Nanostructured Na₂Ti₉O₁₉ for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability

Herein, we report a new Na-insertion electrode material, Na₂Ti₉O₁₉, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured Na₂Ti₉O₁₉, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of Na₂Ti₉O₁₉ are elucidated from neutron Rietveld refinement and pair distribution function (PDF), respectively, to investigate the initial discharge and charge events. Rietveld refinement reveals electrochemical cycling of Na₂Ti₉O₁₉ is driven by single-phase solid solution reaction during (de)­sodiation without any major structural deterioration, keeping the average structure intact. Unit cell volume and lattice evolution on discharge process is inherently related to TiO₆ distortion and Na ion perturbations, while the PDF reveals the deviation in the local structure after sodiation. Raman spectroscopy and X-ray photoelectron spectroscopy studies further corroborate the average and local structural behavior derived from neutron diffraction measurements. Also, Na₂Ti₉O₁₉ shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s^–1, indicating that the material is a good anode candidate for a sodium-ion hybrid capacitor. A full cell hybrid Na-ion capacitor is fabricated by using Na₂Ti₉O₁₉ as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg^–1 and a maximum power density of 5 kW kg^–1. Both structural insights and electrochemical investigation suggest that Na₂Ti₉O₁₉ is a promising negative electrode for sodium-ion batteries and hybrid capacitors

Polymorphism in Photoluminescent KNdW₂O₈: Synthesis, Neutron Diffraction, and Raman Study

Polymorphs of KNdW₂O₈ (α-KNdW₂O₈ and β-KNdW₂O₈) phosphors were synthesized by an efficient solution combustion technique for the first time. The crystal structure of the polymorphs analyzed from Rietveld refinement of neutron diffraction data confirms that α-KNdW₂O₈ crystallizes in the tetragonal system (space group <i>I</i>4̅), and β-KNdW₂O₈ crystallizes in the monoclinic system (space group <i>C</i>2/<i>m</i>)<i>.</i> The local structure of both polymorphs was elucidated using combined neutron pair distribution function (PDF) and Raman scattering techniques. Photoluminescence measurements of the polymorphs showed broadened emission line width and increased intensity for β-KNdW₂O₈ in the visible region compared to α-KNdW₂O₈.This phenomenon is attributed to the increased distortion in the coordination environment of the luminescing Nd³⁺ ion. Combined PDF, Rietveld, and Raman measurements reveal distortions of the WO₆ octahedra and NdO₈ polyhedra in β-KNdW₂O₈. This crystal structure–photoluminescence study suggests that this class of tungstates can be exploited for visible light emitting devices by tuning the crystal symmetry

Odd–Even Structural Sensitivity on Dynamics in Network-Forming Ionic Liquids

As a compelling case of sensitive structure–property relationship, an odd–even effect refers to the alternating trend of physical or chemical properties on odd/even number of repeating structural units. In crystalline or semicrystalline materials, such odd–even effects emerge as manifestations of differences in the periodic packing patterns of molecules. Therefore, due to the lack of long-range order, such an odd–even phenomenon is not expected for dynamic properties in amorphous state. Herein, we report the discovery of a remarkable odd–even effect of dynamical properties in the liquid phase. In a class of glass-forming diammonium citrate ionic liquids, using incoherent quasi-elastic neutron scattering measurements, we measured the dynamical properties including diffusion coefficient and rotational relaxation time. These directly measured molecular dynamics showed pronounced alternating trends with increased number of methylene (−CH₂−) groups in the backbone. Meanwhile, the structure factor <i>S</i>(<i>Q</i>) showed no long-range periodic packing of molecules, while the pair distribution function <i>G</i>(<i>r</i>) revealed subtle differences in the local molecular morphology. The observed dynamical odd–even phenomenon in liquids showed that profound dynamical changes originate from subtle local structural differences

K₃Fe(CN)₆ under External Pressure: Dimerization of CN^– Coupled with Electron Transfer to Fe(III)

The addition polymerization of charged monomers like CC^2– and CN^– is scarcely seen at ambient conditions but can progress under external pressure with their conductivity significantly enhanced, which expands the research field of polymer science to inorganic salts. The reaction pressures of transition metal cyanides like Prussian blue and K₃Fe­(CN)₆ are much lower than that of alkali cyanides. To figure out the effect of the transition metal on the reaction, the crystal structure and electronic structure of K₃Fe­(CN)₆ under external pressure are investigated by <i>in situ</i> neutron diffraction, <i>in situ</i> X-ray absorption fine structure (XAFS), and neutron pair distribution functions (PDF) up to ∼15 GPa. The cyanide anions react following a sequence of approaching–bonding–stabilizing. The Fe­(III) brings the cyanides closer which makes the bonding progress at a low pressure (2–4 GPa). At ∼8 GPa, an electron transfers from the CN to Fe­(III), reduces the charge density on cyanide ions, and stabilizes the reaction product of cyanide. From this study we can conclude that bringing the monomers closer and reducing their charge density are two effective routes to decrease the reaction pressure, which is important for designing novel pressure induced conductor and excellent electrode materials

Intricate Short-Range Ordering and Strongly Anisotropic Transport Properties of Li_1–<i>x</i>Sn_2+<i>x</i>As₂

