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In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 A along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

B-site Os-doped quadruple perovskite oxides LaCu3Fe4-xOsxO12 (x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu3Fe4O12 experiences Cu-Fe intermetallic charge transfer that changes the Cu3+/Fe3+ charge combination to Cu2+/Fe3.75+ at 393 K, in the Os-doped samples, the Cu and Fe charge states are found to be constant 2+ and 3+, respectively, indicating the complete suppression of charge transfer. Correspondingly, Os6+ and mixed Os4.5+ valence states are determined by X-ray absorption spectroscopy for x = 1 and x = 2 compositions, respectively. The x = 1 sample crystallizes in an Fe/Os disordered structure with the Im3 space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn3, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu2+(↑)Fe3+(↑)Os4.5+(↓) ferrimagnetic ordering near 520 K. This work provides a rare example for 5d substitution-suppressed intermetallic charge transfer as well as induced structural and magnetic phase transitions with high spin ordering temperature.

We report the experimental observation of and theoretical explanation for the reduction of dopant ions and enhancement of magnetic properties in Ce-doped TiO2 diluted magnetic semiconductors from UV-light irradiation. Substantial increase in Ce3+ concentration and creation of oxygen vacancy defects in the sample due to UV-light irradiation was observed by X-ray and optical methods. Magnetic measurements demonstrate a combination of paramagnetism and ferromagnetism up to room temperatures in all samples. The magnetization of both paramagnetic and ferromagnetic components was observed to be dramatically enhanced in the irradiated sample. First-principle theoretical calculations show that valence holes created by UV irradiation can substantially lower the formation energy of oxygen vacancies. While the electron spin densities for defect states near oxygen vacancies in pure TiO2 are in antiferromagnetic orientation, they are in ferromagnetic orientations in Ce-doped TiO2. Therefore, the ferromagnetically-oriented spin densities near oxygen vacancies created by UV irradiation are the most probable cause for the experimentally observed enhancement of magnetism in the irradiated Ce-doped TiO2.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

Developing a versatile electrocatalyst with remarkable performance viable for pH-universal overall water splitting is increasingly important for the industrial production of renewable energy conversion. Herein, our theoretical calculations predicate that the limitations in the mean-field behavior from the traditional catalyst designing strategy can be largely overcome by introducing diluted metal nanoclusters, which can give an optimal thermodynamic effect for enhancing electron-transfer capability, and in turn promote the activation of initial water-dissociation for both the hydrogen evolution reaction and oxygen evolution reaction. As a proof of concept, a unique catalyst, namely diluted nickel nanocluster-decorated ruthenium nanowires, was explored as a high-performance electrocatalyst for overall water splitting. The optimized catalyst delivered record activity for overall water splitting in a wide pH range from 0 to 14 with all the potentials lower than 1.454 V to achieve the current density of 10 mA cm−2, largely outperforming the Pt/C–Ir/C integrated couple. It also readily reaches a high current density, of up to 100 mA cm−2, with a low voltage of only 1.55 V applied. It is further demonstrated that the diluted nickel nanoclusters can strongly anchor on the ruthenium nanowires, contributing to the enhanced stability after the long-term tests. The diluted metal nanocluster-enhanced strategy highlights a general pathway for the rational design of catalysts with unprecedented performance for electrocatalysis and beyond.

Cost-effective electrocatalysts with high activity and long durability for the oxygen evolution reaction (OER) are key to water splitting and rechargeable metal–air batteries. Here, we report the development of a superior OER electrocatalyst with outstanding activity, favorable durability, and stable particulate morphology based on an ex situ ultra-fast cation exchange strategy that can result in fine tuning of the atom arrangement inside the oxide lattice, thus optimizing the electrocatalytic performance. O3-phase NaCo0.8Fe0.2O2 (O-NCF) is selected as the starting material, and the sodium in the oxide lattice is rapidly exchanged (several minutes) with hydronium ions (H3O+) in an acidic solution. The as-derived structure fine-tuned sample displays excellent OER performances in alkaline media with an ultra-low overpotential of only 234 mV at 10 mA cm−2 in oxide-based electrocatalysts and an ultra-small Tafel slope of 34 mV dec−1. The exchange of H3O+ with Na+ does not affect the oxidation state of cobalt and iron cations inside the oxide lattice, while protons in the inserted H3O+ promote the formation of the hydroxyl group to improve activity. As a general strategy, such cation exchange strategy can also be applied to many other layered sodium transition metal oxides.

