Search

Copper ferrite (CuFe2O4) possesses an indirect bandgap in the range of 1.54-1.95 eV. It is used as an attractive p-type photocathode in photoelectrochemical (PEC) water splitting, and theoretically it can yield a maximum photocurrent density of ∼27 mA/cm2 and a maximum solar-to-hydrogen conversion efficiency of ∼33%. To date, only a few reports have been published on CuFe2O4 photocathodes with very low-photocurrent densities, with a maximum value of 0.4 mA/cm2 at 0.4 V vs RHE. Herein, we prepared a CuFe2O4 photocathode on FTO glass with the sol-gel method followed by either high-temperature flame annealing or furnace annealing. We found that the flame-annealed CuFe2O4 photocathode generated a photocurrent density of 1.82 mA/cm2 at 0.4 V vs RHE that is approximately 3.5 times higher than the furnace-annealed CuFe2O4 (0.52 mA/cm2). This photocurrent density is also higher than those of all the reported CuFe2O4 photocathodes, and any Cu containing ternary oxide (Cu-M-O, M: Fe, Bi, V, and Nb) photocathode (0.1-1.3 mA/cm2 at 0.4 V vs RHE). An improved PEC performance of the flame-annealed CuFe2O4 photocathode is elicited owing to the beneficial effects of flame annealing on the physical, optical, and electrical properties of CuFe2O4. Flame annealing enhances the light absorption property of the CuFe2O4 photocathode by slightly reducing the bandgap, and by forming a thicker film with increased porosity. Flame annealing also reduces the oxygen vacancy concentration in CuFe2O4, thus facilitating charge transport and interfacial charge transfer processes. Moreover, flame annealing requires only 16 min, which is much shorter than the time required for furnace annealing (∼9 h). These results demonstrate that flame annealing is a rapid and effective means for fabricating metal oxide photoelectrodes with an enhanced PEC water splitting performance.

The olivine Fe2GeS4, featuring non-toxic elements, cost-effective synthesis, and suitable optoelectronic properties, recently emerged as a promising light-absorbing candidate. Fe2GeS4 precursor powders obtained via a simple solution-based process were converted to highly crystalline Fe2GeS4 powders upon a thermal treatment in controlled atmosphere. Thin films fabricated by dip coating in the Fe2GeS4 precursor dispersion and subjected to the same thermal treatment render high-purity Fe2GeS4 thin films with a band gap of 1.4 eV, measured by room-temperature photoluminescence. Using Fe2GeS4 thin films as the sole absorber in a solution-based solar cell, open-circuit voltages of 361 mV are observed, while the use of the Fe2GeS4 films as counter electrodes in dye-sensitized solar cell constructs enhances the overall power conversion efficiency of the cell by a factor of five. This is the first report of a photovoltaic device based on Fe2GeS4.

The evolution of the antiferromagnetic phase across the temperature-driven ferromagnetic (FM) to antiferromagnetic (AF) phase transition in epitaxial FeRh thin films was studied by x-ray magnetic linear and circular dichroism (XMLD and XMCD) and photoemission electron microscopy. By comparing XMLD and XMCD images recorded at the same temperature, the AF phase was identified, its structure directly imaged, and its evolution studied across the transition. A quantitative analysis of the correlation length of the images shows differences between the characteristic length scale of the two phases with the AF phase having a finer feature size. The asymmetry of the transition from FM to AF upon cooling and AF-FM upon heating is evidenced: upon cooling the formation of AF phase is dominated by nucleation at defects, with little subsequent growth, resulting in a small and non-random final AF domain structure, while upon heating, heterogeneous nucleation at different sites followed by significant domain size growth of the FM phase is observed, resulting in a non-reproducible final FM large domain structure.

Off-stoichiometry, epitaxial FexSi1-x thin films (0.5 < x < 1.0) exhibit D03 or B2 chemical order, even far from stoichiometry. Theoretical calculations show the magnetic moment is strongly enhanced in the fully chemically disordered A2 phase, while both theoretical and experimental results show that the magnetization is nearly the same in the B2 and D03 phases, meaning partial chemical disorder does not influence the magnetism. The dependencies of the magnetic moments are directly and nonlinearly linked to the number of Si atoms, primarily nearest neighbor but also to a lesser extent (up to 10%) next nearest neighbor, surrounding Fe, explaining the similarities between B2 and D03 and the strong enhancement for the A2 structure. The calculated electronic density of states shows many similarities in both structure and spin polarization between the D03 and B2 structures, while the A2 structure exhibits disorder broadening and a reduced spin polarization.

