Topochemical Synthesis of LiCoF3 with a High-Temperature LiNbO3-Type Structure.

A novel perovskite fluoride, LixCoF3, which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO3-type fluoride CoF3 using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO3-type structure (space group: R3̅c) involving Li-Co antisite defects and A-site splitting along the c direction. A-site splitting is consistent with the prediction based on hybrid Hartree-Fock density functional theory calculations. Co-L2,3 edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF3 (x ≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO3-type fluoride family.

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability.

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δ containing Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ (A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δ precursors with CaH2 at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δ are due to the simultaneous shift of the midgap states composed of Sn2+ with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δ precursors were essential to obtain H--doped products that have a low density of defects.

Electronic Origin of Non-Zone-Center Phonon Condensation: Octahedral Rotation as a Case Study.

Unstable zone-boundary phonon modes drive atomic displacements linked to a rich array of properties. Yet, the electronic origin of the instability remains to be clearly explained. In this Letter, we propose that bonding interaction between Bloch states belonging to different wave vectors leads to such instability via the pseudo- or second-order Jahn-Teller effect. Our first-principles calculations and representation theory-based analyses show that rotations of anion coordinated octahedra, an archetypal example of zone-boundary phonon condensations, are induced by this bonding mechanism. The proposed mechanism is universal to any non-zone-center phonon condensations and could offer a general approach to understanding the origin of structural phase transitions in crystals.

Gas sorption porosimetry for the evaluation of hard carbons as anodes for Li- and Na-ion batteries.

Hard carbons are promising candidates for high-capacity anode materials in alkali metal-ion batteries, such as lithium- and sodium-ion batteries. High reversible capacities are often coming along with high irreversible capacity losses during the first cycles, limiting commercial viability. The trade-off to maximize the reversible capacities and simultaneously minimizing irreversible losses can be achieved by tuning the exact architecture of the subnanometric pore system inside the carbon particles. Since the characterization of small pores is nontrivial, we herein employ Kr, N2 and CO2 gas sorption porosimetry, as well as H2O vapor sorption porosimetry, to investigate eight hard carbons. Electrochemical lithium as well as sodium storage tests are compared to the obtained apparent surface areas and pore volumes. H2O, and more importantly CO2, sorption porosimetry turned out to be the preferred methods to evaluate the likelihood for excessive irreversible capacities. The methods are also useful to select the relatively most promising active materials within chemically similar materials. A quantitative relation of porosity descriptors to the obtained capacities remains a scientific challenge.

Aging of starting solutions for nanoparticles synthesis with two different ultrasonication.

Novel concept of aging starting solutions for nanoparticle synthesis and the application of two different sonication to aging is introduced. In sonoprocess of nanoparticles from liquid phase, it is usual to irradiate ultrasound to a reacting solution during synthesis. In this work, ultrasound is applied to a starting solution before synthesis. In case of the SiO2 sphere synthesis, the aging process elongated the induction period of the precipitation, enlarge the sphere size, and enhanced the monodispersity of silica spheres. These effects were promoted by soft sonication at subtle power (~1 mW). In case of the TiO2 sphere case, ordinary (cavitating) ultrasound during synthesis was found to be effective in more spherical shape. Also, the sonication to aging of the starting solution was useful for uniform spheres synthesis. Existence of miscible liquid such as water-ethanol and feasible relationship to the nucleation of nanoparticles are discussed.

Thermogravimetric Evolved Gas Analysis and Microscopic Elemental Mapping of the Solid Electrolyte Interphase on Silicon Incorporated in Free-Standing Porous Carbon Electrodes.

Free-standing electrodes, which are free from additives (binders and conductive agents) and even current collectors, are useful in terms of both application research and fundamental study. Here, we demonstrate the preparation of binder-free monolithic carbon electrodes embracing Si nanoparticles in their well-defined porous scaffolds via the one-pot sol-gel reaction followed by carbonization. The free-standing electrodes with a thickness of 150 µm work out as a high-areal-density anode for Li-ion batteries, delivering up to ca. 7 mA h cm-2. As the Si content increases, the capacity decay on cycling becomes pronounced, which is likely to associate with the fracturing and pulverization of Si nanoparticles even with the size smaller than 100 nm after long-term cycles. The thermogravimetry-mass spectrometry profile of the cycled electrode corroborates the successive electrolyte decomposition to grow solid electrolyte interphase (SEI) mainly composed of lithium alkylcarbonates, polymeric species, and LiF, rendering the electrode mass nearly double of its original state after 200 cycles. The elemental mapping analysis reveals that LiF is generated inhomogeneously in the monolithic electrodes unlike the other SEI components, resulting in the concentration gradient depending on the distance from a Li counter electrode.

