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Detecting H 2 S in oxygen-deficient conditions is vital for identifying leaks in SF 6 -insulated electrical equipment. Current infrared-based detection methods are expensive and sensitive to environmental conditions, highlighting the necessity for cost-effective and stable gas sensors. Existing gas sensors based on semiconducting metal oxides (SMOXs) are limited by redox reactions with oxygen and require high operating temperatures. Here, we introduce a room-temperature (RT) H 2 S sensor for oxygen-deficient environments using the intrinsic conducting two-dimensional (2D) metal-organic framework (MOF), Co 1.8 Ni 1.2 (hexaiminotriphenylene) 2 [Co 1.8 Ni 1.2 (HITP) 2 ], overcoming the limitations of SMOX gas sensors. Remarkably, Co 1.8 Ni 1.2 (HITP) 2 sensors exhibit exceptional selectivity for H 2 S with negligible cross-responses and a sensitivity drift of less than 4.13% in an SF 6 atmosphere over 60 days. The Co 1.8 Ni 1.2 (HITP) 2 gas sensor shows significant promise for real-time and stable monitoring of H 2 S gas in oxygen-deficient environments.

Efficient gas sensors are critical for environmental monitoring and industrial safety. While metal oxide semiconductor (MOS) sensors are cost-effective, they struggle with poor selectivity, high operating temperatures, and limited stability. Electrochemical sensors, though selective and energy-efficient, face high costs, and stability issues due to precious metal catalysts like platinum on carbon (Pt/C). Herein, a novel, cost-effective electrochemical sensor using nitrogen-doped indium oxide In 2 O 3- x N 2 x /3 V x /3 (0.01≤x≤0.14), synthesized with varying nitriding times is presented. The optimized In 2 O 3 N-40 min sensor demonstrates a remarkable response current of 771 nA to 10 ppm nitrogen dioxide (NO 2 ) at ambient temperature, with outstanding long-term stability (over 30 days) and rapid response/recovery times (5/16 s). Compared to Pt/C sensors, it shows 84% and 67% reductions in response and recovery times, respectively, and maintains 98% performance after a month, versus 68% for Pt/C. This cost-effective sensor presents a promising alternative for electrochemical gas sensing, eliminating the need for precious metal catalysts., (© 2024 Wiley‐VCH GmbH.)

Layered perovskites with Ruddlesden-Popper-type structures are fundamentally important for low-dimensional properties, for example, photovoltaic hybrid iodides and superconducting copper oxides. Many such halides and oxides are known, but analogous nitrides are difficult to stabilize due to the high cation oxidation states required to balance the anion charges. Here we report the high-pressure synthesis of three single-layer Ruddlesden-Popper (K 2 NiF 4 type) nitrides-Pr 2 ReN 4 , Nd 2 ReN 4 and Ce 2 TaN 4 -along with their structural characterization and properties. The R 2 ReN 4 materials (R = Pr and Nd) are metallic, and Nd 2 ReN 4 has a ferromagnetic Nd 3+ spin order below 15 K. Thermal decomposition gives R 2 ReN 3 with a Peierls-type distortion and chains of Re-Re multiply bonded dimers. Ce 2 TaN 4 has a structural transition driven by octahedral tilting, with local distortions and canted magnetic Ce 3+ order evidencing two-dimensional Ce 3+ /Ce 4+ charge ordering correlations. Our work demonstrates that Ruddlesden-Popper nitrides with varied structural, electronic and magnetic properties can be prepared from high-pressure synthesis, opening the door to related layered nitride materials., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Strategies for discovery of high-performance electrocatalysts are important to advance clean energy technologies. Metastable phases such as low temperature or interfacial structures that are difficult to access in bulk may offer such catalytically active surfaces. We report here that the suboxide Zr 3 O, which is formed at Zr-ZrO 2 interfaces but does not appear in the experimental Zr-O phase diagram exhibits outstanding oxygen reduction reaction (ORR) performance surpassing that of benchmark Pt/C and most transition metal-based catalysts. Addition of Fe 3 C nanoparticles to give a Zr-Zr 3 O-Fe 3 C/NC catalyst (NC=nitrogen-doped carbon) gives a half-wave potential (E 1/2 ) of 0.914 V, outperforming Pt/C and showing only a 3 mV decrease after 20,000 electrochemical cycles. A zinc-air battery (ZAB) using this cathode material has a high power density of 241.1 mW cm -2 and remains stable for over 50 days of continuous cycling, demonstrating potential for practical applications. Zr 3 O demonstrates that interfacial or other phases that are difficult to stabilize may offer new directions for the discovery of high-performance electrocatalysts., (© 2024 Wiley-VCH GmbH.)