Inverse opals are three-dimensional ordered macroporous materials whose pores are arranged in a hexagonal array with interconnected pore channels. These unique structural attributes provide an excessive surface area with facile mass transport. In this work, we fabricate a composite NiCoO2/NiCo2O4 thin layer encapsulating the skeletons of Ni inverse opals serving as the conductive substrate, and explore their electrocatalytic activities for the oxygen evolution reaction (OER) in an alkaline electrolyte. The NiCoO2/NiCo2O4 inverse opals are prepared by electroplating of NiCo alloy on Ni inverse opals followed by a mild oxidation treatment in air at 200 °C for 20 min. The X-ray diffraction patterns indicate mixed phases of crystalline NiCoO2, NiCo2O4, and NiCo, and images from scanning electron microscopy demonstrate the conformal formation of NiCoO2/NiCo2O4/NiCo on the Ni inverse opaline skeletons. Analysis from X-ray photoelectron spectroscopy determines the oxidation states for surface Co and Ni ions to be both +2 and +3. Electrochemical measurements including the active surface area and OER performance reveal a large surface area and strong OER activity with a relatively flat Tafel slope of 95 mV dec−1. In addition, from durability tests in both galvanostatic and potentiostatic modes, the sample shows impressive stability in both profiles while the inverse opaline structure is nicely maintained.

International audience; Synthetic crystals of the ferberite FeWO 4 have been grown by the chemical transport reaction starting from a polycrystalline sample of this phase. Magnetic susceptibility measurements showed an antiferromagnetic Neél temperature of T N = 75 K. The anisotropy in the magnetic susceptibility can be ascribed to the magnetocrystalline anisotropy of the Fe 2+ ion, the oxidation state of which was confirmed by X-ray absorption spectroscopy. While X-ray photoemission analysis indicated that all W ions are in the expected 6+ charge state, the dielectric permittivity of the FeWO 4 crystals was found to be leaky, hindering changes at T N to be detected. Subsequent thermoelectric power measurements suggested the presence of about 1.5% Fe 3+. X-ray diffraction experiments confirmed the basic crystal structure of the wolframite type and revealed some structural disorder in the 1% range. Transmission electron microscopy allowed us to unveil the occurrence of stacking faults attributed to the similarity of the atomic environment of the Fe and W species.

Antimonene is a promising two-dimensional (2D) material that is calculated to have a significant fundamental bandgap usable for advanced applications such as field-effect transistors, photoelectric devices, and the quantum-spin Hall (QSH) state. Herein, we demonstrate a phenomenon termed topological proximity effect, which occurs between a 2D material and a three-dimensional (3D) topological insulator (TI). We provide strong evidence derived from hydrogen etching on Sb2Te3 that large-area and well-ordered antimonene presents a 2D topological state. Delicate analysis with a scanning tunneling microscope of the evolutionary intermediates reveals that hydrogen etching on Sb2Te3 resulted in the formation of a large area of antimonene with a buckled structure. A topological state formed in the antimonene/Sb2Te3 heterostructure was confirmed with angle-resolved photoemission spectra and density-functional theory calculations; in particular, the Dirac point was located almost at the Fermi level. The results reveal that Dirac fermions are indeed realized at the interface of a 2D normal insulator (NI) and a 3D TI as a result of strong hybridization between antimonene and Sb2Te3. Our work demonstrates that the position of the Dirac point and the shape of the Dirac surface state can be tuned by varying the energy position of the NI valence band, which modifies the direction of the spin texture of Sb-BL/Sb2Te3via varying the Fermi level. This topological phase in 2D-material engineering has generated a paradigm in that the topological proximity effect at the NI/TI interface has been realized, which demonstrates a way to create QSH systems in 2D-material TI heterostructures.