Off-stoichiometry, epitaxial FexSi1-x thin films (0.5 < x < 1.0) exhibit D03 or B2 chemical order, even far from stoichiometry. Theoretical calculations show the magnetic moment is strongly enhanced in the fully chemically disordered A2 phase, while both theoretical and experimental results show that the magnetization is nearly the same in the B2 and D03 phases, meaning partial chemical disorder does not influence the magnetism. The dependencies of the magnetic moments are directly and nonlinearly linked to the number of Si atoms, primarily nearest neighbor but also to a lesser extent (up to 10%) next nearest neighbor, surrounding Fe, explaining the similarities between B2 and D03 and the strong enhancement for the A2 structure. The calculated electronic density of states shows many similarities in both structure and spin polarization between the D03 and B2 structures, while the A2 structure exhibits disorder broadening and a reduced spin polarization.

Amorphous FexSi1-x thin films exhibit a striking enhancement in magnetization compared to crystalline films with the same composition (0.45 < x < 0.75), and xray magnetic circular dichroism reveals an enhancement in both spin and orbital moments in the amorphous films. Density functional theory (DFT) calculations reproduce this enhanced magnetization and also show a relatively large spinpolarization at the Fermi energy, also seen experimentally in Andreev reflection. Theory and experiment show that the amorphous materials have a decreased number of nearest neighbors and reduced number density relative to the crystalline samples of the same composition; the associated decrease in Fe-Si neighbors reduces the hybridization of Fe orbitals, leading to the enhanced moment.

Amorphous FexSi1-x thin films exhibit a striking enhancement in magnetization compared to crystalline films with the same composition (0.45 < x < 0.75), and xray magnetic circular dichroism reveals an enhancement in both spin and orbital moments in the amorphous films. Density functional theory (DFT) calculations reproduce this enhanced magnetization and also show a relatively large spinpolarization at the Fermi energy, also seen experimentally in Andreev reflection. Theory and experiment show that the amorphous materials have a decreased number of nearest neighbors and reduced number density relative to the crystalline samples of the same composition; the associated decrease in Fe-Si neighbors reduces the hybridization of Fe orbitals, leading to the enhanced moment.

The magnesiothermic reduction of SiO 2 is an important reaction as it is a bulk method that produces porous Si for a wide range of applications directly from SiO 2 . While its main advantage is potential tunability, the reaction behavior and final product properties are heavily dependent on many parameters including feedstock type. However, a complete understanding of the reaction pathway has not yet been achieved. Here, using in situ X-ray diffraction analysis, for the first time, various pathways through which the magnesiothermic reduction reaction proceeds were mapped. Further, the key parameters and conditions that determine which pathways are favored were determined. It was discovered that the reaction onset temperatures can be as low as 348 ± 7 °C, which is significantly lower when compared to previously reported values. The onset temperature is dependent on the size of Mg particles used in the reaction. Further, Mg 2 Si was identified as a key intermediate rather than a reaction byproduct during the reduction process. Its rate of consumption is determined by the reaction temperature which needs to be >561 °C. These findings can enable process and product optimization of the magnesiothermic reduction process to manufacture and tune porous Si for a range of applications.

This study provides the first experimental polarized intermolecular and intramolecular optical absorption components of field-induced polarons in regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, a polymer semiconductor. Highly aligned rr-P3HT thin films were prepared by a high temperature shear-alignment process that orients polymer backbones along the shearing direction. rr-P3HT in-plane molecular orientation was measured by electron diffraction, and out-of-plane orientation was measured through series of synchrotron X-ray scattering techniques. Then, with molecular orientation quantified, polarized charge modulation spectroscopy was used to probe mid-IR polaron absorption in the ℏω = 0.075 - 0.75 eV range and unambiguously assign intermolecular and intramolecular optical absorption components of hole polarons in rr-P3HT. This data represents the first experimental quantification of these polarized components and allowed long-standing theoretical predictions to be compared to experimental results. The experimental data is discrepant with predictions of polaron absorption based on an adiabatic framework that works under the Born-Oppenheimer approximation, but the data is entirely consistent with a more recent nonadiabatic treatment of absorption based on a modified Holstein Hamiltonian. This nonadiabatic treatment was used to show that both intermolecular and intramolecular polaron coherence break down at length scales significantly smaller than estimated structural coherence in either direction. This strongly suggests that polaron delocalization is fundamentally limited by energetic disorder in rr-P3HT.