High pressure high temperature treatments of the composition CaMnMnTeO 6 are found to yield only an A 2 BB'O 6 -type double perovskite (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 , rather than a AA'BB'O 6 double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO 6 and CaMnMnWO 6 with similar cation sizes. Double perovskite (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 adopts a monoclinic structure in space group P2 1 /n with a framework of highly tilted MnO 6 and TeO 6 octahedra enclosing disordered Ca 2+ and Mn 2+ cations. Magnetic measurements show that (Ca 0.5 Mn 0.5 ) 2 MnTeO 6 is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K., (© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH.)

Three new nitrides La 3 MN 5 (M=Cr, Mn, and Mo) have been synthesized using a high pressure azide route. These are the first examples of ternary Cs 3 CoCl 5 -type nitrides, and show that this (MN 4 )NLa 3 antiperovskite structure type may be used to stabilise high oxidation-state transition metals in tetrahedral molecular [MN 4 ] n- nitridometallate anions. Magnetic measurements confirm that Cr and Mo are in the M 6+ state, but the M=Mn phase has an anomalously small paramagnetic moment and large cell volume. Neutron powder diffraction data are fitted using an anion-excess La 3 MnN 5.30 model (space group I4/mcm, a=6.81587(9) Å and c=11.22664(18) Å at 200 K) in which Mn is close to the +7 state. Excess-anion incorporation into Cs 3 CoCl 5 -type materials has not been previously reported, and this or other substitution mechanisms may enable many other high oxidation state transition metal nitrides to be prepared., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The ocean, a vast hydrogen reservoir, holds potential for sustainable energy and water development. Developing high-performance electrocatalysts for hydrogen production under harsh seawater conditions is challenging. Here, we propose incorporating a protective V 2 O 3 layer to modulate the microcatalytic environment and create in situ dual-active sites consisting of low-loaded Pt and Ni 3 N. This catalyst demonstrates an ultralow overpotential of 80 mV at 500 mA cm -2 , a mass activity 30.86 times higher than Pt-C and maintains at least 500 hours in seawater. Moreover, the assembled anion exchange membrane water electrolyzers (AEMWE) demonstrate superior activity and durability even under demanding industrial conditions. In situ localized pH analysis elucidates the microcatalytic environmental regulation mechanism of the V 2 O 3 layer. Its role as a Lewis acid layer enables the sequestration of excess OH - ions, mitigate Cl - corrosion, and alkaline earth salt precipitation. Our catalyst protection strategy by using V 2 O 3 presents a promising and cost-effective approach for large-scale sustainable green hydrogen production.

Electrocatalytic high-throughput seawater electrolysis for hydrogen production is a promising green energy technology that offers possibilities for environmental and energy sustainability. However, large-scale application is limited by the complex composition of seawater, high concentration of Cl - leading to competing reaction, and severe corrosion of electrode materials. In recent years, extensive research has been conducted to address these challenges. Metal nitrides (MNs) with excellent chemical stability and catalytic properties have emerged as ideal electrocatalyst candidates. This review presents the electrode reactions and basic parameters of the seawater splitting process, and summarizes the types and selection principles of conductive substrates with critical analysis of the design principles for seawater electrocatalysts. The focus is on discussing the properties, synthesis, and design strategies of MN-based electrocatalysts. Finally, we provide an outlook for the future development of MNs in the high-throughput seawater electrolysis field and highlight key issues that require further research and optimization.

