Structural Evolution and Photoluminescence Quenching across the FASnI3-xBrx (x = 0-3) Perovskites.

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnI3-xBrx (x = 0-3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes (x = 0-2.9; x = 2.9-3) based on their average structure with the former having a 3D and the latter having an extended 3D (pseudo 0D) structure. Pair distribution function (PDF) analyses further suggest a dynamic 5s2 lone pair expression resulting in increasing levels of off-centering of the central Sn as the Br concentration is increased. These antiferroelectric distortions indicate that even the x = 0-2.9 phase space behaves as a nonideal solid-solution on a more local scale. Solid-state NMR confirms the difference in local structure yielding greater insight into the chemical nature and local distributions of the FA+ cation. In contrast to the FAPbI3-xBrx series, a drastic photoluminescence (PL) quenching is observed with x ≥ 1.9 compounds having no observable PL. Our detailed studies attribute this quenching to structural transitions induced by the distortions of the [SnBr6] octahedra in response to stereochemically expressed lone pairs of electrons. This is confirmed through density functional theory, having a direct impact on the electronic structure.

Chemically Reversible CO2 Uptake by Dendrimer-Impregnated Metal-Organic Frameworks.

Industrialization over the past two centuries has resulted in a continuous rise in global CO2 emissions. These emissions are changing ecosystems and livelihoods. Therefore, methods are needed to capture these emissions from point sources and possibly from our atmosphere. Though the amount of CO2 is rising, it is challenging to capture directly from air because its concentration in air is extremely low, 0.04%. In this study, amines installed inside metal-organic frameworks (MOFs) are investigated for the adsorption of CO2, including at low concentrations. The amines used are polyamidoamine dendrimers that contain many primary amines. Chemically reversible adsorption of CO2 via carbamate formation was observed, as was enhanced uptake of carbon dioxide, likely via dendrimer-amide-based physisorption. Limiting factors in this initial study are comparatively low dendrimer loadings and slow kinetics for carbon dioxide uptake and release, even at 80 °C.

Nanotube Structure of AsPS4-xSex (x = 0, 1).

Single-wall nanotubes of isostructural AsPS4-xSex (x = 0, 1) are grown from solid-state reaction of stoichiometric amounts of the elements. The structure of AsPS4 was determined using single-crystal X-ray diffraction and refined in space group P1¯. The infinite, single-walled AsPS4 nanotubes have an outer diameter of ≈1.1 nm and are built of corner-sharing PS4 tetrahedra and AsS3 trigonal pyramids. Each nanotube is nearly hexagonal, but the ≈3.4 Å distance between S atoms on adjacent nanotubes allows them to easily slide past one another, resulting in the loss of long-range order. Substituting S with Se disrupted the crystallization of the nanotubes, resulting in amorphous products that precluded the determination of the structure for AsPS3Se. 31P solid-state NMR spectroscopy indicated a single unique tetrahedral P environment in AsPS4 and five different P environments all with different degrees of Se substitution in AsPS3Se. Optical absorption spectroscopy revealed an energy band gap of 2.7 to 2.4 eV for AsPS4 and AsPS3Se, respectively. Individual AsPS4 microfibers showed a bulk conductivity of 3.2 × 10-6 S/cm and a negative photoconductivity effect under the illumination of light (3.06 eV) in ambient conditions. Thus, intrinsic conductivity originates from hopping through empty trap states along the length of the AsPS4 nanotubes.

Tailoring Hydrophobicity and Pore Environment in Physisorbents for Improved Carbon Dioxide Capture under High Humidity.