Solution shearing, a meniscus-guided coating process, can create large-area metal-organic framework (MOF) thin films rapidly, which can lead to the formation of uniform membranes for separations or thin films for sensing and catalysis applications. Although previous work has shown that solution shearing can render MOF thin films, examples have been limited to a few prototypical systems, such as HKUST-1, Cu-HHTP, and UiO-66. Here, we expand on the applicability of solution shearing by making thin films of NU-901, a zirconium-based MOF. We study how the NU-901 thin film properties (i.e., crystallinity, surface coverage, and thickness) can be controlled as a function of substrate temperature and linker concentration. High fractional surface coverage of small-area (∼1 cm 2 ) NU-901 thin films (0.88 ± 0.06) is achieved on a glass substrate for all conditions after one blade pass, while a low to moderate fractional surface coverage (0.73 ± 0.18) is obtained for large-area (∼5 cm 2 ) NU-901 thin films. The crystallinity of NU-901 crystals increases with temperature and decreases with linker concentration. On the other hand, the adjusted thickness of NU-901 thin films increases with both increasing temperature and linker concentration. We also extend the solution shearing technique to synthesize MOF-525 thin films on a transparent conductive oxide that are useful for electrocatalysis. We show that Fe-metalated MOF-525 films can reduce CO 2 to CO, which has implications for CO 2 capture and utilization. The demonstration of thin film formation of NU-901 and MOF-525 using solution shearing on a wide range of substrates will be highly useful for implementing these MOFs in sensing and catalytic applications.

The general-purpose powder diffractometer beamline (BL2-1) at the Stanford Synchrotron Radiation Lightsource (SSRL) is described. The evolution of design and performance of BL2-1 are presented, in addition to current operating specifications, applications and measurement capabilities. Recent developments involve a robotic sample changer enabling high-throughput X-ray diffraction measurements, applicable to mail-in and remote operations. In situ and operando capabilities to measure samples with different form factors ( e.g. capillary, flat plate or thin film, and transmission) and under variable experimental conditions are discussed. Several example datasets and accompanying Rietveld refinements are presented., (© Kevin H. Stone et al. 2023.)

In situ techniques are essential to understanding the behavior of electrocatalysts under operating conditions. When employed, in situ synchrotron grazing-incidence X-ray diffraction (GI-XRD) can provide time-resolved structural information of materials formed at the electrode surface. In situ cells, however, often require epoxy resins to secure electrodes, do not enable electrolyte flow, or exhibit limited chemical compatibility, hindering the study of non-aqueous electrochemical systems. Here, a versatile electrochemical cell for air-free in situ synchrotron GI-XRD during non-aqueous Li-mediated electrochemical N 2 reduction (Li-N 2 R) has been designed. This cell not only fulfills the stringent material requirements necessary to study this system but is also readily extendable to other electrochemical systems. Under conditions relevant to non-aqueous Li-N 2 R, the formation of Li metal, LiOH and Li 2 O as well as a peak consistent with the α-phase of Li 3 N was observed, thus demonstrating the functionality of this cell toward developing a mechanistic understanding of complicated electrochemical systems., (open access.)

Objective: Post-operative atrial fibrillation (POAF) is a common complication of coronary artery bypass graft (CABG) surgery. Previous studies suggest carvedilol is more effective than metoprolol in preventing POAF in on-pump CABG. This study investigated if the same benefit would be seen in off-pump CABG., Methods: This single-center, retrospective review compared rates of new-onset POAF between adult patients who received carvedilol and metoprolol after off-pump CABG surgery. Safety endpoints included hypotension, bradycardia, dyspnea, and the composite. Multivariate logistic regression was conducted to identify associations between demographics, potential confounders, and beta-blocker dose and POAF. Kaplan-Meier plots and Cox proportional-hazards models examined differences in time-to-event for POAF., Results: 134 patients were included (34 carvedilol and 100 metoprolol). The mean age was 63 years, 70.9% were male, 85% had history of hypertension, 3.7% had history of heart failure with reduced ejection fraction, and 38.8% were taking beta blockers prior to admission. POAF developed in 2 patients (5.8%) in the carvedilol group and 24 patients (24.0%) in the metoprolol group (odds ratio 0.17 [95% CI 0.03-0.83], p = 0.023). Safety endpoints occurred in 10 carvedilol (29.4%) and 44 metoprolol (44.0%) patients (p = 0.134). Hypotension and dyspnea rates were similar between groups; bradycardia occurred more commonly among metoprolol-treated patients (p = 0.040). Time-to-event analyses revealed a hazard ratio = 0.22 (95% CI 0.05-0.93, p = 0.040) for carvedilol use., Conclusions: In this single-center, retrospective study of off-pump CABG patients, carvedilol was associated with reduced POAF risk and enhanced safety compared to metoprolol., (© 2022. The Author(s), under exclusive licence to The Japanese Association for Thoracic Surgery.)