Improving the catalytic efficiency of platinum (Pt) for hydrogen evolution reaction (HER) is crucial for water splitting technologies, and hydrogen spillover has emerged as a new frontier in designing the binary-component Pt/support HER electrocatalysts. However, such binary catalysts always suffer from long reaction pathway, undesirable interfacial barrier, and complicated synthesis processes. Here we report a single-phase complex oxide La2Sr2PtO7+δ as a high-performance HER electrocatalysts in acidic media via a unique atomic-scale hydrogen spillover effect between multifunctional catalytic sites. With insights from theoretical calculations, a possible synergistic mechanism involving the hydrogen spillover channel from OLa site→La-Pt bridge site→Pt site is proposed; namely, the OLa site enriches proton, the La-Pt bridge site with thermo-neutral H* adsorption facilitates the hydrogen spillover and H2 generation, and Pt site favors the final H2 desorption. Benefiting from such unusual phenomenon, the resulting La2Sr2PtO7+δ exhibits an exceptional HER electrode activity with low overpotential of 13 mV at 10 mA cm− 2 and small Tafel slope of 22 mV dec− 1, and significantly enhanced intrinsic activity and durability than commercial Pt black catalyst.

A new 3d–5d hybridization oxide, CaCu3Mn2Os2O12 (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered q...

Ultraviolet (UV) light irradiation on CeO2 nanocrystals catalysts has been observed to largely increase the material’s catalytic activity and reactive surface area. As revealed by x-ray absorption near edge structure (XANES) analysis, the concentration of subvalent Ce3+ ions in the irradiated ceria samples progressively increases with the UV-light exposure time. The increase of Ce3+ concentration as a result of UV irradiation was also confirmed by the UV-vis diffuse reflectance and photoluminescence spectra that indicate substantially increased concentration of oxygen vacancy defects in irradiated samples. First-principle formation-energy calculation for oxygen vacancy defects revealed a valence-hole-dominated mechanism for the irradiation-induced reduction of CeO2 consistent with the experimental results. Based on a Mars-van Krevelen mechanism for ceria catalyzed oxidation processes, as the Ce3+ concentration is increased by UV-light irradiation, an increased number of reactive oxygen atoms will be captured from gas-phase O2 by the surface Ce3+ ions, and therefore leads to the observed catalytic activity enhancement. The unique annealing-free defect engineering method using UV-light irradiation provides an ultraconvenient approach for activity improvement in nanocrystal ceria for a wide variety of catalytic applications.

The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. The three-wave Si(222)/(13 {\overline 1}) diffraction at 7.82 keV is used for phase measurements. It is found that laser excitation causes a relative phase change of around 4° in Si(222) in the first 100 ps of excitation and this is gradually recovered over several nanoseconds. This phase change is due to laser excitation of covalent electrons around the silicon atoms in the unit cell and makes the electron density deviate further from the centrosymmetric distribution.

An integrated optical chip that minimizes the size of the energy-tuning single-resonance-mode x-ray monochromator system into a 3 c m × 5 c m silicon wafer is proposed. A Fabry–Perot x-ray resonator and two back-reflecting Si mirrors are employed on the wafer as the optical components, where Si(12 4 0) back reflection is used for both Fabry–Perot resonance and re-diffraction of the x-ray beams from the resonator in the incident direction. We can achieve an energy bandwidth of 3.4 meV in single-mode x rays and tune the energy by temperature variation. Such Si chips can be readily employed at the synchrotron beamlines and conventional x-ray laboratories for high-resolution investigations.

A quadruple perovskite oxide $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{2}{\mathrm{Re}}_{2}{\mathrm{O}}_{12}$ was synthesized by high-pressure annealing. This compound crystallizes in an $A$- and $B$-site ordered quadruple perovskite structure with space group $Pn\text{\ensuremath{-}}3$. The charge combination is determined to be $\mathrm{Ca}{{\mathrm{Cu}}^{2+}}_{3}{{\mathrm{Co}}^{2+}}_{2}{{\mathrm{Re}}^{6+}}_{2}{\mathrm{O}}_{12}$ by bond valence sum analysis and x-ray absorption spectroscopy. In contrast to other isostructural $A{\mathrm{Cu}}_{3}{B}_{2}{B}_{2}^{\ensuremath{'}}{O}_{12}$ compounds with a single magnetic transition, a long-range antiferromagnetic phase transition originating from the ${A}^{\ensuremath{'}}$-site ${\mathrm{Cu}}^{2+}$ sublattice is found to occur at ${T}_{N}\ensuremath{\approx}28\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Subsequently, the spin coupling between the $B$-site ${\mathrm{Co}}^{2+}$ and ${B}^{\ensuremath{'}}$-site ${\mathrm{Re}}^{6+}$ ions contributes to a ferrimagnetic transition around ${T}_{\mathrm{C}}\ensuremath{\approx}20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Strong electrical insulating behavior is identified by optical measurement with an energy gap of approximately 3.75 eV. The mechanisms of the spin interactions are discussed in detail.