Sodium azide (NaN 3 ) is a versatile nitrogen source that can be used for the synthesis of new nitrides under high-pressure and temperature conditions. Reactions between lanthanum nitride (LaN) and sodium azide (NaN 3 ) at 800°C under 8 GPa pressure have led to the discovery of two defect rocksalt phases which are the first reported ternaries in the La-Na-N system. Preliminary structure assignments have been made based on fits to powder X-ray diffraction profiles. One phase is La 1- x Na 3 x N with vacancies at octahedral La sites and interstitial tetrahedral Na cations. This phase has a tetragonally distorted rocksalt structure (space group I 4[Formula: see text] mmm , a  = 3.8704(2) and c  = 5.2098(3) Å for nominal x  = 0.10) and the distortion decreases with increasing Na content (space group I 4[Formula: see text] mmm , a  = 3.8060(2) Å, c  = 5.2470(3) Å for nominal x  = 0.14), further giving a cubic phase ( a  = 5.3055(2) Å) for nominal x  = 0.25. This coexists with another cubic [Formula: see text] phase ( a  = 5.1561 (5) Å), tentatively identified as rocksalt 'NaN 1/3 ' stabilized by a small amount of La; NaLa y N (1+3 y )/3 with y  ≈ 1%. These initial investigations reveal that the high-pressure La-Na-N phase diagram may be rich in defect rocksalt-type materials although further work using neutron diffraction will be needed to confirm the structures. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Ordering of cations is important for controlling properties of ABO 3 perovskites, and CaFeFeNbO 6 is the first example of an Fe-based AA'BB'O 6 double double perovskite, with Ca 2+ /Fe 2+ ordered on A-site columns, and Fe 3+ /Nb 5+ at the octahedral B-sites. Substantial (37%) antisite disorder of the latter cations leads to spin glassy magnetism below a freezing transition at 12 K. The CaMnFeNbO 6 analogue also shows substantial cation disorder and spin glassy behaviour. Comparison of synthesis pressures for ordered materials based on different A-site transition metals, suggests that pressures of at least 14-18 GPa will be required to discover the expected plethora of double double perovskites based on A' cations smaller than Mn 2+ .

Cation ordering in ABX 3 perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO 6 with the tetragonal AA'BB'O 6 double double perovskite structure that was previously only reported for A'=Mn compositions. CaCuFeReO 6 occurs in the same phase field as CaCu 3 Fe 2 Re 2 O 12 demonstrating that different A-cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO 6 has ferrimagnetic order of Fe, Re and Cu spins below T C =567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO 6 displays low-field "butterfly" hysteresis with an unusual change from negative to positive values as field increases. Many more AA'BB'O 6 double double perovskites may be accessible for A'=Cu and other divalent transition metals at high pressure, so the presently known phases likely represent only the "tip of the iceberg" for this family., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO 3 ) and LiNbO 3 type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO 3 derivative with R32 symmetry has never been observed. Here we show that Co 2 InSbO 6 recovered from high pressure has a new, ordered-R32 A 2 BCO 6 variant of the corundum structure. Co 2 InSbO 6 is also remarkable for showing two cation redistributions, to (Co 0.5 In 0.5 ) 2 CoSbO 6 and then Co 2 InSbO 6 variants of the ordered-LiNbO 3 A 2 BCO 6 structure on heating. The cation distributions change magnetic properties as the final ordered-LiNbO 3 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 ABO 3 materials with the R32 structure to be discovered., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Chemical pressure, a strange but familiar concept, is a lattice internal force caused by lattice strain with chemical modifications and arouses great interest due to its diversity and efficiency to synthesize new compounds and tune functional materials. Different from physical pressure loaded by an external force that is positive, chemical pressure can be either positive or negative (contract a lattice or expand it), often through flexible and mild chemical synthesis strategies, which are particularly important as a degree of freedom to manipulate material behaviors. In this tutorial review, we summarize the features of chemical pressure as a methodology and demonstrate its role in synthesizing and discovering some typical magnetically, electrically, and thermally responsive functional materials. The measure of chemical pressure using experimental lattice strain and elastic modulus was proposed, which can be used for quantitative descriptions of the correlation between lattice distortion and properties. From a lattice strain point of view, we classify chemical pressure into different categories: (i) chemical substitution, (ii) chemical intercalation/de-intercalation, (iii) size effect, and (iv) interface constraint, etc. Chemical pressure affects chemical bonding and rationalizes the crystal structure by modifying the electronic structure of solids, regulating the lattice symmetry, local structure, phonon structure effects etc. , emerging as a general and effective method for synthesizing new compounds and tuning functional materials.

A review of progress in understanding the Verwey transition in magnetite (Fe 3 O 4 ) over the past decade is presented. This electronic and structural transition at T V ≈ 125 K was reported in 1939 and has since been a contentious issue in magnetism. Long range Fe 2+ /Fe 3+ charge ordering has been confirmed below the transition from crystal structure refinement, and Fe 2+ orbital ordering and formation of trimerons through weak bonding of Fe 2+ states to two Fe neighbors has been discovered. This model has accounted for many spectroscopic observations such as the 57 Fe NMR frequencies. The trimeron lifetime has been measured, and trimeron soft modes have been observed. The origin of the first to second order crossover of Verwey transitions in doped magnetites has been revealed by a nanoparticle study. Electronic and structural fluctuations are found to persist to temperatures far above T V and local structural distortions track the bulk magnetization, disappearing at the 850 K Curie transition. New binary mixed-valent iron oxides discovered at high pressure are found to have electronic transitions and orbital molecule ground states similar to those of magnetite., Competing Interests: The author declares no competing financial interest., (© 2022 American Chemical Society.)

Atomically dispersed CoNC is a promising material for H 2 O 2 selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN 4 active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN 2 C 2 ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H 2 O 2 electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s -1 ; thus outperforming the state-of-the-art carbon-based catalysts., (© 2022 Wiley-VCH GmbH.)

Efficient, accurate and reliable detection and monitoring of H 2 S is of significance in a wide range of areas: industrial production, medical diagnosis, environmental monitoring, and health screening. However the rapid corrosion of commercial platinum-on-carbon (Pt/C) sensing electrodes in the presence of H 2 S presents a fundamental challenge for fuel cell gas sensors. Herein we report a solution to the issue through the design of a sensing electrode, which is based on Pt supported on mesoporous titanium chromium nitrides (Pt/Ti 0.5 Cr 0.5 N). Its desirable characteristics are due to its high electrochemical stability and strong metal-support interactions. The Pt/Ti 0.5 Cr 0.5 N-based sensors exhibit a much smaller attenuation (1.3%) in response to H 2 S than Pt/C-sensor (40%), after 2 months sensing test. Furthermore, the Pt/Ti 0.5 Cr 0.5 N-based sensors exhibit negligible cross response to other interfering gases compared with hydrogen sulfide. Results of density functional theory calculation also verify the excellent long-term stability and selectivity of the gas sensor. Our work hence points to a new sensing electrode system that offers a combination of high performance and stability for fuel-cell gas sensors., (Copyright © 2021. Published by Elsevier B.V.)

Magnetite (Fe 3 O 4 ) is of fundamental importance for the Verwey transition near T V  = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe 3 O 4 nanoparticles leads to an intriguing variation of the Verwey transition: an initial drop of T V to a minimum at 70 K after 75 days and a followed recovery to 95 K after 160 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution between inhomogeneous to homogeneous doping regimes. This work demonstrates that slow oxidation of nanoparticles can give exquisite control and separation of homogeneous and inhomogeneous doping effects on the Verwey transition and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials., (© 2021. The Author(s).)

Calcium nitridonickelate(II) Ca 2 NiN 2 has been prepared through a high-temperature and high-pressure azide-mediated redox reaction, demonstrating that this method can stabilise nitrides of late transition metals in relatively high oxidation states. Ca 2 NiN 2 crystallizes in the Na 2 HgO 2 structure type and displays low-dimensional antiferromagnetic ordering of Ni 2+ spins.

While halide and oxide perovskites are numerous and many display outstanding properties, ABN 3 perovskite nitrides are extremely rare due to synthetic challenges arising from the low chemical potential of nitrogen and a tendency to form low-coordination nitridometallate anions. We report the preparation of a perovskite nitride LaReN 3 through azide-mediated oxidation at high pressure. High-resolution synchrotron diffraction shows that LaReN 3 has a low-symmetry, triclinic, perovskite superstructure resulting from orbital ordering with strong spin-orbit coupling distortions. Topotactic reduction of LaReN 3 above 500 °C leads to layered tetragonal LaReN 2 via a probable LaReN 2.5 intermediate, which is the first reported example of nitride defect perovskites. Magnetisation and conductivity measurements indicate that LaReN 3 and LaReN 2 are both metallic solids. The two chemical approaches presented are expected to lead to new classes of ABN 3 and defect ABN 3-x nitride perovskite materials., (© 2021 Wiley-VCH GmbH.)