CALF-20, a Zn-triazolate-based metal-organic framework (MOF), is one of the most promising adsorbent materials for CO2 capture. However, competitive adsorption of water severely limits its performance when the relative humidity (RH) exceeds 40%, limiting the potential implementation of CALF-20 in practical settings where CO2 is saturated with moisture, such as postcombustion flue gas. In this work, three newly designed MOFs related to CALF-20, denoted as NU-220, CALF-20M-w, and CALF-20M-e that feature hydrophobic methyltriazolate linkers, are presented. Inclusion of methyl groups in the linker is proposed as a strategy to improve the uptake of CO2 in the presence of water. Notably, both CALF-20M-w and CALF-20M-e retain over 20% of their initial CO2 capture efficiency at 70% RH─a threshold at which CALF-20 shows negligible CO2 uptake. Grand canonical Monte Carlo simulations reveal that the methyl group hinders water network formation in the pores of CALF-20M-w and CALF-20M-e and enhances their CO2 selectivity over N2 in the presence of a high moisture content. Moreover, calculated radial distribution functions indicate that introducing the methyl group into the triazolate linker increases the distance between water molecules and Zn coordination bonds, offering insights into the origin of the enhanced moisture stability observed for CALF-20M-w and CALF-20M-e relative to CALF-20. Overall, this straightforward design strategy has afforded more robust sorbents that can potentially meet the challenge of effectively capturing CO2 in practical industrial applications.

Rational Design and Reticulation of Infinite qbe Rod Secondary Building Units into Metal-Organic Frameworks through a Global Desymmetrization Approach for Inverse C3 H8 /C3 H6 Separation.

The development of reticular chemistry has enabled the construction of a large array of metal-organic frameworks (MOFs) with diverse net topologies and functions. However, dominating this class of materials are those built from discrete/finite secondary building units (SBUs), yet the designed synthesis of frameworks involving infinite rod-shaped SBUs remain underdeveloped. Here, by virtue of a global linker desymmetrization approach, we successfully targeted a novel Cu-MOF (Cu-ASY) incorporating infinite Cu-carboxylate rod SBUs with its structure determined by micro electron diffraction (MicroED) crystallography. Interestingly, the rod SBU can be simplified as a unique cylindric sphere packing qbe tubule made of [43 .62 ] tiles, which further connect the tritopic linkers to give a newly discovered 3,5-connected gfc net. Cu-ASY is a permanent ultramicroporous material featuring 1D channels with highly inert surfaces and shows a preferential adsorption of propane (C3 H8 ) over propene (C3 H6 ). The efficiency of C3 H8 selective Cu-ASY is validated by multicycle breakthrough experiments, giving C3 H6 productivity of 2.2 L/kg. Density functional theory (DFT) calculations reveal that C3 H8 molecules form multiple C-H⋅⋅⋅π and atypical C-H⋅⋅⋅ H-C van der Waals interactions with the inner nonpolar surfaces. This work therefore highlights the linker desymmetrization as an encouraging and intriguing strategy for achieving unique MOF structures and properties.

A Nanocavitation Approach to Understanding Water Capture, Water Release, and Framework Physical Stability in Hierarchically Porous MOFs.

Chemically stable metal-organic frameworks (MOFs) featuring interconnected hierarchical pores have proven to be promising for a remarkable variety of applications. Nevertheless, the framework's susceptibility to capillary-force-induced pore collapse, especially during water evacuation, has often limited practical applications. Methodologies capable of predicting the relative magnitudes of these forces as functions of the pore size, chemical composition of the pore walls, and fluid loading would be valuable for resolution of the pore collapse problem. Here, we report that a molecular simulation approach centered on evacuation-induced nanocavitation within fluids occupying MOF pores can yield the desired physical-force information. The computations can spatially pinpoint evacuation elements responsible for collapse and the chemical basis for mitigation of the collapse of modified pores. Experimental isotherms and difference-electron density measurements of the MOF NU-1000 and four chemical variants validate the computational approach and corroborate predictions regarding relative stability, anomalous sequence of pore-filling, and chemical basis for mitigation of destructive forces.

Modulator-Dependent Dynamics Synergistically Enabled Record SO2 Uptake in Zr(IV) Metal-Organic Frameworks Based on Pyrene-Cored Molecular Quadripod Ligand.