Perovskite oxides are of particular interest for the oxygen evolution reaction (OER) due to their high intrinsic activity. However, low surface area and nonpores in bulk phase generally limit the mass transport and thereby result in unsatisfactory mass activity. Herein, we propose a "molecular-level strategy"with the simultaneous modulation of the ordered pores on the oxygen-deficient sites along with sulfur (S) substitution on oxygen sites at the molecular level to boost the mass transport behavior of perovskite electrocatalyst for enhanced mass activity. As a proof of concept, the elaborately designed brownmillerite oxide Sr2Co1.6Fe0.4O4.8S0.2 (S-BM-SCF) shows approximately fourfold mass activity enhancement in 1 M KOH compared with the pristine SrCo0.8Fe0.2O3-δ (SCF) perovskite. Comprehensive experimental results, in combination with theoretical calculations, demonstrate that the intrinsic molecular-level pores in the brownmillerite structure can facilitate reactive hydroxyl ion (OH-) uptake into the oxygen-vacant sites and that S doping further promotes OH- adsorption by electronic structure modulation, thus accelerating mass transport rate. Meanwhile, the S-BM-SCF can significantly weaken the resistance of O2 desorption on the catalyst surface, facilitating the O2 evolution. This work deepens the understanding of how mass transport impacts the kinetics of the OER process and opens up a new avenue to design high-performance catalysts on the molecular level. Published version This work was supported by National Natural Science Foundation of China (Grant Nos. 21576135 and 21878158) and Jiangsu Natural Science Foundation for Distinguished Young Scholars (Grant No. BK20170043). Dr. H. A. Tahini acknowledges the resources provided by the National Computational Infrastructure (NCI) facility at the Australian National University through the National Computational Merit Allocation Scheme. Dr. Y. Zhu acknowledges the Australian Research Council (Discovery Early Career Researcher Award No. DE190100005, and Discovery Project No. DP200100500). We acknowledge support from the Max Planck-POSTECH/Hsinchu Center for Complex Phase Materials.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of the most classical perovskite-based functional materials. While there are well-known simple perovskites with ferroelectric properties, so far there is no report of ferrolectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites, AA′3B4O12. Here we report the synthesis at high pressure and temperature of a new A-site-ordered perovskite, PbHg3Ti4O12. Remarkably, PbHg3Ti4O12 is the only known quadruple perovskite that transforms from a high-temperature centrosymmetric (Im-3), paraelectric phase to a low-temperature, non-centrosymmetric (Imm2) ferroelectric phase. Moreover, the average ionic radius of A-site cations for PbHg3Ti4O12 is large~ 1.1 Å and the tolerance factor t is about 0.88. Surprisingly the coordination chemistry of Hg2+ is changed from the usual square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12 driven via pressures. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ATiO3 perovskite to AA′3B4O12 type A-site-ordered perovskites, sharing both displacive ferroelectricity with the former and the structure coordination with the latter. This is the first example of ferroelectricity due to a symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskite, and opens a new direction to search for ferroelectric materials in combinatory perovskites.

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials., There are few reports of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here, the authors find one with phase transition from a high-temperature centrosymmetric paraelectric phase to a low-temperature non-centrosymmetric ferroelectric phase in a high pressure synthesized compound.

