Exploring Layered Disorder in Lithium-Ion-Conducting Li3Y1-xInxCl6.

Li3Y1-xInxCl6 undergoes a phase transition from trigonal to monoclinic via an intermediate orthorhombic phase. Although the trigonal yttrium containing the end member phase, Li3YCl6, synthesized by a mechanochemical route, is known to exhibit stacking fault disorder, not much is known about the monoclinic phases of the serial composition Li3Y1-xInxCl6. This work aims to shed light on the influence of the indium substitution on the phase evolution, along with the evolution of stacking fault disorder using X-ray and neutron powder diffraction together with solid-state nuclear magnetic resonance spectroscopy, studying the lithium-ion diffusion. Although Li3Y1-xInxCl6 with x ≤ 0.1 exhibits an ordered trigonal structure like Li3YCl6, a large degree of stacking fault disorder is observed in the monoclinic phases for the x ≥ 0.3 compositions. The stacking fault disorder materializes as a crystallographic intergrowth of faultless domains with staggered layers stacked in a uniform layer stacking, along with faulted domains with randomized staggered layer stacking. This work shows how structurally complex even the "simple" series of solid solutions can be in this class of halide-based lithium-ion conductors, as apparent from difficulties in finding a consistent structural descriptor for the ionic transport.

Investigating the Li+ substructure and ionic transport in Li10GeP2-xSbxS12 (0 ≤ x ≤ 0.25).

Understanding the correlation between ionic motion and crystal structure is crucial for improving solid electrolyte conductivities. Several substitution series in the Li10GeP2S12 structure have shown a favorable impact on the ionic conductivity, e.g. the replacement of P(+V) by Sb(+V) in Li10GeP2S12. However, here the interplay between the structure and ionic motion remains elusive. X-Ray diffraction, high-resolution neutron diffraction, Raman spectroscopy and potentionstatic impedance spectroscopy are employed to explore the impact of Sb(+V) on the Li10GeP2S12 structure. The introduction of antimony elongates the unit cell in the c-direction and increases the M(1)/P(1) and Li(2) polyhedral volume. Over the solid solution range, the Li+ distribution remains similar, an inductive effect seems to be absent and the ionic conductivity is comparable for all compositions. The effect of introducing Sb(+V) in Li10GeP2S12 cannot be corroborated.

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1-xGexS5I as Probed by Neutron Diffraction and NMR.

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li+ ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li6+xP1-xGexS5I served as an application-oriented model system to study the effect of cation substitution (P5+ vs Ge4+) on Li+ ion dynamics. While Li6PS5I is a rather poor ionic conductor (10-6 S cm-1, 298 K), the Ge-containing samples show specific conductivities on the order of 10-2 S cm-1 (330 K). Replacing P5+ with Ge4+ not only causes S2-/I- anion site disorder but also reveals via neutron diffraction that the Li+ ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used 7Li and 31P NMR to show that this Li+ site disorder has a tremendous effect on both local ion dynamics and long-range Li+ transport. For the Ge-rich samples, NMR revealed several new Li+ exchange processes, which are to be characterized by rather low activation barriers (0.1-0.3 eV). Consequently, in samples with high Ge-contents, the Li+ ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Highly Converged Valence Bands and Ultralow Lattice Thermal Conductivity for High-Performance SnTe Thermoelectrics.

A two-step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self-compensated SnTe (that is, Sn1.03 Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn1.03-x Agx Te resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔEv decreases to 0.10 eV in Sn0.83 Ag0.03 Mn0.17 Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 µW cm-1 K-2 at 816 K in Sn0.83 Ag0.03 Mn0.17 Te was attained. The lattice thermal conductivity of Sn0.83 Ag0.03 Mn0.17 Te reached to an ultralow value (ca. 0.3 W m-1 K-1 ) at 865 K, owing to the formation of Ag7 Te4 nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn0.83 Ag0.03 Mn0.17 Te.

All-Solid-State Mechanochemical Synthesis and Post-Synthetic Transformation of Inorganic Perovskite-type Halides.