Sodium titanium oxide bronze nanoparticles synthesized via concurrent reduction and Na+-doping into TiO2(B).

A mixed valence compound, sodium titanium oxide bronze (NaxTiO2-B), combines intriguing properties of high electric conductivity and good chemical stability together with a unique one-dimensional tunnel crystal structure available for cation storage. However, this compound has not been studied for a long period because of the strongly reductive condition at high temperature required for its preparation, which limits the morphological control such as the preparation of nanocrystals. For the first time in this paper, the topotactic synthesis of nano-sized NaxTiO2-B with high specific surface area (>130 m2 g-1) from TiO2(B) nanoparticles has been demonstrated. The reaction of metastable TiO2(B) with NaBH4 allows carrier electrons to be doped simultaneously with incorporation of Na+ ions into the interstitial sites of the host Ti-O lattice at relatively low temperature. An electrochemical investigation of Li+- and Na+-ion storage behaviors suggests that the incorporated Na+ ions are mainly placed in the 6-fold coordination sites of bronze. In addition, optical measurements including time-resolved transient spectroscopy revealed that the doped electrons in the NaxTiO2-B nanoparticles are predominantly in the Ti3+ state and behave as a small polaron. The pelletized NaxTiO2-B nanoparticles shows a good electronic conductivity of 1.4 × 10-2 S cm-1 at 30 °C with an activation energy of 0.17 eV, which is attributable to the thermal barrier for the polaron hopping.

Preparation of Nickel Nanoparticles by Direct Current Arc Discharge Method and Their Catalytic Application in Hybrid Na-Air Battery.

Nickel nanoparticles were prepared by the arc discharge method. Argon and argon/hydrogen mixtures were used as plasma gas; the evaporation of anode material chiefly resulted in the formation of different arc-anode attachments at different hydrogen concentrations. The concentration of hydrogen was fixed at 0, 30, and 50 vol% in argon arc, corresponding to diffuse, multiple, and constricted arc-anode attachments, respectively, which were observed by using a high-speed camera. The images of the cathode and anode jets were observed with a suitable band-pass filter. The relationship between the area change of the cathode/anode jet and the synchronous voltage/current waveform was studied. By investigating diverse arc-anode attachments, the effect of hydrogen concentration on the features of nickel nanoparticles were investigated, finding that 50 vol% H2 concentration has high productivity, fine crystallinity, and appropriate size distribution. The synthesized nickel nanoparticles were then used as catalysts in a hybrid sodium⁻air battery. Compared with commercial a silver nanoparticle catalyst and carbon black, nickel nanoparticles have better electrocatalytic performance. The promising electrocatalytic activity of nickel nanoparticles can be ascribed to their good crystallinity, effective activation sites, and Ni/NiO composite structures. Nickel nanoparticles prepared by the direct current (DC) arc discharge method have the potential to be applied as catalysts on a large scale.

Novel High-Energy-Density Rechargeable Hybrid Sodium-Air Cell with Acidic Electrolyte.

Low-cost, high-energy-density, and highly efficient devices for energy storage have long been desired in our society. Herein, a novel high-energy-density hybrid sodium-air cell was fabricated successfully on the basis of acidic catholytes. Such a hybrid sodium-air cell possess a high theoretical voltage of 3.94 V, capacity of 1121 mAh g-1, and energy density of 4418 Wh kg-1. First, the buffering effect of an acidic solution was demonstrated, which provides relatively long and stable cell discharge behaviors. Second, the catholytes of hybrid sodium-air cells were optimized systematically from the solutions of 0.1 M H3PO4 + 0.1 M Na2SO4 to 0.1 M HAc + 0.1 M NaAc and it was found that the cells with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed a maximum power density of 34.9 mW cm-2. The cell with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed higher discharge capacity of 896 mAh g-1. Moreover, the fabricated acidic hybrid sodium-air cells exhibited stable cycling performance in ambient air and they delivered a low voltage gap around 0.3 V when the current density is 0.13 mA cm-2, leading to a high energy efficiency up to 90%. Therefore, the present study provides new opportunities to develop highly cost-effective energy storage technologies.