Control of cation ordering in ABX 3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA'BB'O 6 double double perovskites, where both A and B sites have 1:1 cation order, to (A 0.5 A' 0.5 ) 2 BB'O 6 double perovskites with fully disordered A/A' cations can be achieved under pressure in CaMnMnWO 6 and SmMnMnTaO 6 , enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO 6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored., (© 2021 Wiley-VCH GmbH.)

The first triple perovskite with Mn in A- and 1 : 2 B-site order Mn 3 MnNb 2 O 9 , prepared using high pressure phase transformation of the magnetodielectric Mn 4 Nb 2 O 9 , is reported herein. It has a complex magnetic behaviour with a transition from a collinear AFM into an evolving incommensurate spin density wave (SDW) further stabilised into a lock-in structure dictated by the B-site order.

An instrument and software algorithm are described for the purpose of characterization of large single crystals at the Alignment Facility of the ISIS spallation neutron source. A method for both characterizing the quality of the sample and aligning it in a particular scattering plane is introduced. A software package written for this instrument is presented, and its utility is demonstrated by way of an example of the structural characterization of large single crystals of Pb(Mg 1/3 Nb 2/3 )O 3 . Extensions and modifications of characterization instruments for future improved beamlines are suggested. It is hoped that this software will be used by the neutron community for pre-characterizing large single crystals for spectroscopy experiments and that in the future such a facility will be included as part of the spectroscopy suite at other spallation neutron sources., (© Zihao Liu et al. 2021.)

Efficient catalysts are required for both oxidative and reductive reactions of hydrogen and oxygen in sustainable energy conversion devices. However, current precious metal-based electrocatalysts do not perform well across the full range of reactions and reported multifunctional catalysts are all complex hybrids. Here, we show that single-phase porous Co 3 Mo 3 N prepared via a facile method is an efficient and reliable electrocatalyst for three essential energy conversion reactions; oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) in alkaline solutions. Co 3 Mo 3 N presents outstanding OER, ORR, and HER activity with high durability, comparable with the commercial catalysts RuO 2 for OER and Pt/C for ORR and HER. In practical demonstrations, Co 3 Mo 3 N gives high specific capacity (850 mA h g Zn -1 at 10 mA cm -2 ) as the cathode in a zinc-air battery, and a low potential (1.63 V at 10 mA cm -2 ) used in a water-splitting electrolyzer. Availability of Co and Mo d-states appear to result in high ORR and HER performance, while the OER properties result from a cobalt oxide-rich activation surface layer. Our findings will inspire further development of bimetallic nitrides as cost-effective and versatile multifunctional catalysts that will enable scalable usage of electrochemical energy devices., Competing Interests: The authors declare no competing financial interests., (© 2021 The Authors.)

Semiconducting metal oxides (SMOXs) are used widely for gas sensors. However, the effect of ambient humidity on the baseline and sensitivity of the chemiresistors is still a largely unsolved problem, reducing sensor accuracy and causing complications for sensor calibrations. Presented here is a general strategy to overcome water-sensitivity issues by coating SMOXs with a hydrophobic polymer separated by a metal-organic framework (MOF) layer that preserves the SMOX surface and serves a gas-selective function. Sensor devices using these nanoparticles display near-constant responses even when humidity is varied across a wide range [0-90 % relative humidity (RH)]. Furthermore, the sensor delivers notable performance below 20 % RH whereas other water-resistance strategies typically fail. Selectivity enhancement and humidity-independent sensitivity are concomitantly achieved using this approach. The reported tandem coating strategy is expected to be relevant for a wide range of SMOXs, leading to a new generation of gas sensors with excellent humidity-resistant performance., (© 2020 Wiley-VCH GmbH.)

New Ni 3 TeO 6 -type (NTO) and double perovskite (DPv) polymorphs of Co 3 TeO 6 are synthesised at pressures of 15 GPa. A complex elliptic helical magnetic order is observed in the NTO polymorph (T N 1 = 58 K) that reorientates (42 K) and further splits (T N 2 = 23.5 K) creating a coexisting helix. Increasing Co content within the Mn 3-x Co x TeO 6 system changes the dominant DPv phase to NTO structural type and drastically modifies the magnetic behaviour. DPv Co 3 TeO 6 is the first A-site double cobaltite.