The resurgence of inter-est in hydrogen-related technologies has stimulated new studies aimed at advancing lesser-developed water-splitting processes, such as solar thermochemical hydrogen production (STCH). Progress in STCH has been largely hindered by a lack of new materials able to efficiently split water at a rate comparable to ceria under identical experimental conditions. BaCe 0.25 Mn 0.75 O 3 (BCM) recently demonstrated enhanced hydrogen production over ceria and has the potential to further our understanding of two-step thermochemical cycles. A significant feature of the 12R hexa-gonal perovskite structure of BCM is the tendency to, in part, form a 6H polytype at high temperatures and reducing environments ( i.e. , during the first step of the thermochemical cycle), which may serve to mitigate degradation of the complex oxide. An analogous compound, namely BaNb 0.25 Mn 0.75 O 3 (BNM) with a 12R structure was synthesized and displays nearly complete conversion to the 6H structure under identical reaction conditions as BCM. The structure of the BNM-6H polytype was determined from Rietveld refinement of synchrotron powder X-ray diffraction data and is presented within the context of the previously established BCM-6H structure., (© Strange et al. 2023.)

γ-Graphyne is the most symmetric sp 2 /sp 1 allotrope of carbon, which can be viewed as graphene uniformly expanded through the insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk γ-graphyne has remained a challenge. We here report the synthesis of multilayer γ-graphyne through crystallization-assisted irreversible cross-coupling polymerization. A comprehensive characterization of this new carbon phase is described, including synchrotron powder X-ray diffraction, electron diffraction, lateral force microscopy, Raman spectroscopy, infrared spectroscopy, and cyclic voltammetry. Experiments indicate that γ-graphyne is a 0.48 eV band gap semiconductor, with a hexagonal a -axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 °C but undergoes transformation at higher temperatures. While conventional 2D polymerization and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family.

Flower-like polyacrylonitrile (PAN) particles have shown promising performance for numerous applications, including sensors, catalysis, and energy storage. However, the detailed formation process of these unique structures during polymerization has not been investigated. Here, we elucidate the formation process of flower-like PAN particles through a series of in situ and ex situ experiments. We have the following key findings. First, lamellar petals within the flower-like particles were predominantly orthorhombic PAN crystals. Second, branching of the lamellae during the particle formation arose from PAN's fast nucleation and growth on pre-existing PAN crystals, which was driven by the poor solubility of PAN in the reaction solvent. Third, the particles were formed to maintain a constant center-to-center distance during the reaction. The separation distance was attributed to strong electrostatic repulsion, which resulted in the final particles' spherical shape and uniform size. Lastly, we employed the understanding of the formation mechanism to tune the PAN particles' morphology using several experimental parameters including incorporating comonomers, changing temperature, adding nucleation seeds, and adjusting the monomer concentration. These findings provide important insights into the bottom-up design of advanced nanostructured PAN-based materials and controlled polymer nanostructure self-assemblies.

The role of strain in metal halide perovskite (MHP) solar cells is still under investigation, showing both beneficial and detrimental effects on the device performance and stability. One crucial component to elucidating the impact of strain in the MHP absorber is a robust method of quantifying the amount of strain in the material. Here, we present a parametric refinement approach based on grazing incidence wide-angle X-ray scattering and demonstrate its use on quantifying strain during thermal annealing and subsequent cooling as a function of substrate and processing route. We use the analysis to reveal the impact of the cubic-to-tetragonal phase transition during cooling on the material's strain and discuss texture formation as a potential strain-relief mechanism. Thereby we present both a robust approach to quantify strain in MHPs and potential mechanisms to control strain in the film, opening the path for further investigations of strain in MHPs.

The resurgence of interest in a hydrogen economy and the development of hydrogen-related technologies has initiated numerous research and development efforts aimed at making the generation, storage, and transportation of hydrogen more efficient and affordable. Solar thermochemical hydrogen production (STCH) is a process that potentially exhibits numerous benefits such as high reaction efficiencies, tunable thermodynamics, and continued performance over extended cycling. Although CeO 2 has been the de facto standard STCH material for many years, more recently 12R-Ba 4 CeMn 3 O 12 (BCM) has demonstrated enhanced hydrogen production at intermediate H 2 /H 2 O conditions compared to CeO 2 , making it a contender for large-scale hydrogen production. However, the thermo-reduction stability of 12R-BCM dictates the oxygen partial pressure ( p O 2 ) and temperature conditions optimal for cycling. In this study, we identify the formation of a 6H-BCM polytype at high temperature and reducing conditions, experimentally and computationally, as a mechanism and pathway for 12R-BCM decomposition. 12R-BCM was synthesized with high purity and then controllably reduced using thermogravimetric analysis (TGA). Synchrotron X-ray diffraction (XRD) data is used to identify the formation of a 6H-Ba 3 Ce 0.75 Mn 2.25 O 9 (6H-BCM) polytype that is formed at 1350 °C under strongly reducing p O 2 . Density functional theory (DFT) total energy and defect calculations show a window of thermodynamic stability for the 6H-polytype consistent with the XRD results. These data provide the first evidence of the 6H-BCM polytype and could provide a mechanistic explanation for the superior water-splitting behaviors of 12R-BCM.

