Structure Transformation of Methylammonium Polyoxomolybdates via In-Solution Acidification and Solid-State Heating from Methylammonium Monomolybdate and Application as Negative Staining Reagents for Coronavirus Observation

We prepared polyoxomolybdates with methylammonium countercations from methylammonium monomolybdate, (CH3NH3)2[MoO4], through two dehydrative condensation methods, acidifying in the aqueous solution and solid-state heating. Discrete (CH3NH3)10[Mo36O112(OH)2(H2O)14], polymeric ((CH3NH3)8[Mo36O112(H2O)14])n, and polymeric ((CH3NH3)4[γ-Mo8O26])n were selectively isolated via pH control of the aqueous (CH3NH3)2[MoO4] solution. The H2SO4-acidified solution of pH < 1 produced "sulfonated α-MoO3", polymeric ((CH3NH3)2[(MoO3)3(SO4)])n. The solid-state heating of (CH3NH3)2[MoO4] in air released methylamine and water to produce several methylammonium polyoxomolybdates in the sequence of discrete (CH3NH3)8[Mo7O24-MoO4], discrete (CH3NH3)6[Mo7O24], discrete (CH3NH3)8[Mo10O34], and polymeric ((CH3NH3)4[γ-Mo8O26])n, before their transformation into molybdenum oxides such as hexagonal-MoO3 and α-MoO3. Notably, some of their polyoxomolybdate structures were different from polyoxomolybdates produced from ammonium molybdates, such as (NH4)2[MoO4] or (NH4)6[Mo7O24], indicating that countercation affected the polyoxomolybdate structure. Moreover, among the tested polyoxomolybdates, (CH3NH3)6[Mo7O24] was the best negative staining reagent for the observation of the SARS-CoV-2 virus using transmission electron microscopy.

Mechanochemical Synthesis of Perovskite Oxyhydrides: Insights from Shear Modulus

Perovskite oxyhydrides have attracted recent attention due to their intriguing properties such as ionic conductivity and catalysis, but their repertoire is still restricted compared to perovskite oxynitrides and oxyfluorides. Historically, perovskite oxyhydrides have been prepared mostly by topochemical reactions and high-pressure (HP) reactions, while in this study, we employed a mechanochemical (MC) approach, which enables the synthesis of a series of ABO2H-type oxyhydrides, including those with the tolerance factor (t) much smaller than 1 (e.g., SrScO2H with t = 0.936) which cannot be obtained by HP synthesis. The octahedral tilting, often present in perovskite oxides, does not occur, suggesting that the lack of π-symmetry of the H 1s orbital and the large polarization destabilize tilted low-symmetry structures. Interestingly, SrCrO2H (t = 0.997), previously reported with the HP method, was not achieved with the MC method. A comparative analysis revealed a correlation between the feasibility of MC reactions and the (calculated) shear modulus of the starting reagents (binary oxides and hydrides). Notably, this indicator is not exclusive to oxyhydride perovskites but extends to oxide perovskites (SrMO3). This study demonstrates that MC synthesis offers unique opportunities not only to expand the compositional space in oxyhydrides in various structural types but also to provide a guide for the choice of starting materials for the synthesis of other compounds.

Anionic Sublattices in Halide Solid Electrolytes: A Case Study with the High-Pressure Phase of Li3ScCl6

The Li3MX6 compounds (M=Sc, Y, In; X=Cl, Br) are known as promising ionic conductors due to their compatibility with typical metal oxide cathode materials. In this study, we have successfully synthesized γ-Li3ScCl6 using high pressure for the first time in this family. Structural analysis revealed that the high-pressure polymorph crystallizes in the polar and chiral space group P63mc with hexagonal close-packing (hcp) of anions, unlike the ambient-pressure α-Li3ScCl6 and its spinel analog with cubic closed packing (ccp) of anions. Investigation of the known Li3MX6 family further revealed that the cation/anion radius ratio, rM/rX, is the factor that determines which anion sublattice is formed and that in γ-Li3ScCl6, the difference in compressibility between Sc and Cl exceeds the ccp rM/rX threshold under pressure, enabling the ccp-to-hcp conversion. Electrochemical tests of γ-Li3ScCl6 demonstrate improved electrochemical reduction stability. These findings open up new avenues and design principles for lithium solid electrolytes, enabling routes for materials exploration and tuning electrochemical stability without compositional changes or the use of coatings.