Metal-support interaction strongly influences the catalytic properties of metal-based catalysts. Here, titanium nitride (TiN) nanospheres are shown to be an outstanding support, for tuning the electronic property of platinum (Pt) nanoparticles and adjusting the morphology of indium sulfide (In 2 S 3 ) active components, forming flower-like core-shell nanostructures (TiN-Pt@In 2 S 3 ). The strong metal-support interaction between Pt and TiN through the formation of Pt-Ti bonds favors the migration of charge carriers and leads to the easy reducibility of TiN-Pt, thus improving the photocatalytic atom efficiency of Pt. The TiN-Pt@In 2 S 3 composite shows reduction of Pt loading by 70% compared to the optimal Pt-based system. In addition, the optimal TiN-Pt@In 2 S 3 composite exhibits a H 2 evolution rate 4 times that of a Pt reference. This increase outperforms all other supports reported thus far.

Transition metal nitrides are an important class of materials with applications as abrasives, semiconductors, superconductors, Li-ion conductors, and thermoelectrics. However, high oxidation states are difficult to attain as the oxidative potential of dinitrogen is limited by its high thermodynamic stability and chemical inertness. Here we present a versatile synthesis route using azide-mediated oxidation under pressure that is used to prepare the highly oxidised ternary nitride Ca 4 FeN 4 containing Fe 4+ ions. This nitridometallate features trigonal-planar [FeN 3 ] 5- anions with low-spin Fe 4+ and antiferromagnetic ordering below a Neel temperature of 25 K, which are characterised by neutron diffraction, 57 Fe-Mössbauer and magnetisation measurements. Azide-mediated high-pressure synthesis opens a way to the discovery of highly oxidised nitrides.

The magnetic ground state of the pyrochlore Yb 2 GaSbO 7 has remained an enigma for nearly a decade. The persistent spin fluctuations observed by muon spin relaxation measurements at low temperatures have not been adequately explained for this material using existing theories for quantum magnetism. Here we report on the synthesis and characterisation of Yb 2 GaSbO 7 to elucidate the central physics at play. Through DC and AC magnetic susceptibility, heat capacity, and neutron scattering experiments, we observe evidence for a dynamical ground state that makes Yb 2 GaSbO 7 a promising candidate for disorder-induced spin-liquid or spin-singlet behaviour. This state is quite fragile, being tuned to a splayed ferromagnet in a modest magnetic field μ 0 H c ∼ 1.5 T .

Mn 2 NiReO 6 , prepared at high pressure and temperature, has a highly-distorted double perovskite structure. Canted antiferromagnetic order is observed below T M1 = 80 K and an unusual continuous spin rotation of Mn spins occurs down to T M2 = 42 K where a collapse in weak ferromagnetism evidences an unprecedented switching of the weak ferromagnetic moment directions.

The oxygen evolution reaction (OER) is key to renewable energy technologies such as water electrolysis and metal-air batteries. However, the multiple steps associated with proton-coupled electron transfer result in sluggish OER kinetics and catalysts are required. Here we demonstrate that a novel nitride, Ni 2 Mo 3 N, is a highly active OER catalyst that outperforms the benchmark material RuO 2 . Ni 2 Mo 3 N exhibits a current density of 10 mA cm -2 at a nominal overpotential of 270 mV in 0.1 m KOH with outstanding catalytic cyclability and durability. Structural characterization and computational studies reveal that the excellent activity stems from the formation of a surface-oxide-rich activation layer (SOAL). Secondary Mo atoms on the surface act as electron pumps that stabilize oxygen-containing species and facilitate the continuity of the reactions. This discovery will stimulate the further development of ternary nitrides with oxide surface layers as efficient OER catalysts for electrochemical energy devices., (© 2020 Wiley-VCH GmbH.)

Cs 4 PbBr 6 is regarded as an outstanding luminescent material with good thermal stability and optical performance. However, the mechanism of green emission from Cs 4 PbBr 6 has been controversial. Here we show that isolated CsPbBr 3 nanoparticles embedded within a Cs 4 PbBr 6 matrix give rise to a "normal" green luminescence while superfluorescence at longer wavelengths is suppressed. High-resolution transmission electron microscopy shows that the embedded CsPbBr 3 nanoparticles are around 3.8 nm in diameter and are well-separated from each other, perhaps by a strain-driven mechanism. This mechanism may enable other efficient luminescent composites to be developed by embedding optically active nanoparticles epitaxially within inert host lattices.

