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Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag3AuTe2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag3AuTe2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the semiconducting behavior of Ag3AuTe2 with high resistivity and an activation energy E a of 0.2 eV. The optical bandgap determined by diffuse reflectance measurements is about three times wider than the experimental E a . Despite the difference, both experimental gaps fall within the range of predicted bandgaps by our first-principles density functional theory (DFT) calculations employing the Perdew–Burke–Ernzerhof and modified Becke–Johnson methods. Furthermore, our DFT simulations predict a progressive narrowing of the bandgap under compressive strain, with a full closure expected at a strain of −4% relative to the lattice parameter. To evaluate the feasibility of gap tunability at such substantial strain, the high-pressure behavior of Ag3AuTe2 was investigated by in situ high-pressure x-ray diffraction up to 47 GPa. Mechanical compression beyond 4% resulted in a pressure-induced structural transformation, indicating the possibility of substantial gap modulation under extreme compression conditions. [ABSTRACT FROM AUTHOR]

Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- (T -) invariant (helical) 3D TCIs—termed higher-order TCIs (HOTIs)—the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetals, and T -doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that β-MoTe2 realizes a spin-Weyl state and that α-BiBr hosts both 3D QSHI and T-DAXI regimes. 3D higher-order topological insulators (HOTIs) exhibit 1D hinge states depending on extrinsic sample details, while intrinsic features of HOTIs remain unknown. Here, K.S. Lin et al. introduce the framework of spin-resolved topology to show that helical HOTIs can realize a doubled axion insulator phase with nontrivial partial axion angles. [ABSTRACT FROM AUTHOR]

Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- (T -) invariant (helical) 3D TCIs—termed higher-order TCIs (HOTIs)—the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetals, and T -doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that β-MoTe2 realizes a spin-Weyl state and that α-BiBr hosts both 3D QSHI and T-DAXI regimes. 3D higher-order topological insulators (HOTIs) exhibit 1D hinge states depending on extrinsic sample details, while intrinsic features of HOTIs remain unknown. Here, K.S. Lin et al. introduce the framework of spin-resolved topology to show that helical HOTIs can realize a doubled axion insulator phase with nontrivial partial axion angles. [ABSTRACT FROM AUTHOR]

Previous band structure calculations predicted Ag3AuSe2 to be a semiconductor with a band gap of approximately 1 eV. Here, we report single crystal growth of Ag3AuSe2 and its transport and optical properties. Single crystals of Ag3AuSe2 were synthesized by slow‐cooling from the melt, and grain sizes were confirmed to be greater than 2 mm using electron backscatter diffraction. Optical and transport measurements reveal that Ag3AuSe2 is a highly resistive semiconductor with a band gap and activation energy around 0.3 eV. Our first‐principles calculations show that the experimentally determined band gap lies between the predicted band gaps from GGA and hybrid functionals. We predict band inversion to be possible by applying tensile strain. The sensitivity of the gap to Ag/Au ordering, chemical substitution, and heat treatment merit further investigation. [ABSTRACT FROM AUTHOR]