Developing innovative porous solid sorbents for the capture and storage of toxic SO2 is crucial for energy-efficient transportation and subsequent processing. Nonetheless, the quest for high-performance SO2 sorbents, characterized by exceptional uptake capacity, minimal regeneration energy requirements, and outstanding recyclability under ambient conditions, remains a significant challenge. In this study, we present the design of a unique tertiary amine-embedded, pyrene-based quadripod-shaped ligand. This ligand is then assembled into a highly porous Zr-metal-organic framework (MOF) denoted as Zr-TPA, which exhibits a newly discovered 3,4,8-c woy net structure. Remarkably, our Zr-TPA MOF achieved an unprecedented SO2 sorption capacity of 22.7 mmol g-1 at 298 K and 1 bar, surpassing those of all previously reported solid sorbents. We elucidated the distinct SO2 sorption behaviors observed in isostructural Zr-TPA variants synthesized with different capping modulators (formate, acetate, benzoate, and trifluoroacetate, abbreviated as FA, HAc, BA, and TFA, respectively) through computational analyses. These analyses revealed unexpected SO2-induced modulator-node dynamics, resulting in transient chemisorption that enhanced synergistic SO2 sorption. Additionally, we conducted a proof-of-concept experiment demonstrating that the captured SO2 in Zr-TPA-FA can be converted in situ into a valuable pharmaceutical intermediate known as aryl N-aminosulfonamide, with a high yield and excellent recyclability. This highlights the potential of robust Zr-MOFs for storing SO2 in catalytic applications. In summary, this work contributes significantly to the development of efficient SO2 solid sorbents and advances our understanding of the molecular mechanisms underlying SO2 sorption in Zr-MOF materials.

Unlocking the mysterious polytypic features within vaterite CaCO3.

Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays a crucial role in various fields such as hydrosphere, biosphere, and climate regulation. Of the four polymorphs, calcite, aragonite, vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an ongoing debate regarding its structure that has persisted for nearly a century. In this work, based on systematic transmission electron microscopy characterizations, crystallographic analysis and machine learning aided molecular dynamics simulations with ab initio accuracy, we reveal that vaterite can be regarded as a polytypic structure. The basic phase has a monoclinic lattice possessing pseudohexagonal symmetry. Direct imaging and atomic-scale simulations provide evidence that a single grain of vaterite can contain three orientation variants. Additionally, we find that vaterite undergoes a second-order phase transition with a critical point of ~190 K. These atomic scale insights provide a comprehensive understanding of the structure of vaterite and offer advanced perspectives on the biomineralization process of calcium carbonate.

Suitability of a diamine functionalized metal-organic framework for direct air capture.

The increase in the atmospheric carbon dioxide level is a significant threat to our planet, and therefore the selective removal of CO2 from the air is a global concern. Metal-organic frameworks (MOFs) are a class of porous materials that have shown exciting potential as adsorbents for CO2 capture due to their high surface area and tunable properties. Among several implemented technologies, direct air capture (DAC) using MOFs is a promising strategy for achieving climate targets as it has the potential to actively reduce the atmospheric CO2 concentration to a safer levels. In this study, we investigate the stability and regeneration conditions of N,N'-dimethylethylenediamine (mmen) appended Mg2(dobpdc), a MOF with exceptional CO2 adsorption capacity from atmospheric air. We employed a series of systematic experiments including thermogravimetric analysis (TGA) coupled with Fourier transformed infrared (FTIR) and gas chromatography mass spectrometer (GCMS) (known as TGA-FTIR-GCMS), regeneration cycles at different conditions, control and accelerated aging experiments. We also quantified CO2 and H2O adsorption under humid CO2 using a combination of data from TGA-GCMS and coulometric Karl-Fischer titration techniques. The quantification of CO2 and H2O adsorption under humid conditions provides vital information for the design of real-world DAC systems. Our results demonstrate the stability and regeneration conditions of mmen appended Mg2(dobpdc). It is stable up to 50% relative humidity when the adsorption temperature varies from 25-40 °C and the best regeneration condition can be achieved at 120 °C under dynamic vacuum and at 150 °C under N2.

Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions.