The synthesis, structures and magnetism of six mixed 3d-5d oxides Ba3BM2O9 (B = Ti, Y, Zn; M = Ru, Os) are described. When prepared at ambient pressure the six oxides display a 6H type perovskite structure comprised of corner sharing BO6 and face sharing M2O9 motifs. Synchrotron X-ray diffraction reveals a small monoclinic distortion in Ba3ZnRu2O9; the remaining oxides exhibit a hexagonal structure. The magnetic properties are dominated by the M-M interactions across the shared face. Only in the mixed valent (M4+/M5+) Y oxides is evidence of long-range magnetic order found. Application of high pressure/high temperature synthetic methods for the Ru containing oxides changes the structure to the archetypical cubic Pm3[combining macron]m perovskite structure, where the B and Ru cations are disordered on the corner sharing BO6 octahedral sites. The magnetic properties of the cubic oxides are dominated by short range antiferromagnetic interactions, the chemical disorder inhibiting long range ordering.

Negative volume expansion between 620 and 800 K in V 2 OPO 4 is discovered to be of electronic origin due to the charge ordering transition at 605 K. Domain reorientation and coexistence of the low and high temperature phases close to the transition are observed in X-ray diffraction data from single crystals grown by chemical vapour transport.

Motivated by the presence of an unquenched orbital angular momentum in CoO, a team at Chalk River, including a recently hired research officer Roger Cowley, performed the first inelastic neutron scattering experiments on the classic Mott insulator [Sakurai et al 1968 Phys. Rev. 167 510]. Despite identifying two magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum below T N using conventional pseudo-bosonic approaches, instead achieving only qualitative agreement. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high- T c cuprate superconductors [Bednorz and Muller 1986 Z. Phys. B 64 189], would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him at the start of his career. Upon his return to CoO, Roger found a system embroiled in controversy, with some of its most fundamental parameters still remaining undetermined. Faced with such a formidable task, Roger performed a series of inelastic neutron scattering experiments in the early 2010s on both CoO and a magnetically dilute structural analogue Mg 0.97 Co 0.03 O. These experiments would prove instrumental in the determination of both single-ion [Cowley et al 2013 Phys. Rev. B 88 205117] and cooperative magnetic parameters [Sarte et al 2018 Phys. Rev. B 98 024415] for CoO. Both these sets of parameters would eventually be used in a spin-orbit exciton model [Sarte et al 2019 Phys. Rev. B 100 075143], developed by his longtime friend and collaborator Bill Buyers, to successfully parameterise the complex spectrum that both measured at Chalk River almost 50 years prior. The story of CoO is of one that has come full circle, one filled with both spectacular failures and intermittent, yet profound, little victories., (Creative Commons Attribution license.)

Charge ordering creates a spontaneous array of differently charged ions and is associated with electronic phenomena such as superconductivity, colossal magnetoresistances (CMR), and multiferroicity. Charge orders are usually suppressed by chemical doping and site selective doping of a charge ordered array has not previously been demonstrated. Here we show that selective oxidation of one out of eight distinct Fe 2+ sites occurs within the complex Fe 2+ /Fe 3+ ordered structure of 2%-doped magnetite (Fe 3 O 4 ), while the rest of the charge and orbitally ordered network remains intact. This 'charge order within a charge order' is attributed to the relative instability of the trimeron distortion surrounding the selected site. Our discovery suggests that similar complex charge ordered arrays could be used to provide surface sites for selective redox reactions, or for storing information by doping specific sites.

As sustainable energy becomes a major concern for modern society, renewable and clean energy systems need highly active, stable, and low-cost catalysts for the oxygen evolution reaction (OER). Mesoporous materials offer an attractive route for generating efficient electrocatalysts with high mass transport capabilities. Herein, we report an efficient hard templating pathway to design and synthesize three-dimensional (3-D) mesoporous ternary nickel iron nitride (Ni3FeN). The as-synthesized electrocatalyst shows good OER performance in an alkaline solution with low overpotential (259 mV) and a small Tafel slope (54 mV dec -1 ), giving superior performance to IrO 2 and RuO 2 catalysts. The highly active contact area, the hierarchical porosity, and the synergistic effect of bimetal atoms contributed to the improved electrocatalytic performance toward OER. In a practical rechargeable Zn-air battery, mesoporous Ni 3 FeN is also shown to deliver a lower charging voltage and longer lifetime than RuO 2 . This work opens up a new promising approach to synthesize active OER electrocatalysts for energy-related devices.