Room-temperature ferrimagnetism was discovered for the anti-site-disordered perovskite $\mathrm{C}{\mathrm{a}}_{2}\mathrm{MnOs}{\mathrm{O}}_{6}$ with ${T}_{\mathrm{c}}=305\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. $\mathrm{C}{\mathrm{a}}_{2}\mathrm{MnOs}{\mathrm{O}}_{6}$ crystallizes into an orthorhombic structure with a space group of Pnma, in which Mn and Os share the oxygen-coordinated-octahedral site at an equal ratio without a noticeable ordered arrangement. The material is electrically semiconducting with variable-range-hopping behavior. X-ray absorption spectroscopy confirmed the trivalent state of the Mn and the pentavalent state of the Os. X-ray magnetic circular dichroism spectroscopy reveals that the Mn and Os magnetic moments are aligned antiferromagnetically, thereby classifying the material as a ferrimagnet which is in accordance with band structure calculations. It is intriguing that the magnetic signal of the Os is very weak, and that the observed total magnetic moment is primarily due to the Mn. The ${T}_{\mathrm{c}}=305\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is the second highest in the material category of so-called disordered ferromagnets such as $\mathrm{CaR}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}{\mathrm{O}}_{3}$, $\mathrm{SrR}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}{\mathrm{O}}_{3}$, and $\mathrm{CaI}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}{\mathrm{O}}_{3}$, and hence, may support the development of spintronic oxides with relaxed requirements concerning the anti-site disorder of the magnetic ions.

This work reports an unconventional defect engineering approach using synchrotron-radiation-based X-rays on ceria nanocrystal catalysts of particle sizes 4.4–10.6 nm. The generation of a large number of oxygen-vacancy defects (OVDs), and therefore an effective reduction of cations, has been found in CeO2 catalytic materials bombarded by high-intensity synchrotron X-ray beams of beam size 1.5 mm × 0.5 mm, photon energies of 5.5–7.8 keV and photon fluxes up to 1.53 × 1012 photons s−1. The experimentally observed cation reduction was theoretically explained by a first-principles formation-energy calculation for oxygen vacancy defects. The results clearly indicate that OVD formation is mainly a result of X-ray-excited core holes that give rise to valence holes through electron down conversion in the material. Thermal annealing and subvalent Y-doping were also employed to modulate the efficiency of oxygen escape, providing extra control on the X-ray-induced OVD generating process. Both the core-hole-dominated bond breaking and oxygen escape mechanisms play pivotal roles for efficient OVD formation. This X-ray irradiation approach, as an alternative defect engineering method, can be applied to a wide variety of nanostructured materials for physical-property modification.

Hydrogenation has recently been proposed for use in modulating the magnetic properties of Pd-rich ferromagnetic (FM) alloy films. This study successfully fabricated Pd-rich Mn/MnPd/Fe antiferromagnetic/FM films with an established exchange bias field at RT, and determined that hydrogenation can enhance the exchange bias coupling in such films. Specifically, magnetic hysteresis loops revealed that the magnetic state of the Pd-rich Mn/MnPd/Fe films was gradually changed from an unbiased state to an exchange-biased state upon exposure to a hydrogen environment; moreover, hydrogenation engendered a larger magnitude of the exchange bias field than conventional field cooling. This phenomenon can be attributed to the enhanced long-range antiferromagnetic ordering of the Pd-rich MnPd film upon uptake of hydrogen atoms.

Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TI-based advanced electronic and spintronic devices.

A dramatic band gap narrowing of 1.61 eV has been observed in Co-doped nanocrystals of CeO2 (ceria), as a result of thermal annealing, without changing the ceria crystal structure and the Co concentration. As demonstrated by x-ray absorption fine structures, thermal annealing incurs an oxygen coordination rearrangement around Co atoms from an octahedral coordination to a square-planar coordination. First principle calculation using density functional theory reveals two stable oxygen coordination types surrounding Co, consistent with the experimental observation. The band gap values calculated for the two stable coordination types differ dramatically, reproducing the experimentally observed band gap narrowing. These prominent effects due to local structure rearrangement around dopant atoms can lead to unprecedented methods for band gap engineering in doped nanocrystal oxides.

A new 3d-5d hybridization oxide, CaCu

A new 3d-5d hybridized quadruple perovskite oxide, CaCu

We report on our investigation on the magnetism of the iridate double perovskite ${\mathrm{Sr}}_{2}{\mathrm{CoIrO}}_{6}$, a nominally ${\mathrm{Ir}}^{5+}$ Van Vleck ${J}_{\mathrm{eff}}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-${L}_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the ${J}_{\mathrm{eff}}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-${L}_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% ${\mathrm{Ir}}^{5+}$ (${J}_{\mathrm{eff}}=0$) and ${\mathrm{Co}}^{3+}$ ($S=2$) and 10% ${\mathrm{Ir}}^{6+}$ ($S=3/2$) and ${\mathrm{Co}}^{2+}$ ($S=3/2$). The magnetic signal of this minority ${\mathrm{Ir}}^{6+}$ component is almost equally as strong as that of the main ${\mathrm{Ir}}^{5+}$ component. We infer that there is a competition between the ${\mathrm{Ir}}^{5+}\ensuremath{-}{\mathrm{Co}}^{3+}$ and the ${\mathrm{Ir}}^{6+}\ensuremath{-}{\mathrm{Co}}^{2+}$ configurations in this stoichiometric compound.