The electrochemical nitrate reduction reaction (NO 3 RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiH x , 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH 2 enrichment with increasing NO 3 RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH 2 /Ti electrodes for use in NO 3 RR, thereby decoupling hydride formation from NO 3 RR performance. A wide range of NO 3 RR activity and selectivity on TiH 2 /Ti electrodes between -0.4 and -1.0 V RHE was observed and analyzed with density functional theory (DFT) calculations on TiH 2 (111). This work underscores the importance of relating NO 3 RR performance with near-surface electrode structure to advance catalyst design and operation.

Laser powder bed fusion (LPBF) is a highly dynamic multi-physics process used for the additive manufacturing (AM) of metal components. Improving process understanding and validating predictive computational models require high-fidelity diagnostics capable of capturing data in challenging environments. Synchrotron x-ray techniques play a vital role in the validation process as they are the only in situ diagnostic capable of imaging sub-surface melt pool dynamics and microstructure evolution during LPBF-AM. In this article, a laboratory scale system designed to mimic LPBF process conditions while operating at a synchrotron facility is described. The system is implemented with process accurate atmospheric conditions, including an air knife for active vapor plume removal. Significantly, the chamber also incorporates a diagnostic sensor suite that monitors emitted optical, acoustic, and electronic signals during laser processing with coincident x-ray imaging. The addition of the sensor suite enables validation of these industrially compatible single point sensors by detecting pore formation and spatter events and directly correlating the events with changes in the detected signal. Experiments in the Ti-6Al-4V alloy performed at the Stanford Synchrotron Radiation Lightsource using the system are detailed with sufficient sampling rates to probe melt pool dynamics. X-ray imaging captures melt pool dynamics at frame rates of 20 kHz with a 2 µm pixel resolution, and the coincident diagnostic sensor data are recorded at 470 kHz. This work shows that the current system enables the in situ detection of defects during the LPBF process and permits direct correlation of diagnostic signatures at the exact time of defect formation.

Synthesis of porous, covalent crystals such as zeolites and metal-organic frameworks (MOFs) cannot be described adequately using existing crystallization theories. Even with the development of state-of-the-art experimental and computational tools, the identification of primary mechanisms of nucleation and growth of MOFs remains elusive. Here, using time-resolved in-situ X-ray scattering coupled with a six-parameter microkinetic model consisting of ∼1 billion reactions and up to ∼100 000 metal nodes, we identify autocatalysis and oriented attachment as previously unrecognized mechanisms of nucleation and growth of the MOF UiO-66. The secondary building unit (SBU) formation follows an autocatalytic initiation reaction driven by a self-templating mechanism. The induction time of MOF nucleation is determined by the relative rate of SBU attachment (chain extension) and the initiation reaction, whereas the MOF growth is primarily driven by the oriented attachment of reactive MOF crystals. The average size and polydispersity of MOFs are controlled by surface stabilization. Finally, the microkinetic model developed here is generalizable to different MOFs and other multicomponent systems., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The performance of three-dimensional (3D) organic-inorganic halide perovskite solar cells (PSCs) can be enhanced through surface treatment with 2D layered perovskites that have efficient charge transport. We maximized hole transport across the layers of a metastable Dion-Jacobson (DJ) 2D perovskite that tuned the orientational arrangements of asymmetric bulky organic molecules. The reduced energy barrier for hole transport increased out-of-plane transport rates by a factor of 4 to 5, and the power conversion efficiency (PCE) for the 2D PSC was 4.9%. With the metastable DJ 2D surface layer, the PCE of three common 3D PSCs was enhanced by approximately 12 to 16% and could reach approximately 24.7%. For a triple-cation–mixed-halide PSC, 90% of the initial PCE was retained after 1000 hours of 1-sun operation at ~40°C in nitrogen.

A key step in the analysis of powder X-ray diffraction (PXRD) data is the accurate determination of unit-cell lattice parameters. This step often requires significant human intervention and is a bottleneck that hinders efforts towards automated analysis. This work develops a series of one-dimensional convolutional neural networks (1D-CNNs) trained to provide lattice parameter estimates for each crystal system. A mean absolute percentage error of approximately 10% is achieved for each crystal system, which corresponds to a 100- to 1000-fold reduction in lattice parameter search space volume. The models learn from nearly one million crystal structures contained within the Inorganic Crystal Structure Database and the Cambridge Structural Database and, due to the nature of these two complimentary databases, the models generalize well across chemistries. A key component of this work is a systematic analysis of the effect of different realistic experimental non-idealities on model performance. It is found that the addition of impurity phases, baseline noise and peak broadening present the greatest challenges to learning, while zero-offset error and random intensity modulations have little effect. However, appropriate data modification schemes can be used to bolster model performance and yield reasonable predictions, even for data which simulate realistic experimental non-idealities. In order to obtain accurate results, a new approach is introduced which uses the initial machine learning estimates with existing iterative whole-pattern refinement schemes to tackle automated unit-cell solution., (© Sathya R. Chitturi et al. 2021.)