Construction of Ideal One-Dimensional Spin Chains by Topochemical Dehydration/Rehydration Route

One-dimensional (1D) Heisenberg antiferromagnets are of great interest due to their intriguing quantum phenomena. However, the experimental realization of such systems with large spin S remains challenging because even weak interchain interactions induce long-range ordering. In this study, we present an ideal 1D S = 5/2 spin chain antiferromagnet achieved through a multistep topochemical route involving dehydration and rehydration. By desorbing three water molecules from (2,2'-bpy)FeF3(H2O)·2H2O (2,2'-bpy = 2,2'-bipyridyl) at 150 °C and then intercalating two water molecules at room temperature (giving (2,2'-bpy)FeF3·2H2O 1), the initially isolated FeF3ON2 octahedra combine to form corner-sharing FeF4N2 octahedral chains, which are effectively separated by organic and added water molecules. Mössbauer spectroscopy reveals significant dynamical fluctuations down to 2.7 K, despite the presence of strong intrachain interactions. Moreover, results from electron spin resonance (ESR) and heat capacity measurements indicate the absence of long-range order down to 0.5 K. This controlled topochemical dehydration/rehydration approach is further extended to (2,2'-bpy)CrF3·2H2O with S = 3/2 1D chains, thus opening the possibility of obtaining other low-dimensional spin lattices.

Double-Layered Perovskite Oxyfluoride Cathodes with High Capacity Involving O-O Bond Formation for Fluoride-Ion Batteries

Developing electrochemical high-energy storage systems is of crucial importance toward a green and sustainable energy supply. A promising candidate is fluoride-ion batteries (FIBs), which can deliver a much higher volumetric energy density than lithium-ion batteries. However, typical metal fluoride cathodes with conversion-type reactions cause a low-rate capability. Recently, layered perovskite oxides and oxyfluorides, such as LaSrMnO4 and Sr3Fe2O5F2, have been reported to exhibit relatively high rate performance and cycle stability compared to typical metal fluoride cathodes with conversion-type reactions, but their discharge capacities (∼118 mA h/g) are lower than those of typical cathodes used in lithium-ion batteries. Here, we show that double-layered perovskite oxyfluoride La1.2Sr1.8Mn2O7-δF2 exhibits (de) intercalation of two fluoride ions to rock-salt slabs and further (de) intercalation of excess fluoride ions to the perovskite layer, leading to a reversible capacity of 200 mA h/g. The additional fluoride-ion intercalation leads to the formation of O-O bond in the structure for charge compensation (i.e., anion redox). These results highlight the layered perovskite oxyfluorides as a new class of active materials for the construction of high-performance FIBs.

Thermal multiferroics in all-inorganic quasi-two-dimensional halide perovskites

Multiferroic materials, particularly those possessing simultaneous electric and magnetic orders, offer a platform for design technologies and to study modern physics. Despite the substantial progress and evolution of multiferroics, one priority in the field remains to be the discovery of unexplored materials, especially those offering different mechanisms for controlling electric and magnetic orders1. Here we demonstrate the simultaneous thermal control of electric and magnetic polarizations in quasi-two-dimensional halides (K,Rb)3Mn2Cl7, arising from a polar-antipolar transition, as evidenced using both X-ray and neutron powder diffraction data. Our density functional theory calculations indicate a possible polarization-switching path including a strong coupling between the electric and magnetic orders in our halide materials, suggesting a magnetoelectric coupling and a situation not realized in oxide analogues. We expect our findings to stimulate the exploration of non-oxide multiferroics and magnetoelectrics to open access to alternative mechanisms, beyond conventional electric and magnetic control, for coupling ferroic orders.

Field-induced bound-state condensation and spin-nematic phase in SrCu2(BO3)2 revealed by neutron scattering up to 25.9 T

In quantum magnetic materials, ordered phases induced by an applied magnetic field can be described as the Bose-Einstein condensation (BEC) of magnon excitations. In the strongly frustrated system SrCu2(BO3)2, no clear magnon BEC could be observed, pointing to an alternative mechanism, but the high fields required to probe this physics have remained a barrier to detailed investigation. Here we exploit the first purpose-built high-field neutron scattering facility to measure the spin excitations of SrCu2(BO3)2 up to 25.9 T and use cylinder matrix-product-states (MPS) calculations to reproduce the experimental spectra with high accuracy. Multiple unconventional features point to a condensation of S = 2 bound states into a spin-nematic phase, including the gradients of the one-magnon branches and the persistence of a one-magnon spin gap. This gap reflects a direct analogy with superconductivity, suggesting that the spin-nematic phase in SrCu2(BO3)2 is best understood as a condensate of bosonic Cooper pairs.

Full-Endoscopic Foraminal Decompression for Foraminal Stenosis Following Osteoporotic Vertebral Fracture in an Elderly Woman Under Local Anesthesia:A Case Report

Osteoporotic vertebral fracture (OVF) is common in the elderly population. In this report, we describe a case with radiculopathy due to foraminal stenosis caused by OVF in a very elderly patient that was treated successfully by full-endoscopic foraminotomy under local anesthesia. The patient was an 89-year-old woman who presented with a chief complaint of left leg pain for 5 years. She visited a couple of hospitals and finally consulted us to determine the exact cause of the pain. Computed tomography scans were obtained and selective nerve root block at L3 was performed. The diagnosis was radiculopathy at L3 due to foraminal stenosis following OVF. The patient had severe heart disease, so we decided to avoid surgery under general anesthesia and planned full-endoscopic spine surgery under local anesthesia. We performed transforaminal full-endoscopic lumbar foraminotomy at L3-L4 to decompress the L3 nerve root. The leg pain disappeared completely immediately after surgery. Postoperative computed tomography confirmed appropriate bone resection. The leg pain did not recur during a year of postoperative follow-up. OVF may cause lumbar radiculopathy as a result of foraminal stenosis, and transforaminal full-endoscopic lumbar foraminotomy under local anesthesia would be the best option in an elderly patient with poor general condition. J. Med. Invest. 71 : 179-183, February, 2024.

Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials

Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n = 2 Ruddlesden-Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors.

Ammonia Synthesis via an Associative Mechanism on Alkaline Earth Metal Sites of Ca3 CrN3 H

Typically, transition metals are considered as the centers for the activation of dinitrogen. Here we demonstrate that the nitride hydride compound Ca3 CrN3 H, with robust ammonia synthesis activity, can activate dinitrogen through active sites where calcium provides the primary coordination environment. DFT calculations also reveal that an associative mechanism is favorable, distinct from the dissociative mechanism found in traditional Ru or Fe catalysts. This work shows the potential of alkaline earth metal hydride catalysts and other related 1 D hydride/electrides for ammonia synthesis.

Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Unveiling new quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2 up to the saturation magnetic field

Under magnetic fields, quantum magnets often undergo exotic phase transitions with various kinds of order. The discovery of a sequence of fractional magnetization plateaus in the Shastry-Sutherland compound SrCu₂(BO₃)₂ has played a central role in the high-field research on quantum materials, but so far this system could only be probed up to half the saturation value of the magnetization. Here, we report the first experimental and theoretical investigation of this compound up to the saturation magnetic field of 140 T and beyond. Using ultrasound and magnetostriction techniques combined with extensive tensor-network calculations (iPEPS), several spin-supersolid phases are revealed between the 1/2 plateau and saturation (1/1 plateau). Quite remarkably, the sound velocity of the 1/2 plateau exhibits a drastic decrease of -50%, related to the tetragonal-to-orthorhombic instability of the checkerboard-type magnon crystal. The unveiled nature of this paradigmatic quantum system is a new milestone for exploring exotic quantum states of matter emerging in extreme conditions.

Potassium-rich antiperovskites K3HTe and K3FTe and their structural relation to lithium and sodium counterparts

Unlike perovskite oxides, antiperovskites M₃HCh and M₃FCh (M = Li, Na; Ch = S, Se, Te) mostly retain their ideal cubic structure over a wide range of compositions owing to anionic size flexibility and low-energy phonon modes that promote their ionic conductivity. In this study, we show the synthesis of potassium-based antiperovskites K₃HTe and K₃FTe and discuss the structural features in comparison with lithium and sodium analogues. It is shown experimentally and theoretically that both compounds maintain a cubic symmetry and can be prepared at ambient pressure, in contrast to most of the reported M₃HCh and M₃FCh which require high pressure synthesis. A systematic comparison of a series of cubic M₃HTe and M₃FTe (M = Li, Na, K) revealed that telluride anions contract in the order of K, Na, Li, with a pronounced contraction in the Li system. This result can be understood in terms of the difference in charge density of alkali metal ions as well as the size flexibility of Ch anions, contributing to the stability of the cubic symmetry.

Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba7S(VS3O)2(S2)3 and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study

Oxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q-Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba-V-Q-O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba₇V₂O₂S₁₃, which can be written as Ba₇S(VS₃O)₂(S₂)₃, was substituted to yield three selenide derivatives Ba₇V₂O₂S_9.304Se_3.696, Ba₇V₂O₂S_7.15Se_5.85, and Ba₇V₂O₂S_6.85Se_6.15. They represent original multiple-anion lattices and first members in the system Ba-V-Se-S-O. They exhibit in the first layer heteroleptic tetrahedra V⁵⁺S₃O and isolated Q^2- anions and in the second layer dichalcogenide pairs (Q₂)^2- with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q^2- or (Q₂)^2- (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO₃S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V⁵⁺, together with the presence/nature of the dichalcogenides (Q₂)^2- and isolated Q^2-, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry.

Pressure-Induced Anion Order-Disorder Transition in Layered Perovskite Sr2LiHOCl2

While cation order-disorder transitions have been extensively investigated because of their decisive impact on chemical and physical properties, only few anion order-disorder transitions are known. Here, we show that Sr₂CuO₂Cl₂-type layered perovskite Sr₂LiHOCl₂ exhibits a pressure-induced H^-/O^2- order-disorder transition. When synthesized at ambient and low pressures (≤2 GPa), Sr₂LiHOCl₂ is isostructural to orthorhombic Eu₂LiHOCl₂ (Cmcm) with a H^-/O^2- order at the equatorial sites. However, applying a higher pressure (5 GPa) during synthesis causes the equatorial anions to be disordered, leading to a tetragonal symmetry (I4/mmm) with a loss of the superstructure. The structural analysis revealed that, in the ambient pressure phase, HLi₂Sr₄ and OLi₂Sr₄ octahedra have distinct sizes to stabilize otherwise underbonded oxide ions, which is less important at the higher pressure. Anion-disordered Sr₂LiHOBr₂ and Ba₂LiHOCl₂ were also obtained at 5 GPa. Given the abundant layer-type anion order in perovskite-based oxyhydrides (e.g., La₂LiHO₃), the inclusion of additional anions (e.g., chloride) expands the frontiers of anion ordering patterns and their distribution control with the benefit of improving ionic conduction in solids.

