Journal of the American Chemical Society

Published by American Chemical Society
Online ISSN: 1520-5126
Print ISSN: 0002-7863
Bright green and red luminescence has been generated with a 980 nm diode laser from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles through a newly described hetero-looping-enhanced energy-transfer (hetero-LEET) up-conversion process, which exhibits a power dependence similar to that of a photon avalanche (PA). The hetero-LEET mechanism is potentially more efficient than PA, ground-state absorption/excited-state absorption (GSA/ESA), and energy-transfer (ETU) mechanisms because it combines resonant ground-state absorption with a looping or feedback process.
We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.
The ability of I...I van der Waals interactions to direct the self-assembly of slabs of the radical cation of ethylenedithio-1,2-diiodo-tetrathiafulvalene, EDT-TTF-I(2), and polymeric lead iodide covalent anionic layers is demonstrated by the synthesis of single crystals of beta-(EDT-TTF-I(2))(2)(.+)[(Pb(5/6) square (1/6)I(2))(1/3-)](3), triclinic, P-1, a = 7.7818(8), b = 7.9760(8), c = 19.9668(2) A, alpha = 82.409(12), beta = 85.964(12), gamma = 61.621(11) degrees, V = 1080.76(19) A(3), R1, wR2 = 0.0459, 0.0947; and beta-(EDT-TTF-I(2))(2)(.+)[(Pb(2/3+x)Ag(1/3-2x)square(x)I(2))(1/3-)](3), x approximately 0.05, triclinic, P-1, a = 7.7744(8), b = 7.9193(8), c = 19.834(2) A, alpha = 87.189(12), beta = 83.534(12), gamma = 61.602(11) degrees, V = 1067.4(2) A(3), R1, wR2 = 0.0508, 0.0997. The state-of-the-art, combined microprobe and structural analysis of the metal site vacancies and occupancies patterns reveal a commensurate organic-inorganic interface and point out the importance of halogen.halogen van der Waals interactions to future studies aiming at directing interface topologies. The electronic structure, room-temperature metallic character and metal-insulator transition at ca. 70 K of the two-dimensional organic slabs are retained upon alloying of the inorganic sublattice with monocations. The room-temperature conductivity of the metallic lead-silver alloy is 2 orders of magnitude larger than that of beta-(EDT-TTF-I(2))(2)(.+)[(Pb(5/6) square (1/6)I(2))(1/3-)](3). This calls for the study of materials with diverse alloying patterns with metal cations of different nature and charge.
We have investigated in situ the crystal structure, oxygen diffusion path, oxygen permeation rate, and electrical conductivity of a doped praseodymium nickel oxide, Pr(2)NiO(4)-based mixed conductor, (Pr(0.9)La(0.1))(2)(Ni(0.74)Cu(0.21)Ga(0.05))O(4+delta) (PLNCG) in air between 27 degrees C and 1015.6 degrees C. The PLNCG has a tetragonal I4/mmm K(2)NiF(4)-type structure which consists of a (Pr(0.9)La(0.1))(Ni(0.74)Cu(0.21)Ga(0.05))O(3) perovskite unit and a (Pr(0.9)La(0.1))O rock salt unit in the whole temperature range. Both experimental and theoretical electron density maps indicated two-dimensional networks of (Ni(0.74)Cu(0.21)Ga(0.05))-O covalent bonds in PLNCG. Highest occupied molecular orbitals (HOMO) in PLNCG demonstrate that the electron-hole conduction occurs via Ni and Cu atoms in the (Ni(0.74)Cu(0.21)Ga(0.05))-O layer. The bulk oxygen permeation rate was high (137 mumol cm(-2) min(-1) at 1000 degrees C), and its activation energy was low (51 kJ mol(-1) at 950 degrees C). The Rietveld method, maximum-entropy method (MEM), and MEM-based pattern fitting analyses of neutron and synchrotron diffraction data indicate a large anisotropic thermal motion of the apical O2 oxygen at the 4e site (0, 0, z; z approximately 0.2) in the (Pr(0.9)La(0.1))(Ni(0.74)Cu(0.21)Ga(0.05))O(3) perovskite unit. Neutron and synchrotron diffraction data and theoretical structural optimization show the interstitial oxygen (O3) atom at (x, 0, z) (x approximately 0.6 and z approximately 0.2). The nuclear density analysis indicates that the bulk oxide-ion diffusion, which is responsible for the high oxygen permeation rate, occurs through the interstitial O3 and anisotropic apical O2 sites. The nuclear density at the bottleneck on the oxygen diffusion path increases with temperature and with the oxygen permeation rate. The activation energy from the nuclear density at the bottleneck decreases with temperature, which is consistent with the decrease of the activation energy from oxygen permeation rate. The extremely low activation energy (12 kJ mol(-1) at 900 degrees C) from the nuclear density at the bottleneck indicates possible higher bulk oxygen permeation rates in quality single crystals and epitaxial thin films.