A new ternary compound, Li_1–<i>x</i>Sn_2+<i>x</i>As₂, 0.2 < <i>x</i> < 0.4, was synthesized via solid-state reaction of elements. The compound crystallizes in a layered structure in the <i>R</i>3̅<i>m</i> space group (No. 166) with Sn–As layers separated by layers of jointly occupied Li/Sn atoms. The Sn–As layers are comprised of Sn₃As₃ puckered hexagons in a chair conformation that share all edges. Li/Sn atoms in the interlayer space are surrounded by a regular As₆ octahedron. Thorough investigation by synchrotron X-ray and neutron powder diffraction indicate no long-range Li/Sn ordering. In contrast, the local Li/Sn ordering was revealed by synergistic investigations via solid-state ^6,7Li NMR spectroscopy, HRTEM, STEM, and neutron and X-ray pair distribution function analyses. Due to their different chemical natures, Li and Sn atoms tend to segregate into Li-rich and Sn-rich regions, creating substantial inhomogeneity on the nanoscale. The inhomogeneous local structure has a high impact on the physical properties of the synthesized compounds: the local Li/Sn ordering and multiple nanoscale interfaces result in unexpectedly low thermal conductivity and highly anisotropic resistivity in Li_1–<i>x</i>Sn_2+<i>x</i>As₂

Synthesis, Structure, and Pressure-Induced Polymerization of Li₃Fe(CN)₆ Accompanied with Enhanced Conductivity

Pressure-induced polymerization of charged triple-bond monomers like acetylide and cyanide could lead to formation of a conductive metal–carbon network composite, thus providing a new route to synthesize inorganic/organic conductors with tunable composition and properties. The industry application of this promising synthetic method is mainly limited by the reaction pressure needed, which is often too high to be reached for gram amounts of sample. Here we successfully synthesized highly conductive Li₃Fe­(CN)₆ at maximum pressure around 5 GPa and used in situ diagnostic tools to follow the structural and functional transformations of the sample, including in situ X-ray and neutron diffraction and Raman and impedance spectroscopy, along with the neutron pair distribution function measurement on the recovered sample. The cyanide anions start to react around 1 GPa and bond to each other irreversibly at around 5 GPa, which are the lowest reaction pressures in all known metal cyanides and within the technologically achievable pressure range for industrial production. The conductivity of the polymer is above 10^–3 S·cm^–1, which reaches the range of conductive polymers. This investigation suggests that the pressure-induced polymerization route is practicable for synthesizing some types of functional conductive materials for industrial use, and further research like doping and heating can hence be motivated to synthesize novel materials under lower pressure and with better performances

Structure and Stability of SnO₂ Nanocrystals and Surface-Bound Water Species

The structure of SnO₂ nanoparticles (avg. 5 nm) with a few layers of water on the surface has been elucidated by atomic pair distribution function (PDF) methods using in situ neutron total scattering data and molecular dynamics (MD) simulations. Analysis of PDF, neutron prompt gamma, and thermogravimetric data, coupled with MD-generated surface D₂O/OD configurations demonstrates that the minimum concentration of OD groups required to prevent rapid growth of nanoparticles during thermal dehydration corresponds to ∼0.7 monolayer coverage. Surface hydration layers not only stabilize the SnO₂ nanoparticles but also induce particle-size-dependent structural modifications and are likely to promote interfacial reactions through hydrogen bonds between adjacent particles. Upon heating/dehydration under vacuum above 250 °C, nanoparticles start to grow with low activation energies, rapid increase of nanoparticle size, and a reduction in the <i>a</i> lattice dimension. This study underscores the value of neutron diffraction and prompt-gamma analysis, coupled with molecular modeling, in elucidating the influence of surface hydration on the structure and metastable persistence of oxide nanomaterials

Mechanical Properties of Nanoscopic Lipid Domains

The lipid raft hypothesis presents insights into how the cell membrane organizes proteins and lipids to accomplish its many vital functions. Yet basic questions remain about the physical mechanisms that lead to the formation, stability, and size of lipid rafts. As a result, much interest has been generated in the study of systems that contain similar lateral heterogeneities, or domains. In the current work we present an experimental approach that is capable of isolating the bending moduli of lipid domains. This is accomplished using neutron scattering and its unique sensitivity to the isotopes of hydrogen. Combining contrast matching approaches with inelastic neutron scattering, we isolate the bending modulus of ∼13 nm diameter domains residing in 60 nm unilamellar vesicles, whose lipid composition mimics the mammalian plasma membrane outer leaflet. Importantly, the bending modulus of the nanoscopic domains differs from the modulus of the continuous phase surrounding them. From additional structural measurements and all-atom simulations, we also determine that nanoscopic domains are in-register across the bilayer leaflets. Taken together, these results inform a number of theoretical models of domain/raft formation and highlight the fact that mismatches in bending modulus must be accounted for when explaining the emergence of lateral heterogeneities in lipid systems and biological membranes

Ionic Conduction in Cubic Na₃TiP₃O₉N, a Secondary Na-Ion Battery Cathode with Extremely Low Volume Change

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na₃TiP₃O₉N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na₃TiP₃O₉N has an average voltage of 2.7 V vs Na⁺/Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na⁺. Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na₃TiP₃O₉N is substantial (4 × 10^–7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na₃TiP₃O₉N and desodiated Na₂TiP₃O₉N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na₃TiP₃O₉N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature