Chemistry of Materials最新文献

Solid-state electrolytes (SSEs) have emerged as promising alternatives to traditional liquid electrolytes due to their enhanced safety, higher stability and energy density in energy storage applications. Among SSEs, cubic Li₇La₃Zr₂O₁₂ (LLZO) is considered particularly promising, offering high lithium ion conductivity, high chemical stability to metal anode and a wide electrochemical stability window. Nevertheless, the cubic phase converts to a less conductive tetragonal phase during cooling in pure LLZO. Doping is one of most effective methods to stabilize the cubic LLZO at lower temperatures and improve the ion conductivity. While there is extensive research on cation site substitutions, studies on anion doping are very limited. We have investigated the effects of fluorine doping on the phase stability and ion conductivity of LLZO, exploring fluorine concentrations ranging from 1 to 10% across a wide temperature range of 300–1400 K using molecular dynamics (MD) simulations based on polarizable shell model potentials. Our results indicate that 3% fluorine doping achieves the highest diffusion coefficient (3.69 × 10^–7 cm² s^–1) at room temperature, while the lowest activation energy (∼0.22 eV) also occurs at around 3% doping, which is in good agreement with experimental observations. Doping at 1% was found to be insufficient to stabilize the cubic phase, while high fluorine concentrations (>4%) inhibited ion migration pathways due to stronger electrostatic interactions between point defects V_Li^′ and F_O^•. Defect formation energies were also calculated to study defect formation and interactions and their effect on lithium ion conduction. Lithium ion diffusion pathways and mechanisms are also explored by using trajectories from MD simulations. This study provides insights into the optimization of fluorine-doped LLZO, suggesting that moderate doping levels (around 3%) offer a balance between phase stability and ionic conductivity.

We investigated tetracyanoanthracenediacenaphthalimides (TCDADIs) as n-type organic semiconductors (OSCs) and assessed their molecular self-assembly in forming monolayers and thin films using optical absorption spectroscopy, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and grazing incidence wide-angle X-ray scattering (GIWAXS). The absorption spectra, along with quantitative GIWAXS analysis, reveal the influence of molecular structure (alkyl chain length) and film processing conditions (annealing temperature and spin-coating speed) on the orientation of TCDADI molecules in films. Our findings indicate that increasing the spin-coating speed and annealing temperatures causes a transition from a mixed phase to a predominantly edge-on molecular orientation. This transition significantly enhances the electron mobility, from 0.01 to 0.05 cm² V^–1 s^–1 for TCDADI-C16 and from 0.13 to 0.20 cm² V^–1 s^–1 for TCDADI-C24. In addition, we highlight the potential of TCDADIs for photodetector applications, showing a photoresponse gain of over 2000 under white light.

A combination of high-resolution powder diffraction techniques and solid-state NMR has been employed to explore the links between crystal structure, orbital ordering, and magnetism in three isostructural double perovskites containing transition metal ions with a 5d¹ configuration. In Ba₂ZnReO₆, both neutron and synchrotron X-ray powder diffraction data reveal a cubic-to-tetragonal transition at 23 K that breaks the degeneracy of the t_2g orbitals and leads to a pattern of orbital ordering that stabilizes magnetic ordering when the sample is cooled below 16 K. Similar behavior is observed in Ba₂MgReO₆, with an orbital ordering temperature of 33 K and a magnetic ordering temperature of 18 K. Prior theoretical works suggest that the pattern of orbital order seen in the P4₂/mnm space group is needed to stabilize the heavily canted antiferromagnetism of these compounds. Unfortunately, powder diffraction data is not sensitive enough to differentiate between the I4/mmm and P4₂/mnm structural models, as the distortions are too subtle to be unambiguously identified from either neutron or synchrotron X-ray powder diffraction methods. In contrast, both diffraction and ⁷Li NMR data indicate that Ba₂LiOsO₆ retains the cubic structure down to 1.7 K. The antiferromagnetic ground state and lack of any sign of orbital ordering in Ba₂LiOsO₆ provide compelling evidence that the electronically driven tetragonal distortion seen in Ba₂ZnReO₆, and Ba₂MgReO₆ is intimately linked to the magnetic ordering seen in those compounds. The absence of magnetic reflections in high intensity neutron powder diffraction data collected on Ba₂MgReO₆ strongly suggests ordering of multipolar moments on Re(VI), likely ferro-octupolar ordering.

Hydrogen bronzes can be used as hydrogen donors for the broad class of reactions involving proton-coupled electron transfer (PCET). Here, we describe a method to prepare platinum-decorated hydrogen tungsten bronzes, Pt@H_xWO₃·nH₂O with n = 0, 1, and 2, by reacting the pristine oxides at modest temperatures with a mild reducing agent, H₃PO₂, and H₂PtCl₆ in an aqueous solution. We explored the tunability and kinetics of this reaction and compared it with that of archetypal gas–solid hydrogen spillover. We demonstrate that the identity of the noble metal affects the extent of bronze reduction. This suggests that the mechanism proceeds via the adsorption of a hydrogen-atom species on the noble metal. Finally, we explored the ability of the Pt-decorated hydrogen tungsten bronzes to hydrogenate a model H⁺/e^– acceptor, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). The bronze phases return to their fully oxidized states along with the subsequent reduction of TEMPO to TEMPOH. Overall, this work demonstrates a solution-phase method to obtain hydrogen bronzes, which can then be used to perform hydrogen transfer reactions, providing a pathway for the use of extended transition metal oxides as stoichiometric reagents for broad classes of hydrogenation reactions.

Rotator phases are rotationally disordered yet crystalline stable states found in many materials. The presence of a rotator phase leads to unique properties that influence processing methods and offer potential applications in areas such as thermal energy storage, lubrication, and sensing. Recently, a novel family of chemically recyclable oligomers, (1,n′-divinyl)oligocyclobutane (DVOCB(n)), has shown evidence of rotator phases. This study combines experimental characterization and molecular dynamics simulations to confirm and elucidate the rotator phases in DVOCB(n). Compared with well-studied n-alkanes, DVOCB(n) exhibits distinct structural, thermodynamic, and dynamical characteristics. The crystal-to-rotator phase transition of DVOCB(n) involves a shift from stretched to isotropic hexagonal lamellar packing, captured by a rotational order parameter tracking local chain orientations orthogonal to the chain axis. Unlike n-alkanes, where rotational relaxation times are constant and long in the crystal phase before dropping dramatically during the crystal-to-rotator phase transition, relaxation times decrease more gradually upon heating in DVOCB(n), including continuously throughout the transition. This behavior is attributed to its unique enchained-ring architecture, which allows for semi-independent rotation of chain segments that promotes overall rotational disorder. This work provides a fundamental understanding of rotator phases in DVOCB(n) and highlights differences from those of conventional materials. The analyses and insights herein will inform future studies and applications of DVOCB(n) as well as other materials with rotator phases.