Ultraviolet (UV) light irradiation on CeO2 nanocrystals catalysts has been observed to largely increase the material’s catalytic activity and reactive surface area. As revealed by x-ray absorption near edge structure (XANES) analysis, the concentration of subvalent Ce3+ ions in the irradiated ceria samples progressively increases with the UV-light exposure time. The increase of Ce3+ concentration as a result of UV irradiation was also confirmed by the UV-vis diffuse reflectance and photoluminescence spectra that indicate substantially increased concentration of oxygen vacancy defects in irradiated samples. First-principle formation-energy calculation for oxygen vacancy defects revealed a valence-hole-dominated mechanism for the irradiation-induced reduction of CeO2 consistent with the experimental results. Based on a Mars-van Krevelen mechanism for ceria catalyzed oxidation processes, as the Ce3+ concentration is increased by UV-light irradiation, an increased number of reactive oxygen atoms will be captured from gas-phase O2 by the surface Ce3+ ions, and therefore leads to the observed catalytic activity enhancement. The unique annealing-free defect engineering method using UV-light irradiation provides an ultraconvenient approach for activity improvement in nanocrystal ceria for a wide variety of catalytic applications.

A new 3d-5d hybridized quadruple perovskite oxide, CaCu3Mn2Ir2O12, was synthesized by high-pressure and high-temperature methods. The Rietveld structure analysis reveals that the compound crystallizes in an [Formula: see text]-type perovskite structure with space group Im-3, where the Ca and Cu are 1:3 ordered at fixed atomic positions. At the B site the 3d Mn and the 5d Ir ions are disorderly distributed due to the rare equal +4 charge states for both of them as determined by x-ray absorption spectroscopy. The competing antiferromagnetic and ferromagnetic interactions among Cu2+, Mn4+, and Ir4+ ions give rise to spin glass behavior, which follows a conventional dynamical slowing down model.

MgO-based magnetic heterostructures are crucial building blocks in state-of-the-art spintronic devices. In this study, we performed thermal annealing to control the formation and distribution of FeO across the interface of a Fe-MgO system. The exchange bias phenomenon (i.e., a horizontal shift of magnetic hysteresis loops) was extended from involving only interface spins to involving the full-volume magnetization of the Fe film. The extended and reinforced exchange bias coupling in the annealed Fe/MgO(0 0 1) films can be attributed to the formation of a Fe-FeO matrix-like structure at the Fe/MgO interface under an increasing degree of interface roughness and the reduction or diffusion of oxygen from the interfacial oxides, as indicated by X-ray absorption near edge structures and X-ray reflectivity measurements. Moreover, the thermal stability of exchange bias coupling in the annealed Fe/MgO(0 0 1) films (maximum value at approximately 600 K) was considerably higher than that in conventional thin-film-based Fe/FeO structures (approximately 50 K).

Constructing catalysts with a low overpotential for the oxygen evolution reaction (OER) is critical for the increased application of rechargeable Zn-air batteries. Herein, an in-situ cobalt extraction and simultaneous coordination growth strategy is adopted to develop an active and robust interface between nanostructural Co-metal-organic framework (Co-MOF) pillars and perovskite LaCoO3-δ (LC). Moreover, the multi-active cobalt sites, including Co3+ in the LC and coordinatively unsaturated cobalt sites in the Co-MOF, could promote the deprotonation of oxygen adsorption intermediates and act as catalytic active centres. Therefore, the optimal sample exhibits excellent OER activity with a low overpotential of 330 mV. Notably, the Zn-air battery with Co-MOF/LC-0.5 as the OER catalyst exhibits a low charge potential of 2.03 V and excellent cycling stability. This work illustrates efficient electrochemical catalysts based on perovskites with low overpotentials that are prepared by an in-situ growth strategy, and have great potential in rechargeable Zn-air batteries.