Na-ion conducting solid electrolytes can enable both the enhanced safety profile of all-solid-state-batteries and the transition to an earth-abundant charge-carrier for large-scale stationary storage. In this work, we developed new perovskite-structured Na-ion conductors from the analogous fast Li-ion conducting Li3xLa2/3−xTiO3 (LLTO), testing strategies of chemo-mechanical and defect engineering. NaxLa2/3−1/3xZrO3 (NLZ) and NaxLa1/3−1/3xBa0.5ZrO3 (NLBZ) were prepared using a modified Pechini method with varying initial stoichiometries and sintering temperatures. With the substitution of larger framework cations Zr4+ and Ba2+ on B- and A-sites respectively, NLZ and NLBZ both had larger lattice parameters compared to LLTO, in order to accommodate and potentially enhance the transport of larger Na ions. Additionally, we sought to introduce Na vacancies through (a) sub-stoichiometric Na : La ratios, (b) Na loss during sintering, and (c) donor doping with Nb. AC impedance spectroscopy and DC polarization experiments were performed on both Na0.5La0.5ZrO3 and Na0.25La0.25Ba0.5ZrO3 in controlled gas environments (variable oxygen partial pressure, humidity) at elevated temperatures to quantify the contributions of various possible charge carriers (sodium ions, holes, electrons, oxygen ions, protons). Our results showed that the lattice-enlarged NLZ and NLBZ exhibited ∼19× (conventional sintering)/49× (spark plasma sintering) and ∼7× higher Na-ion conductivities, respectively, compared to unexpanded Na0.42La0.525TiO3. Moreover, the Na-ion conductivity of Na0.5La0.5ZrO3 is comparable with that of NaNbO3, despite having half the carrier concentration. Additionally, more than 96% of the total conductivity in dry conditions was contributed by sodium ions for both compositions, with negligible electronic conductivity and little oxygen ion conductivity. We also identified factors that limited Na-ion transport: NLZ and NLBZ were both challenging to densify using conventional sintering without the loss of Na because of its volatility. With spark plasma sintering, higher density can be achieved. In addition, the NLZ perovskite phase appeared unable to accommodate significant Na deficiency, whereas NLBZ allowed some. Density functional theory calculations supported a thermodynamic limitation to creation of Na-deficient NLZ in favor of a pyrochlore-type phase. Humid environments generated different behavior: in Na0.25La0.25Ba0.5ZrO3, incorporated protons raised total conductivity, whereas in Na0.5La0.5ZrO3, they lowered total conductivity. Ultimately, this systematic approach revealed both effective approaches and limitations to achieving super-ionic Na-ion conductivity, which may eventually be overcome through alternative processing routes. [ABSTRACT FROM AUTHOR]

Effective control on chemoselectivity in the catalytic hydrogenation of C=O over C=C bonds is uncommon with Pd‐based catalysts because of the favored adsorption of C=C bonds on Pd surface. Here we report a unique orthorhombic PdSn intermetallic phase with unprecedented chemoselectivity toward C=O hydrogenation. We observed the formation and metastability of this PdSn phase in situ. During a natural cooling process, the PdSn nanoparticles readily revert to the favored Pd3Sn2 phase. Instead, using a thermal quenching method, we prepared a pure‐phase PdSn nanocatalyst. PdSn shows an >96 % selectivity toward hydrogenating C=O bonds of various α,β‐unsaturated aldehydes, highest in reported Pd‐based catalysts. Further study suggests that efficient quenching prevents the reversion from PdSn‐ to Pd3Sn2‐structured surface, the key to the desired catalytic performance. Density functional theory calculations and analysis of reaction kinetics provide an explanation for the observed high selectivity. [ABSTRACT FROM AUTHOR]

Effective control on chemoselectivity in the catalytic hydrogenation of C=O over C=C bonds is uncommon with Pd‐based catalysts because of the favored adsorption of C=C bonds on Pd surface. Here we report a unique orthorhombic PdSn intermetallic phase with unprecedented chemoselectivity toward C=O hydrogenation. We observed the formation and metastability of this PdSn phase in situ. During a natural cooling process, the PdSn nanoparticles readily revert to the favored Pd3Sn2 phase. Instead, using a thermal quenching method, we prepared a pure‐phase PdSn nanocatalyst. PdSn shows an >96 % selectivity toward hydrogenating C=O bonds of various α,β‐unsaturated aldehydes, highest in reported Pd‐based catalysts. Further study suggests that efficient quenching prevents the reversion from PdSn‐ to Pd3Sn2‐structured surface, the key to the desired catalytic performance. Density functional theory calculations and analysis of reaction kinetics provide an explanation for the observed high selectivity. [ABSTRACT FROM AUTHOR]

Flash sintering has been established as a method to rapidly densify or induce phase transformation in materials by applying an electric field. Understanding rapid transformations is challenging in a laboratory setting, where phase and defect populations may change with sub‐second time scales. In this work, we apply in situ Raman spectroscopy to investigate defect complexes and phase transformations during flash sintering. We demonstrate that the oxygen vacancy complexes in Ce0.85Gd0.15O1.925 and the evolving phases in a SrCO3 + V2O5 reaction can be probed with acquisition times sufficient to monitor stage II of flash sintering. We establish how thermal effects can be separated from effects of the electric field and determine the resolution of the defect concentration. The fast characterization we demonstrate here can be combined with well‐developed models of thermal and phase evolution to elucidate the mechanism of densification during flash sintering. [ABSTRACT FROM AUTHOR]