Hybrid perovskites are a promising class of materials for a range of optoelectronic applications. Many material properties are dictated by the details of the synthetic process, yet a mechanistic understanding is lacking for the majority of these materials. We have studied the formation of methylammonium lead iodide films derived from a lead chloride precursor to understand both the casting solution chemistry and its influence on the final, largely chlorine-free, film. Using solution-phase extended X-ray absorption spectroscopy, we observe a halide exchange with the primary solution plumbate species identified as PbI 2.5 Cl 0.33 . The mixed halide plumbate solution species leads to formation of the crystalline intermediate phase of (CH 3 NH 3 ) 2 PbI 3 Cl. Using in situ synchrotron X-ray diffraction, we show that compositional control of the casting solution can control the annealing kinetics of film formation. Our study demonstrates the importance of solution-phase chemistry and its impact on lead halide perovskite synthesis.

Laser powder bed fusion (LPBF) is a method of additive manufacturing characterized by the rapid scanning of a high powered laser over a thin bed of metallic powder to create a single layer, which may then be built upon to form larger structures. Much of the melting, resolidification, and subsequent cooling take place at much higher rates and with much higher thermal gradients than in traditional metallurgical processes, with much of this occurring below the surface. We have used in situ high speed X-ray diffraction to extract subsurface cooling rates following resolidification from the melt and above the β-transus in titanium alloy Ti-6Al-4V. We observe an inverse relationship with laser power and bulk cooling rates. The measured cooling rates are seen to correlate to the level of residual strain borne by the minority β-Ti phase with increased strain at slower cooling rates. The α-Ti phase shows a lattice contraction which is invariant with cooling rate. We also observe a broadening of the diffraction peaks which is greater for the β-Ti phase at slower cooling rates and a change in the relative phase fraction following LPBF. These results provide a direct measure of the subsurface thermal history and demonstrate its importance to the ultimate quality of additively manufactured materials.

Electrochemical reduction of CO 2 to useful chemicals has been actively pursued for closing the carbon cycle and preventing further deterioration of the environment/climate. Since CO 2 reduction reaction (CO 2 RR) at a cathode is always paired with the oxygen evolution reaction (OER) at an anode, the overall efficiency of electrical energy to chemical fuel conversion must consider the large energy barrier and sluggish kinetics of OER, especially in widely used electrolytes, such as the pH-neutral CO 2 -saturated 0.5 M KHCO 3 OER in such electrolytes mostly relies on noble metal (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover that by anodizing a metallic Ni-Fe composite foam under a harsh condition (in a low-concentration 0.1 M KHCO 3 solution at 85 °C under a high-current ∼250 mA/cm 2 ), OER on the NiFe foam is accompanied by anodic etching, and the surface layer evolves into a nickel-iron hydroxide carbonate (NiFe-HC) material composed of porous, poorly crystalline flakes of flower-like NiFe layer-double hydroxide (LDH) intercalated with carbonate anions. The resulting NiFe-HC electrode in CO 2 -saturated 0.5 M KHCO 3 exhibited OER activity superior to IrO 2 , with an overpotential of 450 and 590 mV to reach 10 and 250 mA/cm 2 , respectively, and high stability for >120 h without decay. We paired NiFe-HC with a CO 2 RR catalyst of cobalt phthalocyanine/carbon nanotube (CoPc/CNT) in a CO 2 electrolyzer, achieving selective cathodic conversion of CO 2 to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O 2 The device showed a low cell voltage of 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a current density of 10 mA/cm 2 ., Competing Interests: The authors declare no competing interest.

Laser powder bed fusion additive manufacturing is an emerging 3D printing technique for the fabrication of advanced metal components. Widespread adoption of it and similar additive technologies is hampered by poor understanding of laser-metal interactions under such extreme thermal regimes. Here, we elucidate the mechanism of pore formation and liquid-solid interface dynamics during typical laser powder bed fusion conditions using in situ X-ray imaging and multi-physics simulations. Pores are revealed to form during changes in laser scan velocity due to the rapid formation then collapse of deep keyhole depressions in the surface which traps inert shielding gas in the solidifying metal. We develop a universal mitigation strategy which eliminates this pore formation process and improves the geometric quality of melt tracks. Our results provide insight into the physics of laser-metal interaction and demonstrate the potential for science-based approaches to improve confidence in components produced by laser powder bed fusion.