Stabilization Mechanism of the Tetragonal Structure in a Hydrothermally Synthesized BaTiO3 Nanocrystal.

Higher OH concentration is identified in tetragonal barium titanate (BaTiO3) nanorods synthesized by a hydrothermal method with a 10 vol % ethylene glycol solvent (Inada, M.; et al. Ceram. Int. 2015, 41, 5581-5587). This is apparently inconsistent with the known fact that higher OH concentration in the conventional hydrothermal synthesis makes pseudocubic BaTiO3 nanocrystals more stable than the tetragonal one. To understand where and how the introduced OH anions are located and behave in the nanocrystals, we applied ab initio analysis to several possible microscopic geometries of OH locations, confirming the relative stability of the tetragonal distortion over the pseudocubic one because of the preference of trans-type configurations of OH anions. We also performed Fourier transform infrared and X-ray diffraction analysis, all being consistent with the microscopic picture established by the ab initio geometrical optimizations.

Expanding frontiers in materials chemistry and physics with multiple anions.

During the last century, inorganic oxide compounds laid foundations for materials synthesis, characterization, and technology translation by adding new functions into devices previously dominated by main-group element semiconductor compounds. Today, compounds with multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offer a new materials platform from which superior functionality may arise. Here we review the recent progress, status, and future prospects and challenges facing the development and deployment of mixed-anion compounds, focusing mainly on oxide-derived materials. We devote attention to the crucial roles that multiple anions play during synthesis, characterization, and in the physical properties of these materials. We discuss the opportunities enabled by recent advances in synthetic approaches for design of both local and overall structure, state-of-the-art characterization techniques to distinguish unique structural and chemical states, and chemical/physical properties emerging from the synergy of multiple anions for catalysis, energy conversion, and electronic materials.

Crystallization behavior of iron-based amorphous nanoparticles prepared sonochemically.

In general, a rapid quenching is required to obtain an amorphous metal. It is known that an intensive ultrasonication generates a very high temperature within cavitation bubbles in a very short moment, which enables a rapid quenching process in a liquid phase synthesis. In this study, the sonochemically-derived "amorphous iron" from Fe(CO)5 was carefully examined by XRD, TEM, TG-DTA. The product was found to be an amorphous containing a certain amount (∼15%) of volatile component that can be removed by heating in a nitrogen flow. After annealed in the inert atmosphere at 600°C, cooled down to room temperature, and then exposed in air (oxygen), the sample showed a strong exotherm accompanied by a weight gain. This is due to oxidation of fine metallic iron. Experimental operations of such a reactive material were examined.

Green apatites: hydride ions, electrons and their interconversion in the crystallographic channel.

Hydride (H(-)) ions and electrons in channel sites of the lattice of calcium phosphate apatites are characterized. Solid-state chemical reduction using TiH2 is effective for doping of H(-) ions into apatites. Irradiation of the H(-) ion-doped apatite with ultraviolet (UV) light induces green coloration. Electron paramagnetic resonance (EPR) reveals that this colour centre is attributed to electrons captured at a vacant anion site in the crystallographic channel, forming F(+) centres. Transient H(0) atoms are detected at low temperatures by EPR. The concentration of UV-induced electrons in the apatite at room temperature decays according to second-order kinetics because of the chemical reactions involving two electrons; overall, electron generation and thermal decay can be described as: H(-) + O(2-) ↔ 2e(-) + OH(-). (1)H magic angle spinning nuclear magnetic resonance spectroscopy is used to identify H(-) ions in the apatite, which are characterized by a chemical shift of +3.4 ppm. Various types of O-H groups including OH(-) ions in the channel and protons bound to phosphate groups are concurrently formed, and are identified by considering the relationship between the O-H stretching frequency and the (1)H chemical shift. The complementary results obtained by EPR and NMR reveal that the H(-) ions and transient H(0) atoms are located at the centre of Ca3 triangles in the apatite, while the electrons are located in the centre of Ca6 octahedra. These findings provide an effective approach for identifying new classes of mixed-oxide-hydride or -electride crystals.

Hydride ions in oxide hosts hidden by hydroxide ions.