Two-dimensional covalent organic frameworks (2D COFs) form as layered 2D polymers whose sheets stack through high-surface-area, noncovalent interactions that can give rise to different interlayer arrangements. Manipulating the stacking of 2D COFs is crucial since it dictates the effective size and shape of the pores as well as the specific interactions between functional aromatic systems in adjacent layers, both of which will strongly influence the emergent properties of 2D COFs. However, principles for tuning layer stacking are not yet well understood, and many 2D COFs are disordered in the stacking direction. Here, we investigate effects of pendant chain length through a series of 2D imine-linked COFs functionalized with n-alkyloxy chains varying in length from one carbon (C1 COF) to 11 carbons (C11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C1 COF adopts an averaged eclipsed geometry with no apparent offset between layers. In contrast, all subsequent chain lengths lead to some degree of unidirectional slip stacking. As pendant chain length is increased, trends show average layer offset increasing to a maximum of 2.07 Å in C5 COF and then decreasing as chain length is extended through C11 COF. Counterintuitively, shorter chains (C2-C4) give rise to lower yields of weakly crystalline materials, while longer chains (C6-C9) produce greater yields of highly crystalline materials, as confirmed by powder X-ray diffraction and scanning electron microscopy. Molecular dynamics simulations corroborate these observations, suggesting that long alkyl chains can interact favorably to promote the self-assembly of sheets. In situ proton NMR spectroscopy provides insights into the reaction equilibrium as well as the relationship between low COF yields and low crystallinity. These results provide fundamental insights into principles of supramolecular assembly in 2D COFs, demonstrating an opportunity for harnessing favorable side-chain interactions to produce highly crystalline materials.

Cyclic versus Linear Alkylammonium Cations: Preventing Phase Transitions at Operational Temperatures in 2D Perovskites.

Two-dimensional lead halide perovskites offer numerous attractive features for optoelectronics owing to their soft, deformable lattices and high degree of chemical tunability. While alteration of the metal and halide ions gives rise to significant modification of the bandgap energy, the organic spacer cations offer in-roads to tuning phase behavior and more subtle functionalities in ways that remain to be understood. Here, we study six variations of 2D perovskites changing only the organic spacer cations and demonstrate that these components intrinsically impact material response in important ways such as altering crystallographic structure, temperature-induced phase transitions, and photoluminescence emission. Two-dimensional perovskites containing commonly utilized aliphatic linear spacers, such as butylammonium, undergo phase transitions near room temperature. These transitions and temperature changes induce spacer-dependent variations in the emission spectra. Conversely, 2D perovskites comprising cyclic aliphatic spacers, such as cyclobutylammonium, are found to lack first-order phase transitions. These cyclic molecules are more sterically hindered within the crystal lattice, leading to temperature-induced contraction or expansion along certain crystallographic planes but no other significant thermal effects; additionally, they undergo changes in their emission spectra that cannot be explained by simple thermal expansion. Given the similarities in the dielectric and chemical makeup of this set of six alkylammonium molecules, these results are unexpected and suggest a large structural and thermal phase space via spacer manipulation that could lead to improved 2D perovskite functionalization.

Unraveling the Role of Entropy in Thermoelectrics: Entropy-Stabilized Quintuple Rock Salt PbGeSnCdxTe3+x.

Entropy-engineered materials are garnering considerable attention owing to their excellent mechanical and transport properties, such as their high thermoelectric performance. However, understanding the effect of entropy on thermoelectrics remains a challenge. In this study, we used the PbGeSnCdxTe3+x family as a model system to systematically investigate the impact of entropy engineering on its crystal structure, microstructure evolution, and transport behavior. We observed that PbGeSnTe3 crystallizes in a rhombohedral structure at room temperature with complex domain structures and transforms into a high-temperature cubic structure at ∼373 K. By alloying CdTe with PbGeSnTe3, the increased configurational entropy lowers the phase-transition temperature and stabilizes PbGeSnCdxTe3+x in the cubic structure at room temperature, and the domain structures vanish accordingly. The high-entropy effect results in increased atomic disorder and consequently a low lattice thermal conductivity of 0.76 W m-1 K-1 in the material owing to enhanced phonon scattering. Notably, the increased crystal symmetry is conducive to band convergence, which results in a high-power factor of 22.4 µW cm-1 K-1. As a collective consequence of these factors, a maximum ZT of 1.63 at 875 K and an average ZT of 1.02 in the temperature range of 300-875 K were obtained for PbGeSnCd0.08Te3.08. This study highlights that the high-entropy effect can induce a complex microstructure and band structure evolution in materials, which offers a new route for the search for high-performance thermoelectrics in entropy-engineered materials.

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites.

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)2 PbI4 , and phenethylammonium lead iodide, (PEA)2 PbI4 , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)2 PbI4 would suggest a transient phase transition, however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P21 21 21 . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Synthesis of the Candidate Topological Compound Ni3Pb2.