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li + in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li 2-x Ir 1-y Sn y O 3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short approximately 1.8 Å metal-oxygen π bonds and approximately 1.4 Å O-O dimers during oxygen redox, which occurs in Li 2-x Ir 1-y Sn y O 3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI 3 ) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl 2  + 3MAI) (MA = CH 3 NH 3 ). The as-spun film consists of crystalline MA 2 PbI 3 Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.

Hydrothermal synthesis is challenging in metal oxide systems with diverse polymorphism, as reaction products are often sensitive to subtle variations in synthesis parameters. This sensitivity is rooted in the non-equilibrium nature of low-temperature crystallization, where competition between different metastable phases can lead to complex multistage crystallization pathways. Here, we propose an ab initio framework to predict how particle size and solution composition influence polymorph stability during nucleation and growth. We validate this framework using in situ X-ray scattering, by monitoring how the hydrothermal synthesis of MnO 2 proceeds through different crystallization pathways under varying solution potassium ion concentrations ([K + ] = 0, 0.2, and 0.33 M). We find that our computed size-dependent phase diagrams qualitatively capture which metastable polymorphs appear, the order of their appearance, and their relative lifetimes. Our combined computational and experimental approach offers a rational and systematic paradigm for the aqueous synthesis of target metal oxides.

In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement.

The ability of a material to adopt multiple structures, known as polymorphism, is a fascinating natural phenomenon. Various polymorphs with unusual properties are routinely synthesized by compression under positive pressure. However, changing a material's structure by applying tension under negative pressure is much more difficult. We show how negative-pressure polymorphs can be synthesized by mixing materials with different crystal structures-a general approach that should be applicable to many materials. Theoretical calculations suggest that it costs less energy to mix low-density structures than high-density structures, due to less competition for space between the atoms. Proof-of-concept experiments confirm that mixing two different high-density forms of MnSe and MnTe stabilizes a Mn(Se,Te) alloy with a low-density wurtzite structure. This Mn(Se,Te) negative-pressure polymorph has 2× to 4× lower electron effective mass compared to MnSe and MnTe parent compounds and has a piezoelectric response that none of the parent compounds have. This example shows how heterostructural alloying can lead to negative-pressure polymorphs with useful properties-materials that are otherwise nearly impossible to make.

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li 1.17-x Ni 0.21 Co 0.08 Mn 0.54 O 2 , these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

Structure-specific synthesis processes are of key importance to the growth of polymorphic functional compounds such as TiO 2 , where material properties strongly depend on structure as well as chemistry. The robust growth of the brookite polymorph of TiO 2 , a promising photocatalyst, has been difficult in both powder and thin-film forms due to the disparity of reported synthesis techniques, their highly specific nature, and lack of mechanistic understanding. In this work, we report the growth of high-fraction (~95%) brookite thin films prepared by annealing amorphous titania precursor films deposited by pulsed laser deposition. We characterize the crystallization process, eliminating the previously suggested roles of substrate templating and Na helper ions in driving brookite formation. Instead, we link phase selection directly to film thickness, offering a novel, generalizable route to brookite growth that does not rely on the presence of extraneous elements or particular lattice-matched substrates. In addition to providing a new synthesis route to brookite thin films, our results take a step towards resolving the problem of phase selection in TiO 2 growth, contributing to the further development of this promising functional material.

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La&#95;{1-x}Sr&#95;{x}MnO&#95;{3} and SrIrO&#95;{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La&#95;{1-x}Sr&#95;{x}MnO&#95;{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6}  erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

Surface sensitive X-ray reflectivity (XRR) measurements were performed to investigate the electrochemical lithiation of a native oxide terminated single crystalline silicon (100) electrode in real time during the first galvanostatic discharge cycle. This allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI). We describe an electrochemistry cell specifically designed for in situ XRR studies and have determined the evolution of the electron density profile of the lithiated Si layer (Li x Si) and the SEI layer with subnanometer resolution. We propose a three-stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the Si at the Li x Si/Si interface.

Amorphous films and coatings are rapidly growing in importance. Yet, there is a dearth of high-quality structural data on sub-micron films. Not understanding how these materials assemble at atomic scale limits fundamental insights needed to improve their performance. Here, we use grazing-incidence x-ray total scattering measurements to examine the atomic structure of the top 50-100 nm of Ta2O5 films; mirror coatings that show high promise to significantly improve the sensitivity of the next generation of gravitational-wave detectors. Our measurements show noticeable changes well into medium range, not only between crystalline and amorphous, but also between as-deposited, annealed and doped amorphous films. It is a further challenge to quickly translate the structural information into insights into mechanisms of packing and disorder. Here, we illustrate a modeling approach that allows translation of observed structural features to a physically intuitive packing of a primary structural unit based on a kinked Ta-O-Ta backbone. Our modeling illustrates how Ta-O-Ta units link to form longer 1D chains and even 2D ribbons, and how doping and annealing influences formation of 2D order. We also find that all the amorphousTa2O5 films studied in here are not just poorly crystalline but appear to lack true 3D order.