We employ, for the first time, a unique combinatorial chemical vapor deposition (CVD) technique to isolate a previously unreported transition-metal mixed-anion phase. The new oxynitride phase, Ti(3-delta)O4N (where 0.06 < delta < 0.25), is the first example of a complex titanium oxynitride and was synthesized within composition graduated films formed from atmospheric pressure CVD of TiCl4, NH3, and ethyl acetate. Characterization was performed by X-ray diffraction, X-ray photoelectron spectroscopy, UV-visible spectra, and SQUID magnetometry. The material crystallizes in the Cmcm space group, with the ordered nitrogen ions stabilizing the orthorhombic analogue of the monoclinic anosovite structure, beta-Ti3O5. The lattice parameters are sensitive to composition, but were determined to be a = 3.8040(1) A, b = 9.6486(6) A, and c = 9.8688(5) A for Ti(2.85(2))O4N. Powder samples were prepared through delamination of the thin films for synchrotron X-ray diffraction and magnetic measurements. It is the first example of a new phase to be synthesized using such a combinatorial CVD approach and clearly demonstrates how such techniques can provide access to new materials. This metastable phase with unusual nitrogen geometry has proved to be elusive to conventional solid-state chemistry techniques and highlights the value of the surface growth mechanism present in CVD. Furthermore, the ease and speed of the synthesis technique, combined with rapid routes to characterization, allow for large areas of phase space to be probed effectively. These results may have major implications in the search for new complex mixed-anion phases in the future.
Li(x)Mg(0.1)Ni(0.4)Mn(1.5)O(4) spinel (P4(3)32) was chemically and electrochemically lithiated in the range 1 < x <or= 2.25 and subjected to detailed X-ray and neutron diffraction analysis to understand the electrochemical behavior in the 3 V region. Extensive migration of Ni and Mn during lithium insertion was found, resulting in the disappearance of the initial Ni-Mn ordering and the formation of Ni-rich and Ni-poor domains, leading to two Jahn-Teller-distorted tetragonal phases with different Ni:Mn ratios. Such extensive Ni and Mn migration was not known for these spinels, and strongly influences the initial cycling behavior. The newly formed tetragonal phase with a Ni:Mn ratio of approximately 0.07 has a higher cyclability and a higher capacity, and is therefore suggested to have a favorable composition for this intercalation range, as is confirmed in the literature. In addition, lithium is found to occupy multiple positions inside the distorted oxygen octahedron of this phase, a finding previously known only for lithium in anatase TiO(2).
Hybrid nanostructure Ag−Zn0.9Co0.1O was prepared via a solution chemistry method. Part of the electrons from the Ag tip was introduced and accumulated in Zn0.9Co0.1O nanorods, shifting the Fermi level to induce a robust room-temperature ferromagnetism.
The high capacity of Ni-rich Li[Ni(1-x)M(x)]O(2) (M = Co, Mn) is very attractive, if the structural instability and thermal properties are improved. Li[Ni(0.5)Mn(0.5)]O(2) has good thermal and structural stabilities, but it has a low capacity and rate capability relative to the Ni-rich Li[Ni(1-x)M(x)]O(2). We synthesized a spherical core-shell structure with a high capacity (from the Li[Ni(0.8)Co(0.1)Mn(0.1)]O(2) core) and a good thermal stability (from the Li[Ni(0.5)Mn(0.5)]O(2) shell). This report is about the microscale spherical core-shell structure, that is, Li[Ni(0.8)Co(0.1)Mn(0.1)]O(2) as the core and a Li[Ni(0.5)Mn(0.5)]O(2) as the shell. A high capacity was delivered from the Li[Ni(0.8)Co(0.1)Mn(0.1)]O(2) core, and a high thermal stability was achieved by the Li[Ni(0.5)Mn(0.5)]O(2) shell. The core-shell structured Li[(Ni(0.8)Co(0.1)Mn(0.1))(0.8)(Ni(0.5)Mn(0.5))(0.2)]O(2)/carbon cell had a superior cyclability and thermal stability relative to the Li[Ni(0.8)Co(0.1)Mn(0.1)]O(2) at the 1 C rate for 500 cycles. The core-shell structured Li[(Ni(0.8)Co(0.1)Mn(0.1))(0.8)(Ni(0.5)Mn(0.5))(0.2)]O(2) as a new positive electrode material is a significant breakthrough in the development of high-capacity lithium batteries.