The non-stoichiometric double perovskite oxide La2Ni1.19Os0.81O6 was synthesized by solid state reaction and its crystal and magnetic structures were investigated by powder x-ray and neutron diffraction. La2Ni1.19Os0.81O6 crystallizes in the monoclinic double perovskite structure (general formula A2BB'O6) with space group P21/n, where the B site is fully occupied by Ni and the B' site by 19 % Ni and 81 % Os atoms. Using x-ray absorption spectroscopy an Os4.5+ oxidation state was established, suggesting presence of about 50 % paramagnetic Os5+ (5d3, S = 3/2) and 50 % non-magnetic Os4+ (5d4, Jeff = 0) ions at the B' sites. Magnetization and neutron diffraction measurements on La2Ni1.19Os0.81O6 provide evidence for a ferrimagnetic transition at 125 K. The analysis of the neutron data suggests a canted ferrimagnetic spin structure with collinear Ni2+ spin chains extending along the c axis but a non-collinear spin alignment within the ab plane. The magnetization curve of La2Ni1.19Os0.81O6 features a hysteresis with a very high coercive field, HC = 41 kOe, at T = 5 K, which is explained in terms of large magnetocrystalline anisotropy due to the presence of Os ions together with atomic disorder. Our results are encouraging to search for rare earth free hard magnets in the class of double perovskite oxides.

This work reports an unconventional defect engineering approach using synchrotron-radiation-based X-rays on ceria nanocrystal catalysts of particle sizes 4.4-10.6 nm. The generation of a large number of oxygen-vacancy defects (OVDs), and therefore an effective reduction of cations, has been found in CeO

We have identified an often observed yet unresolved intermediate structure in a popular processing with dimethylformamide solutions of lead chloride and methylammonium iodide for perovskite solar cells. With subsecond time-resolved grazing-incidence X-ray scattering and X-ray photoemission spectroscopy, supplemental with ab initio calculation, the resolved intermediate structure (CH3NH3)2PbI2Cl2·CH3NH3I features two-dimensional (2D) perovskite bilayers of zigzagged lead-halide octahedra and sandwiched CH3NH3I layers. Such intermediate structure reveals a hidden correlation between the intermediate phase and the composition of the processing solution. Most importantly, the 2D perovskite lattice of the intermediate phase is largely crystallographically aligned with the [110] planes of the three-dimensional perovskite cubic phase; consequently, with sublimation of Cl ions from the organo-lead octahedral terminal corners in prolonged annealing, the zigzagged octahedral layers of the intermediate phase can merge with the intercalated methylammonium iodide layers for templated growth of perovskite crystals. Regulated by annealing temperature and the activation energies of the intermediate and perovskite, deduced from analysis of temperature-dependent structural kinetics, the intermediate phase is found to selectively mature first and then melt along the layering direction for epitaxial conversion into perovskite crystals. The unveiled epitaxial conversion under growth kinetics controls might be general for solution-processed and intermediate-templated perovskite formation.

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.

An overview is given of the study on X-ray focusing using the Fabry-Perot type multi-plate silicon crystal cavities consisting of compound refractive lenses. Silicon (12 4 0) is used as the back reflection for cavity resonance at the photon energy of 14.4388 keV. Measurements of focal length of the transmitted beam through the crystal cavities show enhanced focusing effect due to the presence of back diffraction. Also, an incident beam with ultrahigh energy resolution can improve the focusing owing to the wider acceptance angle of the back diffraction. Considerations based on the excitation of dispersion surface within the framework of X-ray dynamical diffraction theory are also presented to reveal the origin of this enhanced focusing.

Cathode buffer layers (CBLs) can effectively further the efficiency of polymer solar cells (PSCs), after optimization of the active layer. Hidden between the active layer and cathode of the inverted PSC device configuration is the critical yet often unattended vertical diffusion of the active layer components across CBL. Here, a novel methodology of contrast variation with neutron and anomalous X-ray reflectivity to map the multicomponent depth compositions of inverted PSCs, covering from the active layer surface down to the bottom of the ZnO-based CBL, is developed. Uniquely revealed for a high-performance model PSC are the often overlooked porosity distributions of the ZnO-based CBL and the differential diffusions of the polymer PTB7-Th and fullerene derivative PC71 BM of the active layer into the CBL. Interface modification of the ZnO-based CBL with fullerene derivative PCBEOH for size-selective nanochannels can selectively improve the diffusion of PC71 BM more than that of the polymer. The deeper penetration of PC71 BM establishes a gradient distribution of fullerene derivatives over the ZnO/PCBE-OH CBL, resulting in markedly improved electron mobility and device efficiency of the inverted PSC. The result suggests a new CBL design concept of progressive matching of the conduction bands.