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO 3 with a simple ABO 3 composition under high pressure. PbCoO 3 possesses a peculiar A- and B-site ordered charge distribution Pb 2+ Pb 4+ 3 Co 2+ 2 Co 3+ 2 O 12 with insulating behavior at ambient conditions. The high spin Co 2+ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb 4+ and Co 2+ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

Platinum (Pt)-based materials are important components of microelectronic sensors, anticancer drugs, automotive catalytic converters and electrochemical energy conversion devices 1 . Pt is currently the most common catalyst used for the oxygen reduction reaction (ORR) in devices such as fuel cells and metal-air batteries 2,3 , although a scalable use is restricted by the scarcity, cost and vulnerability to poisoning of Pt (refs 4-6 ). Here we show that nanoparticulate zirconium nitride (ZrN) can replace and even surpass Pt as a catalyst for ORR in alkaline environments. As-synthesized ZrN nanoparticles (NPs) exhibit a high oxygen reduction performance with the same activity as that of a widely used Pt-on-carbon (Pt/C) commercial catalyst. Both materials show the same half-wave potential (E 1/2  = 0.80 V) and ZrN has a higher stability (ΔE 1/2  = -3 mV) than the Pt/C catalyst (ΔE 1/2  = -39 mV) after 1,000 ORR cycles in 0.1 M KOH. ZrN is also shown to deliver a greater power density and cyclability than Pt/C in a zinc-air battery. Replacement of Pt by ZrN is likely to reduce costs and promote the usage of electrochemical energy devices, and ZrN may also be useful in other catalytic systems.

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 Pb 2 FeTeO 6.5 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 Pb 2 FeTeO 6 . 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.

A new double perovskite Mn2MnTeO6 has been obtained by high pressure phase transformation of a corundum-related precursor. It is antiferromagnetic below 36 K and develops a magnetic structure with magnetic moments of 4.8 μB and 3.8 μB for Mn2+ at the A and B sites respectively. This new polymorph accounts for a recently reported decrease in the bandgap of Mn3TeO6 under pressure that may lead to useful light-harvesting properties.

Two new double double perovskites of ideal compositions CaMnMReO6 (M = Co, Ni) are reported. The M = Co material has refined composition CaMn0.7Co1.3ReO6 and orders ferrimagnetically below TC = 188 K with a relatively large saturated magnetisation of 4.5 μB. The M = Ni product, CaMn1.2Ni0.8ReO6, is a remarkable example of a ferromagnetic oxide with four distinct spin sublattices all collinearly ordered below TC = 152 K.

Ordering of cations in different structural types occurs when there is a significant difference in the oxidation states and ionic radii of the ions involved. Herein we report an unusual ordering of isovalent cations Fe 3+ and Al 3+ in the polar rhombohedral R3 double perovskite structure of Bi 2 FeAlO 6 synthesized at high-pressure (6 GPa) and high-temperature (1000 °C). This ordered structure is derived from the 1:1 combination of the polar oxides BiFeO 3 (R3c) and BiAlO 3 (R3c), which results in reduction of symmetry to an R3 structure where the Fe 3+ and Al 3+ ions are ordered in a rock salt manner. However, these ions remain disordered in BiFe 1-x Al x O 3 (x=0.2, 0.3, 0.4) perovskites with R3c structure. The ordered compound undergoes antiferromagnetic ordering at T N ≈280 K. The butterfly nature of piezoelectric displacement loop further confirms the polar nature of the cation-ordered Bi 2 FeAlO 6 ., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A ferrite in the Sr 2 Tl 2 O 5 -type MFe 3 O 5 family with M = Co has been synthesized at 12 GPa pressure. Neutron diffraction shows the sample to be Co deficient with composition Co 0.6 Fe 3.4 O 5 . The Co/Fe cation distribution is found to be profoundly different from those of MFe 3 O 5 analogs and lies between normal and inverse limits, as Co 2+ substitutes across trigonal prismatic and one of the two octahedral sites. CoFe 3 O 5 shows complex magnetic behavior with weak ferromagnetism below T C1 ≈ 300 K and a second transition to ferrimagnetic order at T C2 ≈100 K. Spin scattering of carriers leads a substantial increase in the hopping activation energy below T C1 , and a small negative magnetoresistance is observed at low temperatures.