All-inorganic and hybrid perovskite type halides are generally synthesized by solution-based methods, with the help of long chain organic capping ligands, complex organometallic precursors, and high boiling organic solvents. Herein, a room temperature, solvent-free, general, and scalable all-solid-state mechanochemical synthesis is demonstrated for different inorganic perovskite type halides, with versatile structural connectivity in three (3D), two (2D), and zero (0D) dimensions. 3D CsPbBr3 , 2D CsPb2 Br5 , 0D Cs4 PbBr6 , 3D CsPbCl3 , 2D CsPb2 Cl5 , 0D Cs4 PbCl6 , 3D CsPbI3 , and 3D RbPbI3 have all been synthesized by this method. The all-solid-state synthesis is materialized through an inorganic retrosynthetic approach, which directs the decision on the solid-state precursors (e.g., CsX and PbX2 (X=Cl/Br/I) with desired stoichiometric ratios. Moreover, post-synthetic structural transformations from 3D to 2D and 0D perovskite halides were performed by the same mechanochemical synthetic approach at room temperature.

Accessing Heterobiaryls through Transition-Metal-Free C-H Functionalization.

Herein we report a transition-metal-free synthetic protocol for heterobiaryls, one of the most important pharmacophores in the modern drug industry, employing a new multidonor phenalenyl (PLY)-based ligand. The current procedure offers a wide substrate scope (24 examples) with a low catalyst loading resulting in an excellent product yield (up to 95%). The reaction mechanism involves a single electron transfer (SET) from a phenalenyl-based radical to generate a reactive heteroaryl radical. To establish the mechanism, we have isolated the catalytically active SET initiator, characterizing by a magnetic study.

Synthetic Nanosheets of Natural van der Waals Heterostructures.

Creation of new van der Waals heterostructures by stacking different two dimensional (2D) crystals on top of each other in a chosen sequence is the next challenge after the discovery of graphene, mono/few layer of h-BN, and transition-metal dichalcogenides. However, chemical syntheses of van der Waals heterostructures are rarer than the physical preparation techniques. Herein, we demonstrate the kinetic stabilization of 2D ultrathin heterostructure (ca. 1.13-2.35 nm thick) nanosheets of layered intergrowth SnBi2 Te4 , SnBi4 Te7 , and SnBi6 Te10 , which belong to the Snm Bi2n Te3n+m homologous series, by a simple solution based synthesis. Few-layer nanosheets exhibit ultralow lattice thermal conductivity (κlat ) of 0.3-0.5 W m-1 K-1 and semiconducting electron-transport properties with high carrier mobility.

High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence Band Convergence.

Understanding the basis of electronic transport and developing ideas to improve thermoelectric power factor are essential for production of efficient thermoelectric materials. Here, we report a significantly large thermoelectric power factor of ∼31.4 µW/cm·K2 at 856 K in Ag and In co-doped SnTe (i.e., SnAgxInxTe1+2x). This is the highest power factor so far reported for SnTe-based material, which arises from the synergistic effects of Ag and In on the electronic structure and the improved electrical transport properties of SnTe. In and Ag play different but complementary roles in modifying the valence band structure of SnTe. In-doping introduces resonance levels inside the valence bands, leading to a significant improvement in the Seebeck coefficient at room temperature. On the other hand, Ag-doping reduces the energy separation between light- and heavy-hole valence bands by widening the principal band gap, which also results in an improved Seebeck coefficient. Additionally, Ag-doping in SnTe enhances the p-type carrier mobility. Co-doping of In and Ag in SnTe yields synergistically enhanced Seebeck coefficient and power factor over a broad temperature range because of the synergy of the introduction of resonance states and convergence of valence bands, which have been confirmed by first-principles density functional theory-based electronic structure calculations. As a consequence, we have achieved an improved thermoelectric figure of merit, zT ≈ 1, in SnAg0.025In0.025Te1.05 at 856 K.

Few-Layer Nanosheets of n-Type SnSe2.

