallel to the magnetic power lines. Critically, even non-pigment cells were similarly remote-controlled by external magnetic fields once engineered to express tyrosinase and treated with m-YR, suggesting the versatility of the approach. The present methodology may potentially provide a new avenue for mechanobiology and magnetogenetic studies and a framework for magnetic control of specific cells.

High-Pressure Synthesized Perovskite CdMnO3 with C-Type Antiferromagnetic Spin Configuration.

CdMnO3 had not been previously reported and was a missing piece in the A2+Mn4+O3 series. We succeeded in synthesizing this compound by a high-pressure method and confirmed that it is crystallized in a distorted perovskite structure with a Cd2+Mn4+O3 charge configuration. The obtained insulating CdMnO3 exhibits an antiferromagnetic transition at about 86 K. First-principles calculations revealed that the Mn4+ (t2g3) spins form a C-type antiferromagnetic structure, which is in sharp contrast to the G-type antiferromagnetism in the isostructural and isoelectronic CaMnO3. Significant overlap of the Mn-3d and O(2)-2p orbitals produces distorted octahedra with a large Mn-O(1)-Mn tilt and induces antiferromagnetic couplings in the ac plane and the ferromagnetic couplings along the b axis.

A New Cation-Ordered Structure Type with Multiple Thermal Redistributions in Co2 InSbO6.

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO3 ) and LiNbO3 type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO3 derivative with R32 symmetry has never been observed. Here we show that Co2 InSbO6 recovered from high pressure has a new, ordered-R32 A2 BCO6 variant of the corundum structure. Co2 InSbO6 is also remarkable for showing two cation redistributions, to (Co0.5 In0.5 )2 CoSbO6 and then Co2 InSbO6 variants of the ordered-LiNbO3 A2 BCO6 structure on heating. The cation distributions change magnetic properties as the final ordered-LiNbO3 product has a sharp ferrimagnetic transition unlike the initial ordered-R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation-redistribution pathways, and may enable ABO3 materials with the R32 structure to be discovered.

LiNbO3 -Type Polar Antiferromagnet InVO3 Synthesized under High-Pressure Conditions.

The ambient pressure cation disordered InVO3 bixbyite has been predicted to form a GdFeO3 -type perovskite phase under high pressure and high temperature. Contrary to the expectation, InVO3 was found to crystallize in the polar LiNbO3 -type structure with a calculated spontaneous polarization as large as 74 µC cm-2 . Antiferromagnetic coupling of V3+ magnetic moments and a cooperative magnetic ground state below about 10 K coupled with a polar structure suggest an intriguing ground state of the novel LiNbO3 -type high-pressure InVO3 structure.

Geometrical Spin Frustration and Monoclinic-Distortion-Induced Spin Canting in the Double Perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with Unusually High Valence Fe5.

We discovered new B-site-ordered double perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with most likely unusually high valence Fe5+, which was stabilized by strong oxidizing high-pressure synthesis. Despite large antiferromagnetic interactions between Fe5+ spins in these compounds, the magnetic ordering is strongly suppressed due to the geometrical frustration of Fe5+ located in a face-centered cubic lattice. In addition, canted magnetic structures are stabilized only in those with Ln = Sm and Eu, which is most likely due to significant Dzyaloshinskii Moriya interaction caused by large monoclinic structural distortion. These results provide a deep understanding of the structure-property relationships in geometrically frustrated B-site-ordered double perovskites.

Layered Hexagonal Perovskite Oxides 21R Ba7Fe5Ge2O20 and 12H Ba6Fe3Ge3O17.

Two hexagonal-perovskite-structure oxides, 21R Ba7Fe5Ge2O20 and 12H Ba6Fe3Ge3O17, were obtained by synthesis with a high-pressure and high-temperature technique. The Fe-containing hexagonal-perovskite-structure units are sandwiched by nonmagnetic GeO4 tetrahedral layers in the structures, and thus both compounds show two-dimensional ferrimagnetic behaviors due to intra- and interunit magnetic interactions. 21R Ba7Fe5Ge2O20 has the ionic formula Ba7Fe123+Fe24+Fe324+Ge424+O20 at room temperature, and unusually high valence Fe4+ in the trimers undergoes charge disproportionation, Fe24+ + 2Fe34+ → Fe2(4+2δ)+ + 2Fe3(4-δ)+, at low temperatures. In contrast, 12H Ba6Fe3Ge3O17 with ionic formula Ba6Fe123+(Fe20.54+Ge20.54+)2Ge324+O17 does not show a charge transition.