Simultaneous measurement of specific heat and thermal conductivity in pulsed magnetic fields

We report an experimental setup for simultaneously measuring specific heat and thermal conductivity in feedback-controlled pulsed magnetic fields of 50 ms duration at cryogenic temperatures. A stabilized magnetic field pulse obtained by the feedback control, which dramatically improves the thermal stability of the setup and sample, is used in combination with the flash method to obtain absolute values of thermal properties up to 37.2 T in the 22-16 K temperature range. We describe the experimental setup and demonstrate the performance of the present method with measurements on single-crystal samples of the geometrically frustrated quantum spin-dimer system SrCu2(BO3)2. Our proof-of-principle results show excellent agreement with data taken using a standard steady-state method, confirming the validity and convenience of the present approach.

Structural Transformation in LnHS (Ln = La, Nd, Gd, and Er) with Coordination Change between an S-Centered Octahedron and a Trigonal Prism

Lanthanide hydride chalcogenides LnHSe and LnHTe (Ln = lanthanides) crystallize in two polymorphs, 2H and 1H structures (ZrBeSi-type and filled-WC-type structures, respectively), but the chemical origin of the structural selection is unknown. Here, we have expanded the LnHCh (Ch = O, Se, and Te) family to include LnHS (Ln = La, Nd, Gd, and Er) using high-pressure synthesis. LnHS adopts the 2H structure for large Ln (La, Nd, and Gd) and the 1H structure for small Er. We compared the two polymorphs using anion-centered polyhedra and found that in the compounds with large ionicity, the 2H structure with ChLn₆ octahedra is stabilized over the 1H structure with ChLn₆ trigonal prisms due to relatively small electrostatic repulsion, supported by analysis of Madelung energy, crystal orbital Hamilton population (COHP), and density of energy (DOE).

Evolutionary Algorithm Directed Synthesis of Mixed Anion Compounds LaF2X (X = Br, I) and LaFI2

Mixed-anion compounds have attracted growing attentions, but their synthesis is challenging, making a rational search desirable. Here, we explored LaF3-LaX3 (X = Cl, Br, I) system using ab initio structure searches based on evolutionary algorithms, and predicted LaF2X and LaFX2 (X = Br, I), which are respectively isostructural with LaHBr2 and YH2I, consisting of layered La-F blocks with single and double ordered honeycomb lattices, separated by van der Waals gaps. We successfully synthesized these compounds: LaF2Br and LaFI2 crystallized in the predicted structure, while LaF2I is similar to the predicted one but with different layer stacking. LaF2I exhibits fluoride ion conductivity comparable to that of non-doped LaF3, and has the potential to show better ionic conductivity upon appropriate doping, given the theoretically lower diffusion energy barrier and the presence of soft iodine anions. This study shows the structure prediction using evolutionary algorithms will accelerate the discovery of mixed-anion compounds in future, in particular those with an ordered anion arrangement.

Generation of third-harmonic spin oscillation from strong spin precession induced by terahertz magnetic near fields

The ability to drive a spin system to state far from the equilibrium is indispensable for investigating spin structures of antiferromagnets and their functional nonlinearities for spintronics. While optical methods have been considered for spin excitation, terahertz (THz) pulses appear to be a more convenient means of direct spin excitation without requiring coupling between spins and orbitals or phonons. However, room-temperature responses are usually limited to small deviations from the equilibrium state because of the relatively weak THz magnetic fields in common approaches. Here, we studied the magnetization dynamics in a HoFeO₃ crystal at room temperature. A custom-made spiral-shaped microstructure was used to locally generate a strong multicycle THz magnetic near field perpendicular to the crystal surface; the maximum magnetic field amplitude of about 2 T was achieved. The observed time-resolved change in the Faraday ellipticity clearly showed second- and third-order harmonics of the magnetization oscillation and an asymmetric oscillation behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic vector L plays an important role in the nonlinear dynamics of spin systems far from equilibrium.