A poly(vinylidene fluoride-co-trifluoro-ethylene) piezoelectric polymer is blended with nano particles of titanium oxide phthalocyanine to bridge photoconductive and piezoelectric effects. In this study, a system is examined by in situ synchrotron x-ray to test a three-way piezo-phototronic soft-material design. The sample is heated for in situ phase transformation characterization. The semi-crystalline poly (vinylidene fluoride-co-trifluoro-ethylene) polymer gradually transforms to an amorphous structure. A complementary piezoelectric experiment before and after the heating experiment shows that the piezoelectric performance is proportional to the phase ratio. Secondly, the system is examined to test its phototronic effect. Piezoelectric responses are measured by controlling the light illumination. The positive and negative controls of light illumination which validate this newly-designed system can be modulated by a three-way piezo-phototronic effect. In-situ synchrotron x-ray diffraction experiments are employed to measure the microstructure evolution as a function of applied voltage up to 800 V. We then turned off both the light and the applied voltage to examine the kinetic behavior of the system. There is orientation-dependent anisotropic relaxation. We compared the lattice-strain evolutions. Piezo-phototronic creep is found in the (110), but not the (310) planes.

Charge disproportion at octahedral Fe sites in magnetite was observed at low temperature using two inversion-symmetry related three-wave resonant x-ray diffraction, 022-311 and $002\mathrm{\text{\ensuremath{-}}}\overline{3}\overline{1}1$, near the iron $K$ edge. Both of the three-wave cases involve the (002) forbidden-weak reflection. The self-normalized three-wave to two-wave (002) diffraction intensity ratio automatically cancels the self-absorption effect and leads to direct determination of charge disproportion for magnetite below 120 K. This approach provides a more direct and effective way for extracting charge-ordering information.

Unusual x-ray focusing effect is reported for parabolic curved multi-plate x-ray crystal cavities of silicon consisting of compound refractive lenses (CRL). The transmitted beam of the (12 4 0) back reflection near 14.4388 keV from these monolithic silicon crystal devices exhibits extraordinary focusing enhancement, such that the focal length is reduced by as much as 18% for 2-beam and 56% for 24-beam diffraction from the curved crystal cavity. This effect is attributed to the presence of the involved Bragg diffractions, in which the wavevector of the transmitted beam is bent further when traversing several curved crystal surfaces.

Local structure around Co impurity atoms in $\mathrm{Hf}{\mathrm{O}}_{2}:\mathrm{Co}$ thin films prepared by molecular beam epitaxy has been investigated using the extended x-ray absorption fine structure (EXAFS) technique. These films were found to be ferromagnetic at room temperature. Progressive formation of Co clusters was observed accompanied by interstitial doping of Co in samples with Co concentration of 1%--20% grown at a substrate temperature of $\ensuremath{\sim}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. However, films with a relatively high Co concentration of $\ensuremath{\sim}10%$ can be prepared without clustering of Co metal in a low-temperature growth mode with a substrate temperature $\ensuremath{\sim}100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. These low-temperature-grown films were found to be thermally stable up to an annealing temperature of $\ensuremath{\sim}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Our EXAFS results have therefore verified the feasibility for preparing Co-cluster-free room-temperature-ferromagnetic $\mathrm{Hf}{\mathrm{O}}_{2}:\mathrm{Co}$-diluted magnetic oxide thin films with great potential for possible spintronic applications.

A new method, multiple-wave diffraction anomalous fine structure, combining the x-ray multiple-wave diffraction and diffraction anomalous fine structure techniques, is proposed. The real part of dispersion correction {delta}f{sup '} and fine structure {chi} function can be obtained directly by multiple diffraction analysis without using Kramers-Kroenig relations and kinematical fitting of diffracted intensity. Better wave vector sensitivity of the fine structure is expected. The multiple-wave diffraction anomalous fine structure experiment for a GaAs single crystal is reported as an example.