Spin-orbit coupling enables the realization of topologically nontrivial ground states. As spin-orbit coupling increases with increasing atomic number, compounds featuring heavy elements, such as lead, offer a pathway toward creating new topologically nontrivial materials. By employing a high-pressure flux synthesis method, we synthesized single crystals of Ni3Pb2, the first structurally characterized bulk binary phase in the Ni-Pb system. Combining experimental and theoretical techniques, we examined structure and bonding in Ni3Pb2, revealing the impact of chemical substitutions on electronic structure features of importance for controlling topological behavior. From these results, we determined that Ni3Pb2 completes a series of structurally related transition-metal-heavy main group intermetallic materials that exhibit diverse electronic structures, opening a platform for synthetically tunable topologically nontrivial materials.

2D Homologous Series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1) and Commensurately Modulated Sr2F2Bi2/3S2.

We report three new mixed-anion two-dimensional (2D) compounds: SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2. Their structures as well as the parent compound SrFBiS2 were refined using single-crystal X-ray diffraction data, with the sequence of SrFBiS2, SrFPbBiS3, and SrFAg0.5Bi1.5S3 defining the new homologous series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1). Sr2F2Bi2/3S2 has a different structure, which is modulated with a q vector of 1/3b* and was refined in superspace group X2/m(0ß0)00 as well as in the 1 × 3 × 1 superstructure with space group C2/m (with similar results). Sr2F2Bi2/3S2 features hexagonal layers of alternating [Sr2F2]2+ and [Bi2/3S2]2-, and the modulated structure arises from the unique ordering pattern of Sr2+ cations. SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2 are semiconductors with band gaps of 1.31, 1.21, and 1.85 eV, respectively. The latter compound exhibits room temperature red photoluminescence at ∼700 nm.

Low Thermal Conductivity in Heteroanionic Materials with Layers of Homoleptic Polyhedra.

Although BiAgOSe, an analogue of a well-studied thermoelectric material BiCuOSe, is thermodynamically stable, its synthesis is complicated by the low driving force of formation from the stable binary and ternary intermediates. Here we have developed a "subtraction strategy" to suppress byproducts and produce pure phase BiAgOSe using hydrothermal methods. Electronic structure calculations and optical characterization show that BiAgOSe is an indirect bandgap semiconductor with a bandgap of 0.95 eV. The prepared sample exhibits lower lattice thermal conductivities (0.61 W·m-1·K-1 at room temperature and 0.35 W·m-1·K-1 at 650 K) than BiCuOSe. Lattice dynamical simulations and variable temperature diffraction measurements demonstrate that the low lattice thermal conductivity arises from both the low sound velocity and high phonon-phonon scattering rates in BiAgOSe. These in turn result primarily from the soft Ag-Se bonds in the edge-sharing AgSe4 tetrahedra and large sublattice mismatch between the quasi-two-dimensional [Bi2O2]2+ and [Ag2Se2]2- layers. These results highlight the advantages of manipulating the chemistry of homoleptic polyhedra in heteroanionic compounds for electronic structure and phonon transport control.

Perovskite-like K3TiOF5 Exhibits (3 + 1)-Dimensional Commensurate Structure Induced by Octahedrally Coordinated Potassium Ions.

Elpasolite- and cryolite-type oxyfluorides can be regarded as superstructures of perovskite and exhibit structural diversity. While maintaining a similar structural topology with the prototype structures, changes in the size, electronegativity, and charge of cation and/or anion inevitably lead to structural evolution. Therefore, the nominal one-to-one relation suggested by a doubled formula of perovskite does not guarantee a simple 2-fold superstructure for many cases. Herein, the commensurately modulated perovskite-like K3TiOF5 was refined at 100 K from single-crystal X-ray diffraction data by using a pseudotetragonal subcell with lattice parameters of a = b = 6.066(2) Å and c = 8.628(2) Å. The length of the modulation vector was refined to 0.3a* + 0.1b* + 0.25c*. In the commensurate supercell of K3TiOF5, the B-site Ti4+ and K+ cations in [TiOF5]3- and [KOF5]6- octahedral units were found to be significantly displaced from the average atomic positions refined in the subcell. The displacements of the K+ cations are ±0.76 Å, and those for the Ti4+ cations are approximately ±0.13 Å. One- and two-dimensional solid-state 19F NMR measurements revealed two tightly clustered groups of resonances in a ratio of ca. 4:1, assigned to equatorial and axial fluorine, respectively, consistent with local [TiOF5]3- units. S/TEM results confirmed the average structure. Electronic structure calculations of the idealized I4mm subcell indicate the instability to a modulated structure arises from soft optical modes that is controlled by the octahedrally coordinated B-site potassium ions in the cryolite-type structure.