Correlated Rattling of Sodium-Chains Suppressing Thermal Conduction in Thermoelectric Stannides

Tin-based intermetallics with tunnel frameworks containing zigzag Na chains that excite correlated rattling impinging on the framework phonons are attractive as thermoelectric materials owing to their low lattice thermal conductivity. The correlated rattling of Na atoms in the zigzag chains and the origin of the low thermal conductivity is uncovered via experimental and computational analyses. The Na atoms behave as oscillators along the tunnel, resulting in substantial interactions between Na atoms in the chain and between the chain and framework. In these intermetallic compounds, a shorter inter-rattler distance results in lower thermal conductivity, suggesting that phonon scattering by the correlated rattling Na-chains is enhanced. These results provide new insights into the behavior of thermoelectric materials with low thermal conductivity and suggest strategies for the development of such materials that utilize the correlated rattling.

Honeycomb-Layered Oxides With Silver Atom Bilayers and Emergence of Non-Abelian SU(2) Interactions

Honeycomb-layered oxides with monovalent or divalent, monolayered cationic lattices generally exhibit myriad crystalline features encompassing rich electrochemistry, geometries, and disorders, which particularly places them as attractive material candidates for next-generation energy storage applications. Herein, global honeycomb-layered oxide compositions, Ag₂ M₂ TeO₆ ( M = Ni , Mg , etc $M = \rm Ni, Mg, etc$ .) exhibiting Ag $\rm Ag$ atom bilayers with sub-valent states within Ag-rich crystalline domains of Ag₆ M₂ TeO₆ and Ag $\rm Ag$ -deficient domains of Ag 2 - x Ni 2 TeO 6 ${\rm Ag}_{2 - x}\rm Ni_2TeO_6$ ( 0 < x < 2 $0 &lt; x &lt; 2$ ). The Ag $\rm Ag$ -rich material characterized by aberration-corrected transmission electron microscopy reveals local atomic structural disorders characterized by aperiodic stacking and incoherency in the bilayer arrangement of Ag $\rm Ag$ atoms. Meanwhile, the global material not only displays high ionic conductivity but also manifests oxygen-hole electrochemistry during silver-ion extraction. Within the Ag $\rm Ag$ -rich domains, the bilayered structure, argentophilic interactions therein and the expected Ag $\rm Ag$ sub-valent states ( 1 / 2 + , 2 / 3 + $1/2+, 2/3+$ , etc.) are theoretically understood via spontaneous symmetry breaking of SU(2)× U(1) gauge symmetry interactions amongst 3 degenerate mass-less chiral fermion states, justified by electron occupancy of silver 4 d z 2 $4d_{z^2}$ and 5s orbitals on a bifurcated honeycomb lattice. This implies that bilayered frameworks have research applications that go beyond the confines of energy storage.

Magnetically Hidden State on the Ground Floor of the Magnetic Devil's Staircase

We investigated the low-temperature and high-field thermodynamic and ultrasonic properties of SrCu_{2}(BO_{3})_{2}, which exhibits various plateaux in its magnetization curve above 27 T, called a magnetic Devil's staircase. The results of the present study confirm that magnetic crystallization, the first step of the staircase, occurs above 27 T as a first-order transition accompanied by a sharp singularity in heat capacity C_{p} and a kink in the elastic constant. In addition, we observe a thermodynamic anomaly at lower fields around 26 T, which has not been previously detected by any magnetic probes. At low temperatures, this magnetically hidden state has a large entropy and does not exhibit Schottky-type gapped behavior, which suggests the existence of low-energy collective excitations. Based on our observations and theoretical predictions, we propose that magnetic quadrupoles form a spin-nematic state around 26 T as a hidden state on the ground floor of the magnetic Devil's staircase.

Highly Electron-Doped TaON Single-Crystal Growth by a High-Pressure Flux Method

Transition-metal oxynitrides have a variety of functions such as visible light-responsive catalysts and dielectric materials, but acquiring single crystals necessary to understand inherent properties is difficult and is limited to relatively small sizes (<10 μm) because they easily decompose at high temperatures. Here, we have succeeded in growing platelet single crystals of TaON with a typical size of 50 × 100 × 10 μm³ under a high pressure and high temperature (6 GPa and 1400 °C) using a LiCl flux. Such a harsh condition, in contrast to powder samples synthesized under mild conditions, resulted in the introduction of a large amount of oxygen vacancies (x = 0.06 in TaO_1-xN) into the crystal, providing a metallic behavior with a large anisotropy of ρ_c/ρ_ab ∼ 10³. Low-temperature oxygen annealing allows for a single-crystal-to-single-crystal transformation to obtain fully oxidized TaON (yellow) crystals. Needle-like crystals can be obtained when NH₄Cl is used as a flux. Furthermore, black Hf₂ON₂ single crystals are also grown, suggesting that the high-pressure flux method is widely applicable to other transition-metal oxynitrides, with extensive carrier control.