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3), and NCS (4)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1-4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]EF6 (E = P, Sb), which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1-4 and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3, and 4 were measured by electron-spin resonance with no evidence for zero-field splitting (ZFS) being observed. The magnetism of 1-4 spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for 2 and 4 as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound 3 is characterized as a 3D AFM with the interlayer interaction (J⊥) being slightly stronger than the intralayer interaction along Ni-pyz-Ni segments (J(pyz)) within the two-dimensional [Ni(pyz)2](2+) square planes. Regardless of X, J(pyz) is similar for the four compounds and is roughly 1 K.

The reaction of Hg(OAc)(2) with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC(6)H(4)S)(en)](n), while the reaction of Pb(OAc)(2) with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb(SC(6)H(4)S)(dien)](n). Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal-organic polymers with -M-SC(6)H(4)S- repeat units. Diffuse reflectance UV-visible spectroscopy indicates band gaps of 2.89 eV for [Hg(SC(6)H(4)S)(en)](n) and 2.54 eV for [Pb(SC(6)H(4)S)(dien)](n), while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb(SC(6)H(4)S)(dien)](n) calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg(SC(6)H(4)S)(en)](n) does have moderate dispersion and a calculated electron effective mass of 0.29·m(e). Hybridization of the empty Hg 6s orbital with SC(6)H(4)S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.

The crystal structures of the isostructural title compounds poly[(μ-benzene-1,4-dithiolato)dithallium], Tl(2)(SC(6)H(4)S), and poly[(μ-benzene-1,4-diselenolato)dithallium], Tl(2)(SeC(6)H(4)Se), were solved by simulated annealing from high-resolution synchrotron X-ray powder diffraction. Rietveld refinements of an initial structure with one formula unit per triclinic cell gave satisfactory agreement with the data, but led to a structure with impossibly close non-bonded contacts. A disordered model was proposed to alleviate this problem, but an alternative supercell structure leads to slightly improved agreement with the data. The isostructural superlattice structures were confirmed for both compounds through additional data collection, with substantially better counting statistics, which revealed the presence of very weak superlattice peaks not previously seen. Overall, each structure contains Tl-S or Tl-Se two-dimensional networks, connected by phenylene bridges. The sulfur (or selenium) coordination sphere around each thallium is a highly distorted square pyramid or a 'see-saw' shape, depending upon how many Tl-S or Tl-Se interactions are considered to be bonds. In addition, the two compounds contain pairs of Tl(I) ions that interact through a closed-shell 'thallophilic' interaction: in the sulfur compound there are two inequivalent pairs of Tl atoms with Tl-Tl distances of 3.49 and 3.58 Å, while in the selenium compound those Tl-Tl interactions are at 3.54 and 3.63 Å.

Metal-organic frameworks based on metal-sulfur or metal-selenium bonds are relatively rare; herein we describe the synthesis and structural characterization of several examples, including, for example, [Cd(en)3][Cd(SC6H4S)2], which contains the anionic two-dimensional square-grid network [Cd(SC6H4S)2]n(2n-).

X-ray powder diffraction and magnetic susceptibility measurements show that Ag(pyz)(2)(S(2)O(8)) consists of 2D square nets of Ag(2+) ions resulting from the corner-sharing of axially elongated AgN(4)O(2) octahedra and exhibits characteristic 2D antiferromagnetism. Nevertheless, mu(+)SR measurements indicate that Ag(pyz)(2)(S(2)O(8)) undergoes 3D magnetic ordering below 7.8(3) K.

The reactions of Pb(OAc)2 with 1,2,4,5-benzenetetrathiol, 1,4-benzenedithiol, and benzenehexathiol in ethylenediamine yield bright yellow [Pb2(S2C6H2S2)(en)]n, orange-red [Pb3(SC6H4S)3(en)2]n, and brown [Pb3C6S6]n, respectively. The structures of [Pb2(S2C6H2S2)(en)]n and [Pb3C6S6]n were solved by synchrotron X-ray powder diffraction, while the structure of [Pb3(SC6H4S)3(en)2]n was solved by single-crystal X-ray diffraction. The bonding in [Pb2(S2C6H2S2)(en)]n indicates the presence of "molecular" units, while in [Pb3C6S6]n, the bonding most resembles that in an inorganic solid such as PbS. The differences in bonding are reflected in the optical and electrical properties of the materials; [Pb3C6S6]n is a semiconductor.

A simple method of ankle stabilization using a long trifin nail, inserted from below, is described. The method is particularly suitable in the elderly, when a short period of bed rest and early weight bearing is desirable. The procedure is contraindicated if subtalar joint motion is present and should be preserved.