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures =700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion-based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75%dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (~10-12 cm²·s-1), and high reversible areal capacities in the range ~0.25 to 0.76 mA·h·cm-2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks. [ABSTRACT FROM AUTHOR]

Using in situ methods to characterize the state of a system during reactions is critical to understanding and improving solvothermal syntheses. This work demonstrates the use of in situ energy‐dispersive X‐ray diffraction (EDXRD) to investigate the local dynamics during solvothermal formation of Cu4O3 using a general‐purpose full‐sized laboratory oven. This allows for direct comparison of in situ data with laboratory‐based reactions. Using in situ EDXRD, changes in the local amounts of Cu4O3, Cu2O and CuO within approximately 100 × 100 × 700 µm gauge volumes during solvothermal Cu4O3 formation were recorded. Fast conversion between Cu2O and CuO was observed in the solvothermal environment, whereas Cu4O3 was found to be chemically stable against disturbances once formed. The observed differences in local dynamics give further support to the differences in formation mechanisms between Cu4O3 and Cu2O/CuO proposed here. [ABSTRACT FROM AUTHOR]

Reactive flash sintering has been demonstrated as a method to rapidly densify and synthesize ceramic materials, but determining the extent of chemical reactions can be complex since the maximum temperature reached by the sample may be brief in time. The black body radiation (BBR) model has been shown to accurately predict the sample temperature during the steady state of flash (stage III). This work demonstrates situations where the BBR model alone does not accurately predict when a phase transformation will occur. We examine the model reactions of CuO reduction to Cu2O during stage II and Mn2O3 reduction to Mn3O4 in stage III. In CuO, highly resistive samples result in initially localized current flow, a stochastic process resulting in inhomogeneous heating and error in the BBR model during stage II. CuO reduction does not occur in constant heating rate experiments with 6.25 V/mm fields, even though the sample temperature momentarily exceeds the phase transformation temperature. Increased furnace heating to 950°C before application of a field is required to drive the transition. In Mn2O3, the calculated sample temperature of the gauge is less than the transformation temperature, but localized heating at the contact will exceed the transformation temperature, causing the transformation to propagate away from the electrode during stage III. This work demonstrates two forms of inhomogeneity (local, stochastic current flow, and local contact resistance) that result in a complex thermal profile of the sample. This profile should be interrogated to understand reaction kinetics, and can be beneficial when engineered. [ABSTRACT FROM AUTHOR]

As the field of flash sintering expands, more diverse flash processes are emerging that exhibit complex mechanisms and kinetics. Reactive flash sintering studies have been performed using precursor oxides and have yet to explore redox reactions. We show that Mn2O3 transforms into Mn3O4 during stage III of flash sintering via a moving reaction front, propagating from an electrode if sufficient energy is supplied. The power density and sample temperature increases as the transformation progresses due to the lower resistivity of Mn2O3 vs Mn3O4, a secondary thermal runaway effect, further confirming the presence of a transformation front. Additionally, in many studies, the contact resistance is accounted for, but not utilized. The energy for the transformation may either be supplied by the contact resistance–induced Joule heating or the furnace. Room‐temperature impedance measurements demonstrate that Pt electrodes provide substantial contact resistance while Ag electrodes do not. The impedance study demonstrates that it is critical to select the appropriate electrode material to maximize or minimize contact resistance. The contact resistance may be used to create a hot spot and propagate a transformation front in any endothermic reduction reaction that occurs below 950°C in electronic conductors. [ABSTRACT FROM AUTHOR]

The compound (ethylenediamine-N)-bis(ethylenediamine-N,N′)-copper(ii) bis(nitrate) is reported. It has an asymmetric unit containing Cu(ii) penta-coordinating to two chelating ethylenediamine (en) and one monodentate en ligands with square-pyramidal-based geometry having severe trigonal distortion, together with two nitrate anions. It is the first crystallographic confirmation of the existence of penta-coordination with exclusively en ligands for any metal center, as well as the first monodentate en attached to Cu(ii) seen in the solid state. It can be visualized as the penta-ammine effect realized solely with bi-amine ligands, defying the presumed necessity of mono-amines. X-ray crystallography establishes it to have P21/c space group with a = 12.83 Å, b = 9.77 Å, c = 11.91 Å, and β = 94.8° (at 100 K). Spectroscopic analysis of this compound is consistent with the crystallographic assessment of the coordination chemistry, and it is found to be a S = 1/2 paramagnet. [ABSTRACT FROM AUTHOR]