Lone-Pair-Induced Intra- and Interlayer Polarizations in Sillén-Aurivillius Layered Perovskite Bi4NbO8Br

Sillén-Aurivillius layered perovskite oxyhalides Bi₄MO₈X (M = Nb, Ta; X = Cl, Br) are of great interest because of their potential as lead-free ferroelectrics in addition to their function as visible-light-responsive photocatalysts. In this work, we revisited the crystal structure of Bi₄NbO₈Br (space group: P2₁cn), revealing that the intralayer polarization is not based on the reported NbO₆ octahedral tilting but is derived from the stereochemically active Bi³⁺ lone pair electrons (LPEs) and the octahedral off-centering of Nb⁵⁺ cations. The revised structure (space group: Ic) has additional interlayer polarizations (calculated: 0.6 μC/cm²), in agreement with recent experiments on Bi₄NbO₈Br. The occurrence of polarization due to stereochemically active LPEs and Nb-site off-centering is similar to Aurivillius-type ferroelectrics (e.g., Bi₂WO₆), with comparable spontaneous polarizations in the in-plane direction (calculated: 43.5 μC/cm²). This, together with the out-of-plane polarization, indicates that Sillén-Aurivillius compounds have great potential as ferroelectric materials.

Polyoxocationic antimony oxide cluster with acidic protons

The success and continued expansion of research on metal-oxo clusters owe largely to their structural richness and wide range of functions. However, while most of them known to date are negatively charged polyoxometalates, there is only a handful of cationic ones, much less functional ones. Here, we show an all-inorganic hydroxyiodide [H_10.7Sb_32.1O₄₄][H_2.1Sb_2.1I₈O₆][Sb_0.76I₆]₂·25H₂O (HSbOI), forming a face-centered cubic structure with cationic Sb₃₂O₄₄ clusters and two types of anionic clusters in its interstitial spaces. Although it is submicrometer in size, electron diffraction tomography of HSbOI allowed the construction of the initial structural model, followed by powder Rietveld refinement to reach the final structure. The cationic cluster is characterized by the presence of acidic protons on its surface due to substantial Sb³⁺ deficiencies, which enables HSbOI to serve as an excellent solid acid catalyst. These results open up a frontier for the exploration and functionalization of cationic metal-oxo clusters containing heavy main group elements.

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi₄NbO₈Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

An efficient spatial separation of photoexcited carriers is demonstrated in a layered-oxyhalide nanoplate by controlling the direction of carrier flows upon Rh cocatalyst loading, which leads to drastic photocatalytic H₂ evolution activity.

Dehydration of Electrochemically Protonated Oxide: SrCoO2 with Square Spin Tubes

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-T_c superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH₂) are known as the most powerful method to obtain highly reduced oxides including Nd_0.8Sr_0.2NiO₂ superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO_2.5 thin films are converted to SrCoO₂ by dehydration of HSrCoO_2.5 at 350 °C. SrCoO₂ forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO_2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H_1-xSrCoO_2.5-x/2) for the SrCoO₂ formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.

Bi4AO6Cl2 (A = Ba, Sr, Ca) with Double and Triple Fluorite Layers for Visible-Light Water Splitting

Layered oxyhalides containing double or triple fluorite layers are promising visible-light-responsive water-splitting photocatalysts with unique band structures. Herein, we report on the synthesis, structure, and photocatalytic property of Bi₄BaO₆Cl₂ (I4/mmm) with alternating double (Bi₂O₂) and triple (Bi₂BaO₄) fluorite layers, which was extracted from the crystallographic database on the basis of Madelung potential calculations. Rietveld refinements from powder X-ray and neutron diffraction data revealed the presence of cationic disorder between Bi₂O₂ and Bi₂BaO₄ layers, leading to electrostatic stabilization. DFT calculations suggested that photogenerated electrons and holes flow through the double and triple layers, respectively, which may suppress electron-hole recombination. We expanded this double-triple system to include Bi₄CaO₆Cl₂ and Bi₄SrO₆Cl₂ with orthorhombic distortions and different degrees of cationic disorder, which allow band gap tuning. All the double-triple compounds Bi₄AO₆Cl₂ showed stable water-splitting photocatalysis in the presence of a sacrificial reagent.

Electrochemical Crystal Growth of Titanium Oxyfluorides-A Strategy for Development of Electron-Doped Materials

We report on the growth of single crystals of an electron-doped titanium oxyfluoride, Li₂Ti(O,F)₃, employing high-temperature electrolysis of TiO₂ with a eutectic Li₂MoO₄-LiF melt. Greenish octahedral-shaped crystals (∼30 μm in size) with a cubic rocksalt-type structure were successfully obtained by precisely tuning the applied voltage. The temperature-dependent magnetic susceptibility data revealed a paramagnetic behavior at low temperatures, ensuring the presence of Ti³⁺ ions (mean valence number of +3.78; F/Ti ∼ 0.15). The crystals exhibited clear visible-light absorption and produced H₂ from water in the presence of a sacrificial reagent under UV-light irradiation. Li₂Ti(O,F)₃ more efficiently produced H₂ compared with a nondoped oxyfluoride Li₅Ti₂O₆F, likely due to the doped electrons for the former. This work highlights a promising electrochemical approach toward growing electron-doped oxyfluoride crystals.