The true oxidation state of formally 'H(-)' ions incorporated in an oxide host is frequently discussed in connection with chemical shifts of (1)H nuclear magnetic resonance spectroscopy, as they can exhibit values typically attributed to H(+). Here we systematically investigate the link between geometrical structure and chemical shift of H(-) ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches, in an attempt to resolve this issue. We demonstrate that the electron density near the hydrogen nucleus in an OH(-) ion (formally H(+) state) exceeds that in an H(-) ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H(-) and the distance from the H(-) ion to the coordinating electropositive cation. This relationship is pivotal for resolving H(-) species that are masked by various states of H(+) ions.

An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity.

In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

Solid-state source of atomic oxygen for low-temperature oxidation processes: application to pulsed laser deposition of TiO2:N films.

An atomic oxygen (AO) source has been redesigned to coordinate with a pulsed laser deposition system and used to grow nitrogen-doped TiO(2) films by deposition of TiN and simultaneous irradiation of the substrate with AO. The AO source uses an incandescently heated thin tube of zirconia as an oxygen permeation media to generate pure AO of low kinetic energy. The emission flux is calibrated using a silver-coated quartz crystal microbalance. The thin shape of the probe and transverse emission geometry of this emission device allow the emission area to be positioned close to the substrate surface, enhancing the irradiation flux at the substrate. AO irradiation is crucial for formation of TiO(2) phases via oxidation of the deposited TiN laser plume, and is effective for decrease of the substrate temperature for crystallization of anatase phase to as low as around 200 °C.

New functionalities in abundant element oxides: ubiquitous element strategy.

While most ceramics are composed of ubiquitous elements (the ten most abundant elements within the Earth's crust), many advanced materials are based on rare elements. A 'rare-element crisis' is approaching owing to the imbalance between the limited supply of rare elements and the increasing demand. Therefore, we propose a 'ubiquitous element strategy' for materials research, which aims to apply abundant elements in a variety of innovative applications. Creation of innovative oxide materials and devices based on conventional ceramics is one specific challenge. This review describes the concept of ubiquitous element strategy and gives some highlights of our recent research on the synthesis of electronic, thermionic and structural materials using ubiquitous elements.

Metallic state in a lime-alumina compound with nanoporous structure.

We report a metallic state in a nanostructured porous crystal 12CaO x 7Al2O3 by incorporating electrons in the inherent subnanometer-sized cages, in which a three-dimensionally closely packed cage structure acts as an electronic conduction path. High-density electron doping ( approximately 2 x 10(21) cm(-3)), which was achieved by a thermal treatment in Ti metal vapor at approximately 1100 degrees C, induces homogenization of the cage geometry to a symmetric state, resulting in an insulator-metal transition with a sharp enhancement of the electron drift mobility from approximately 0.1 to 4 cm(2) V(-1) s(-1). The results provide an approach for the realization of electroactive functions in materials composed only of environmentally benign elements by utilizing the appropriate nanostructures.

Nanoporous crystal 12CaO.7Al2O3: a playground for studies of ultraviolet optical absorption of negative ions.

A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.

Hydride ion as a two-electron donor in a nanoporous crystalline semiconductor 12CaO.7Al2O3.

The 12CaO.7Al2O3 (C12A7) crystal with a nanoporous lattice framework exhibits high electrical conductivity with an activation energy of approximately 1.5 eV when equilibrated in a hydrogen atmosphere above approximately 800 degrees C. The high conductivity is preserved in a quenched state below approximately 600 degrees C with a reduced activation energy of approximately 0.8 eV. Such complex behavior in electrical conductivity is associated with incorporation of hydride ions (H-) in cages of the lattice framework. Electromotive force measurements reveal that the major carrier for the conductivity is electron with a small contribution by proton (H+), ruling out the possibility of direct intercage migration of the H- ion. A combination of these observations with the ab initio calculations leads to the conclusion that the electrons are thermally generated from the H- ion by the dissociation into two electrons and an proton, which is further converted to an OH- ion via reaction with an extraframework oxide ion (O2-). The energy difference between the initial (H- + O2-) and the final (2e- + OH-) states as evaluated by the theoretical calculation is as small as approximately 1 eV, which agrees well with an experimentally obtained enthalpy change, approximately 1.4 eV. Thus, internal equilibration between the extraframework hydrogen and the oxygen species is responsible for the thermal generation of the carrier electron. It is also suggested that the same conductive (2e- + OH-) state is reached by the photoirradiation of H- -containing C12A7. In this case the photoionization of H- forms an electron and an Ho atom, which then forms an OH- ion and another electron with thermal assistance. The persistence of photoinduced conductivity is explained by the slow kinetics of the reverse process at room temperature.