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

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, 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 (TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers.

Successive and Site-Selective Oxygen Release from B-Site-Layer-Ordered Double Perovskite Ca2FeMnO6 with Unusually High Valence Fe4.

B-site-layer-ordered double perovskite Ca2FeMnO6 with unusually high valence Fe4+ was found to exhibit unusual oxygen-release behaviors, contrasting with those of the B-site-disordered perovskite having the identical chemical composition. During heating, the B-site-layer-ordered compound shows a stepwise oxygen release with successive valence changes from Fe4+ to Fe3+ through an intermediate Fe3.5+, whereas the B-site-disordered compound releases oxygen in a single step. The oxygen in Ca2FeMnO6 is released only from the two-dimensional Fe layers, and this selective oxygen release stabilizes the intermediate Fe3.5+ phase with in-plane-oxygen-vacancy ordering. Therefore, the B-site order/disorder strongly affects the oxygen-release behaviors associated with the oxygen-vacancy ordering.

Conversion of a Defect Pyrochlore into a Double Perovskite via High-Pressure, High-Temperature Reduction of Te6.

High-pressure, high-temperature reaction conditions can be useful to stabilize metastable polymorphs of complex transition metal oxides. We successfully prepare a new defect pyrochlore Pb2FeTeO6.5 with B-site disordered Fe and Te cations under ambient conditions. Treatment of this material under 8 GPa and 950 °C results in a reductive transformation into the B-site cation-ordered double perovskite Pb2FeTeO6. Mössbauer and EELS spectroscopy confirm the iron cations are in the +3 oxidation state in both phases indicating that this transformation proceeds via reduction of the tellurium cations under apparently oxidizing conditions. This reaction demonstrates that for a suitably chosen system, it is possible to carry out chemical reactions under pressure in unexpected ways.

Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction.

A key challenge in carrying out an efficient oxygen reduction reaction (ORR) is the design of a highly efficient electrocatalyst that must have fast kinetics, low cost and high stability for use in an energy-conversion device (e.g. metal-air batteries). Herein, we developed a platinum-free ORR electrocatalyst with a high surface area and pore volume via a molten salt method along with subsequent KOH activation. The activation treatment not only increases the surface area to 940.8 m2 g-1 by generating lots of pores, but also promotes the formation of uniform Fe3C nanoclusters within the atomic dispersed Fe-Nx carbon matrix in the final material (A-FeNC). A-FeNC displays excellent activity and long-term stability for the ORR in alkaline media, and shows a greater half-wave potential (0.85 V) and faster kinetics toward four-electron ORR as compared to those of 20 wt% Pt/C (0.83 V). As a cathode catalyst for the Zn-air battery, A-FeNC presents a peak power density of 102.2 mW cm-2, higher than that of the Pt/C constructed Zn-air battery (57.2 mW cm-2). The superior ORR catalytic performance of A-FeNC is ascribed to the increased exposure of active sites, active single-atom Fe-N-C centers, and enhancement by Fe3C nanoclusters.

Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and Ni Ions.

BaFe xNi1- xO3 with end members of BaNiO3 ( x = 0) and BaFeO3 ( x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba4Fe3NiO12 ( x = 0.75), that adopts the 12R perovskite structure with the space group R3̅ m ( a = 5.66564(7) Å and c = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. The magnetic properties of the compound are also discussed.

Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5.

A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R3̅c crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3+ spins align antiferromagnetically and one-fourth of the Fe3+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide.

Strong correlations between electrons, spins and lattices--stemming from strong hybridization between transition metal d and oxygen p orbitals--are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.

Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa2Fe3O9 with Unusually High Valence Fe3.67.

A-site-ordered layer-structured perovskite LaCa2Fe3O9 with unusually high valence Fe3.67+ was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa2Fe3O8. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows charge disproportionation of Fe3+ and Fe5+ first along the layer-stacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers. The two-dimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ charge ordering pattern. Below 170 K, the charge ordering pattern changes, and the 2:1 charge-disproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible.

Crystal Structures at Atomic Resolution of the Perovskite-Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6.

Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mössbauer spectroscopy, which also reveals the different magnetic behavior of these oxides.

Successive Charge Transitions of Unusually High-Valence Fe3.5+ : Charge Disproportionation and Intermetallic Charge Transfer.

A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5 Bi0.5 FeO3 is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.