Mixed alkali-ion transport and storage in atomic-disordered honeycomb layered NaKNi2TeO6

Honeycomb layered oxides constitute an emerging class of materials that show interesting physicochemical and electrochemical properties. However, the development of these materials is still limited. Here, we report the combined use of alkali atoms (Na and K) to produce a mixed-alkali honeycomb layered oxide material, namely, NaKNi2TeO6. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of Na and K atoms. We also investigate the possibility of mixed electrochemical transport and storage of Na+ and K+ ions in NaKNi2TeO6. In particular, we report an average discharge cell voltage of about 4 V and a specific capacity of around 80 mAh g-1 at low specific currents (i.e., < 10 mA g-1) when a NaKNi2TeO6-based positive electrode is combined with a room-temperature NaK liquid alloy negative electrode using an ionic liquid-based electrolyte solution. These results represent a step towards the use of tailored cathode active materials for "dendrite-free" electrochemical energy storage systems exploiting room-temperature liquid alkali metal alloy materials.

Enhanced Magnetic Interaction by Face-Shared Hydride Anions in 6H-BaCrO2H

Studies on magnetic oxyhydrides have been almost limited to perovskite-based lattices with corner-sharing octahedra with a M-H-M (M: transition metal) angle of θ ∼ 180°. Using a high-pressure method, we prepared BaCrO₂H with a 6H-type hexagonal perovskite structure with corner- and face-sharing octahedra, offering a unique opportunity to investigate magnetic interactions based on a θ ∼ 90° case. Neutron diffraction for BaCrO₂H revealed an antiferromagnetic (AFM) order at T_N ∼ 375 K, which is higher than ∼240 K in BaCrO_3-xF_x. The relatively high T_N of BaCrO₂H can be explained by the preferred occupancy of H^- at the face-sharing site that provides AFM superexchange in addition to AFM direct exchange interactions. First-principles calculations on BaCrO₂H in comparison with BaCrO₂F and BaMnO₃ further reveal that the direct Cr-Cr interaction is significantly enhanced by shortening the Cr-Cr distance due to the covalent nature of H^-. This study provides a useful strategy for the extensive control of magnetic interactions by exploiting the difference in the covalency of multiple anions.

Alkali-Rich Antiperovskite M3FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M₃HCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the M₃FCh family, in which the polarizable H^- anion at the octahedral center is replaced by the ionic F^- anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F^- and M⁺ ions does not slow down the M⁺ ion diffusion, with the calculated energy barrier being as low as that of M₃HCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic M₃FCh (and M₃HCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor t and the ionic transport: as t decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped Li₃FSe has a room-temperature ionic conductivity of 5 × 10^-5 S/cm with a bulk activation energy of 0.18 eV.

Formation of PbCl2-type AHF (A = Ca, Sr, Ba) with partial anion order at high pressure

The high-pressure structures of alkaline earth metal hydride-fluorides (AHFs) (A = Ca, Sr, Ba) were investigated up to 8 GPa. While AHF adopts the fluorite-type structure (Fm3[combining macron]m) at ambient pressure without anion ordering, the PbCl2-type (cotunnite-type) structure (Pnma) is formed by pressurization, with a declining trend of critical pressure as the ionic radius of the A2+ cation increases. In contrast to PbCl2-type LaHO and LaOF whose anions are fully ordered, the H-/F- anions in the high-pressure polymorph of SrHF and BaHF are partially ordered, with a preferential occupation of H- at the square-pyramidal site (vs. tetrahedral site). First-principles calculations partially support the preferential anion occupation and suggest occupation switching at higher pressure. These results provide a strategy for controlling the anion ordering and local structure in mixed-anion compounds.

Spin Frustration in Double Perovskite Oxides and Oxynitrides: Enhanced Frustration in La2MnTaO5N with a Large Octahedral Rotation

The B-site sublattice in the double perovskite oxides A2BB'O6 (B: magnetic cation; B': nonmagnetic cation) causes spin frustration, but the relationship between the structure and spin frustration remains unclear although a number of compounds have been studied. The present study systematically investigated A2MnIIB'O6 (S = 5/2) and found that the frustration factor, defined by f = |θW|/TN (θW: Weiss temperature; TN: Néel temperature), scales linearly with the tolerance factor t, i.e., octahedral rotation. Unexpectedly, La2MnTaO5N (space group: P21/n) synthesized under high pressure is more frustrated (f = 6) than oxides with similar t values, despite the large octahedral rotation due to the small t value of 0.914. Structural analysis suggests that the enhanced frustration can be attributed to the site preference of nitride anions at the equatorial positions, which reduces the variance of neighboring Mn-Mn distances. Our findings provide a new guide to control and improve spin frustration in double perovskites with multiple anions.

Anion ordering enables fast H- conduction at low temperatures

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba_2-δH_3-2δ X (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm^-1 at 200°C, with low activation barriers allowing H^- conduction even at room temperature. In contrast to structurally related BaH₂ (i.e., Ba₂H₄), the layered anion order in Ba_2-δH_3-2δ X, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.