Molecular Coordination, Structure, and Stability of Metal-Polyphosphate Complexes Resolved by Molecular Modeling and X-ray Scattering: Structural Insights on the Biological Fate of Polyphosphate.

Polyphosphate-accumulating organisms (PAOs), which can store high levels of phosphate (Pi) in the form of polyphosphate (polyP), are employed to engineer enhanced biological P removal (EBPR) from wastewaters. Co-localization of Mg and K in polyP granules of PAOs has been reported, and higher abundance of Mg-polyP granules relative to other metal complexes was correlated positively with EBPR performance stability. However, the underlying mechanism remains unknown. Here, we obtained molecular structural information of hydrated polyP complexes with four physiologically relevant metal cations (Na+, K+, Ca2+, and Mg2+) using computational and experimental techniques. Molecular dynamics simulations revealed that Mg-polyP and K-polyP complexes were the most and least stable of the complexes, respectively, suggesting that the co-occurrence of these complexes facilitates variable polyP bioavailability. The relative thermodynamic stability reflected the strength of metal chelation whereby the coordination distance between the polyP ligand O and the metal was 1.71-2.01 Å for Mg2+ but this distance was 2.64-2.70 Å for K+. Pair distribution function analysis of X-ray scattering data obtained with a Mg-polyP solution corroborated the theoretical Mg-polyP coordination geometry. These findings implied a possible mechanistic role of metal complexation in the P cycling traits of PAOs in engineered and natural systems.

A two-dimensional type I superionic conductor.

Superionic conductors possess liquid-like ionic diffusivity in the solid state, finding wide applicability from electrolytes in energy storage to materials for thermoelectric energy conversion. Type I superionic conductors (for example, AgI, Ag2Se and so on) are defined by a first-order transition to the superionic state and have so far been found exclusively in three-dimensional crystal structures. Here, we reveal a two-dimensional type I superionic conductor, α-KAg3Se2, by scattering techniques and complementary simulations. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations confirm that the superionic Ag+ ions are confined to subnanometre sheets, with the simulated local structure validated by experimental X-ray powder pair-distribution-function analysis. Finally, we demonstrate that the phase transition temperature can be controlled by chemical substitution of the alkali metal ions that compose the immobile charge-balancing layers. Our work thus extends the known classes of superionic conductors and will facilitate the design of new materials with tailored ionic conductivities and phase transitions.

In Situ Mechanistic Studies of Two Divergent Synthesis Routes Forming the Heteroanionic BiOCuSe.

Heteroanionic materials are a burgeoning class of compounds that offer new properties via the targeted selection of anions. However, understanding the design principles and achieving successful syntheses of new materials in this class are in their infancy. To obtain mechanistic insight and a panoramic view of the reaction progression from beginning to end of the formation of a heteroanionic material, we selected BiOCuSe, a well-known thermoelectric compound, and utilized in situ synchrotron powder diffraction as a function of temperature and time. BiOCuSe is a layered material, which crystallizes in a common mixed anion structure type: ZrSiAsFe. Two reactions of starting materials (Bi2O2Se + Cu2Se and Bi2O3 + Bi + 3Cu + 3Se) were studied to determine the effect of precursors on the reaction pathway. Our in situ investigation shows that the ternary-binary Bi2O2Se + Cu2Se reaction proceeds without intermediates to directly form BiOCuSe, while the binary-elemental Bi2O3 + Bi + 3Cu + 3Se reaction generates many intermediates before the final product forms. These intermediates include CuSe, Bi3Se4, Bi2Se3, and Cu2Se. While the stoichiometric loading of the precursors necessarily dictates the identity of the first intermediates, kinetics also plays a contributing role in stabilizing unexpected intermediates such as CuSe and Bi3Se4. Understanding and establishing a link between the selection of precursors and the reaction pathways improves the potential for rational synthesis of heteroanionic materials and solid-state reactions in general.