In permanent magnet applications, response often scales with volume or dimension in power-conversion and magnetostrictive applications, even in film form. In microelectromechanical devices it is necessary to explore versatile methods of dense film deposition with film thicknesses approaching one micron. In this study, we present a wet chemical route to hard magnetic cobalt ferrite (CoFe2O4) films to produce films with large coercivity, controllable thickness, saturation approaching that of the bulk, and smoother morphology than state-of-the art sputtered or pulsed-laser-deposited films. The development of etching and releasing processes demonstrates how these films are suitable for precise engineering in a variety of form factors and applications. [ABSTRACT FROM AUTHOR]

Non‐stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS2, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS2−δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line‐compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS2 even at different Fe/S ratios. By properly standardizing in situ gas‐flow X‐ray diffraction measurements, no significant changes in the lattice parameter of FeS2 can be resolved, which portrays iron pyrite as prone to forming sulfur‐deficient compounds, but not intrinsic defects in the manner of NiS2−δ. The stoichiometry of iron pyrite, FeS2, is rigorously studied using high‐resolution and in situ X‐ray diffraction. The upper bound on the concentration of sulfur vacancies in FeS2 is investigated. [ABSTRACT FROM AUTHOR]

Navigating the kinetic landscape is a key to reaching complex inorganic phases efficiently. High-temperature treatment of elemental mixtures is the traditional method of solid-state syntheses, where thermodynamic driving forces are large, but success can be hampered by kinetic barriers. Successful inorganic crystal preparation relies on methods to either surmount such barriers, or direct the reaction to avoid them. Both approaches were utilized in our synthetic investigation with in situ X-ray diffraction (XRD) using Fe2SiS4 as the target compound. In this system, Si sulfidation is the limiting factor preventing thermodynamic Fe2SiS4 crystal formation. Through in situ XRD reaction maps, we established that superheated S liquid from the FeS2 peritectic at 743 °C was responsible for the onset of rapid Fe2SiS4 formation. Alternatively, by pre-reacting Si with Fe, we directed the chemical system via intermediate states that expedited Fe2SiS4 formation at temperatures as low as 550 °C. The utilization of these kinetic expediting factors, and those that can be uncovered by similar methods, can improve the potential of the solid state method for creating new materials and refining chemical syntheses. [ABSTRACT FROM AUTHOR]

The Kanatzidis group entered this research area early, with their initial reports on the solution processing of CsPbI SB 3 sb for all-solid state thin film solar cells in 2012, and the organic/inorganic hybrid I R i PbI SB 3 sb the next year. Mercouri G. Kanatzidis has had a huge influence on the field of solid state chemistry. For the limited space in this laudatio, we will focus on three highly impactful scientific areas that he has dominated: synthesis in polychalcogenide salt fluxes, discovery of new thermoelectric materials, and development of halide perovskites for solar cells. [Extracted from the article]

Rapid shifts in the energy, technological, and environmental demands of materials science call for focused and efficient expansion of the library of functional inorganic compounds. To achieve the requisite efficiency, we need a materials discovery and optimization paradigm that can rapidly reveal all possible compounds for a given reaction and composition space. Here we provide such a paradigm via in situ X-ray diffraction measurements spanning solid, liquid flux, and recrystallization processes. We identify four new ternary sulfides from reactive salt fluxes in a matter of hours, simultaneously revealing routes for ex situ synthesis and crystal growth. Changing the flux chemistry, here accomplished by increasing sulfur content, permits comparison of the allowable crystalline building blocks in each reaction space. The speed and structural information inherent to this method of in situ synthesis provide an experimental complement to computational efforts to predict new compounds and uncover routes to targeted materials by design. [ABSTRACT FROM AUTHOR]