Layered Perovskite Oxyiodide with Narrow Band Gap and Long Lifetime Carriers for Water Splitting Photocatalysis

The development of semiconductors with narrow band gap and high stability is crucial for achieving solar to chemical energy conversion. Compounds with iodine, which has a high polarizability, have attracted attention because of their narrow band gap and long carrier lifetime, as typified by halide perovskite solar cells; however, they have been regarded as unsuitable for harsh photocatalytic water splitting because iodine is prone to self-oxidation. Here, we demonstrate that Ba₂Bi₃Nb₂O₁₁I, a layered Sillén-Aurivillius oxyiodide, not only has access to a wider range of visible light than its chloride and bromide counterparts, but also functions as a stable photocatalyst, efficiently oxidizing water. Density functional theory calculations reveal that the oxygen 2p orbitals in the perovskite block, rather than the fluorite Bi₂O₂ block as previously pointed out, anomalously push up the valence band maximum, which can be explained by a modified Madelung potential analysis that takes into account the high polarizability of iodine. In addition, the highly polarizable iodide contributes to longer carrier lifetime of Ba₂Bi₃Nb₂O₁₁I, allowing for a significantly higher quantum efficiency than its chloride and bromide counterparts. Visible-light-driven Z-scheme water splitting was achieved for the first time in an iodine-based system using Ba₂Bi₃Nb₂O₁₁I as an oxygen-evolution photocatalyst. The present study provides a novel approach for incorporating polarizable "soft" anions into building blocks of layered materials to manipulate the band structure and improve the carrier dynamics for visible-light responsive functions.

Integrating molecular design and crystal engineering approaches in non-humidified intermediate-temperature proton conductors based on a Dawson-type polyoxometalate and poly(ethylene glycol) derivatives

Anionic metal-oxygen clusters known as polyoxometalates (POMs) have been widely researched as components of proton conductors. While proton conduction under non-humidified intermediate-temperature (100-250 °C) conditions is advantageous from the viewpoint of kinetics, few solid-state materials, not to mention POM-based crystals, show truly effective proton conduction without the aid of water vapor. In this context, non-volatile proton-conductive polymers have been confined into POM-based frameworks, while fast proton conduction was infeasible. Herein, we demonstrate a new strategy to synthesize POM-polymer composites exhibiting fast proton conduction under non-humidified intermediate-temperature conditions. Specifically, a molecular design approach utilizing poly(ethylene glycol)s (PEGs) of different terminal groups or chain lengths controls the proton carrier density, and a crystal engineering approach using a large Dawson-type POM ([α-P₂W₁₈O₆₂]^6-) with an anisotropic molecular shape and alkali metal ions as counter cations fine-tunes the mobility of the confined PEGs as proton carriers. By integrating these approaches, proton conductivity over 10^-4 S cm^-1 at 150 °C, comparable to the well-known highly proton-conductive solid-state materials, is achieved. The proton conduction mechanism is discussed with alternative current impedance spectroscopy jointly with specific heat capacity measurements and solid-state NMR spectroscopy.

Pressure-Induced Collapse Transition in BaTi2Pn2O (Pn = As, Sb) with an Unusual Pn-Pn Bond Elongation

Making and breaking bonds in a solid-state compound greatly influences physical properties. A well-known playground for such bonding manipulation is the ThCr₂Si₂-type structure AT₂X₂, allowing a collapse transition where a X-X dimer forms by a chemical substitution or external stimuli. Here, we report a pressure-induced collapse transition in the structurally related BaTi₂Pn₂O (Pn = As, Sb) at a transition pressure P_c of ∼15 GPa. The Pn-Pn bond formation is related with Pn-p band filling, which is controlled by charge transfer from the Ti-3d band. At P_c, the Sb-Sb distance in BaTi₂Sb₂O shrinks due to bond formation, but interestingly, the Sb-Sb expands with increasing pressure above P_c. This expansion, which was not reported in ThCr₂Si₂-type compounds, may arise from heteroleptic coordination geometry around titanium, where a compression of the Ti-O bond plays a role. Electrical resistivity measurements of BaTi₂Sb₂O up to 55 GPa revealed an increasing trend of the superconducting transition temperature with pressure. This study presents structure motifs that allow flexible bonding manipulation and property control with heteroleptic coordination geometry.

Structure and Optical Properties of Layered Perovskite (MA)2PbI2-xBrx(SCN)2 (0 ≤ x < 1.6)

The layered perovskite (MA)₂PbI₂(SCN)₂ (MA = CH₃NH₃⁺) is a member of an emerging series of compounds derived from hybrid organic-inorganic perovskites. Here, we successfully synthesized (MA)₂PbI_2-xBr_x(SCN)₂ (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN^- anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)₂PbI₂(SCN)₂ tolerates a wide range of bromide substitution toward tuning the band gap energy.