Layered p-block metal chalcogenides are renowned for thermoelectric energy conversion due to their low thermal conductivity caused by bonding asymmetry and anharmonicity. Recently, single crystalline layered SnSe has created sensation in thermoelectrics due to its ultralow thermal conductivity and high thermoelectric figure of merit. Tin diselenide (SnSe2 ), an additional layered compound belonging to the Sn-Se phase diagram, possesses a CdI2 -type structure. However, synthesis of pure-phase bulk SnSe2 by a conventional solid-state route is still remains challenging. A simple solution-based low-temperature synthesis is presented of ultrathin (3-5 nm) few layers (4-6 layers) nanosheets of Cl-doped SnSe2 , which possess n-type carrier concentration of 2×1018  cm-3 with carrier mobility of about 30 cm2 V-1 s-1 at room temperature. SnSe2 has a band gap of about 1.6 eV and semiconducting electronic transport in the 300-630 K range. An ultralow thermal conductivity of about 0.67 Wm-1  K-1 was achieved at room temperature in a hot-pressed dense pellet of Cl-doped SnSe2 nanosheets due to the anisotropic layered structure, which gives rise to effective phonon scattering.

C-Alkylation by Phenalenyl-Based Molecule via a Borrowing Hydrogen Pathway.

The present study demonstrates the first transition-metal-free catalytic C-alkylation via a borrowing hydrogen pathway for the α-alkylation of ketone, synthesis of substituted quinoline, and 9-monoalkylation of fluorene. With applications on diversification of biologically active molecules and gram-scale synthesis, a preliminary investigation of the reaction mechanism has been carried out, suggesting a radical-mediated borrowing hydrogen pathway.

Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering.

Layered Bi-misfit cobaltates, such as Bi2Sr2Co2Oy, are the natural superlattice of an electrically insulating rocksalt (RS) type Bi2Sr2O4 layer and electrically conducting CoO2 layer, stacked along the crystallographic c-axis. RS and CoO2 layers are related through charge compensation reactions (or charge transfer). Therefore, thermoelectric transport properties are affected when doping or substitution is carried out in the RS layer. In this work, we have shown improved thermoelectric properties of spark plasma sintered Bi2Sr2-xCaxCo2Oy alloys (x = 0, 0.3 and 0.5). The substitution of Ca atoms affects the thermal properties by introducing point-defect phonon scattering, while the electronic conductivity and thermopower remain unaltered.

Mimicking transition metals in borrowing hydrogen from alcohols.

Borrowing hydrogen from alcohols, storing it on a catalyst and subsequent transfer of the hydrogen from the catalyst to an in situ generated imine is the hallmark of a transition metal mediated catalytic N-alkylation of amines. However, such a borrowing hydrogen mechanism with a transition metal free catalytic system which stores hydrogen molecules in the catalyst backbone is yet to be established. Herein, we demonstrate that a phenalenyl ligand can imitate the role of transition metals in storing and transferring hydrogen molecules leading to borrowing hydrogen mediated alkylation of anilines by alcohols including a wide range of substrate scope. A close inspection of the mechanistic pathway by characterizing several intermediates through various spectroscopic techniques, deuterium labelling experiments, and DFT study concluded that the phenalenyl radical based backbone sequentially adds H+, H˙ and an electron through a dearomatization process which are subsequently used as reducing equivalents to the C-N double bond in a catalytic fashion.

On the underestimated influence of synthetic conditions in solid ionic conductors.

The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis - (micro)structure - transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials.

The journey of tin chalcogenides towards high-performance thermoelectrics and topological materials.

Lead chalcogenides and their alloys belong at the heart of thermoelectrics due to their large thermoelectric figure of merit (zT). However, recent research has shown a limitation in the use of lead (Pb)-based materials due to their toxicity and efforts have been made to produce non-toxic analogues of lead chalcogenides. Tin chalcogenides have been predicted to be promising for this purpose due to their unique electronic structure and phonon dispersion properties. Here, we discuss the journey of tin chalcogenides in the field of thermoelectrics and topological materials with the main emphasis on the bonding, crystal structures, electronic band structures, phonon dispersion and thermoelectric properties. The thermal transport properties of tin chalcogenides are explained based on lattice dynamics, where resonant bonding and local structural distortion play an important role in creating lattice anharmonicity, thereby lowering the lattice thermal conductivity. Since thermoelectric and topological materials, especially topological insulators and topological crystalline insulators, share similar material features, such as a narrow band gap, heavy constituent elements and significant spin-orbit coupling, we have discussed the thermoelectric properties of several topological tin chalcogenides from a chemistry perspective. This feature article serves as a useful reference for researchers who strive to improve the properties of tin chalcogenides and advance the field of thermoelectric and topological materials.