The thermal, mechanical and electric properties of hybrid membranes based on Nafion that contain a [(ZrO2)∙(Ta2O5)0.119] "core-shell" nanofiller are elucidated. DSC investigations reveal the presence of four endothermic transitions between 50 and 300°C. The DMA results indicate improved mechanical stability of the hybrid materials. The DSC and DMA results are consistent with our previous suggestion of dynamic R-SO3H•••[ZrTa] cross-links in the material. These increase the thermal stability of the -SO3H groups and the temperature of thermal relaxation events occurring in hydrophobic domains of Nafion. The broadband electric spectroscopic analysis reveals two electric relaxations associated with the materials' interfacial (σIP) and bulk proton conductivities (σDC). The wet [Nafion/(ZrTa)1.042] membrane has a conductivity of 7.0×10-2 Scm-1 at 115°C, while Nafion has a conductivity of 3.3×10-2 Scm-1 at the same temperature and humidification conditions. σDC shows VTF behaviour, suggesting that the long-range conductivity is closely related to the segmental motion of the Nafion host matrix. Long range conduction (σDC) occurs when the dynamics of the fluorocarbon matrix induces contact between different delocalization bodies (DB), which results in proton exchange processes between these DBs.
Magnesium hydride (MgH(2)) is an attractive candidate for solid-state hydrogen storage applications. To improve the kinetics and thermodynamic properties of MgH(2) during dehydrogenation-rehydrogenation cycles, a nanostructured MgH(2)-0.1TiH(2) material system prepared by ultrahigh-energy-high-pressure mechanical milling was investigated. High-resolution transmission electron microscope (TEM) and scanning TEM analysis showed that the grain size of the milled MgH(2)-0.1TiH(2) powder is approximately 5-10 nm with uniform distributions of TiH(2) among MgH(2) particles. Pressure-composition-temperature (PCT) analysis demonstrated that both the nanosize and the addition of TiH(2) contributed to the significant improvement of the kinetics of dehydrogenation and hydrogenation compared to commercial MgH(2). More importantly, PCT cycle analysis demonstrated that the MgH(2)-0.1TiH(2) material system showed excellent cycle stability. The results also showed that the DeltaH value for the dehydrogenation of nanostructured MgH(2)-0.1TiH(2) is significantly lower than that of commercial MgH(2). However, the DeltaS value of the reaction was also lower, which results in minimum net effects of the nanosize and the addition of TiH(2) on the equilibrium pressure of dehydrogenation reaction of MgH(2).
The open-framework uranium fluorosilicate [(CH3)4N][(C5H5NH)0.8((CH3)3NH)0.2]U2Si9O23F4 (USH-8) has been synthesized hydrothermally by using tetramethylammonium hydroxide and pyridine-HF. The compound has a framework composition U2Si9O23F4 based on silicate double layers that are linked by chains of UO3F4 pentagonal bipyramids. The framework has 12-ring channels along [010] and 7-ring channels along [100]. The [010] 12-ring channels have a calabash-shape with the middle part partially blocked by the uranyl oxygen atoms. The narrow side of the 12-ring channels is occupied by well-ordered TMA cations while the wide side is occupied by disordered pyridinium and trimethylammonium cations.
The cathode in rechargeable lithium-ion batteries operates by conventional intercalation; Li+ is extracted from LiCoO2 on charging accompanied by oxidation of Co3+ to Co4+; the process is reversed on discharge. In contrast, Li+ may be extracted from Mn4+-based solids, e.g., Li2MnO3, without oxidation of Mn4+. A mechanism involving simultaneous Li and O removal is often proposed. Here, we demonstrate directly, by in situ differential electrochemical mass spectrometry (DEMS), that O2 is evolved from such Mn4+ -containing compounds, Li[Ni(0.2)Li(0.2)Mn(0.6)]O2, on charging and using powder neutron diffraction show that O loss from the surface is accompanied by diffusion of transition metal ions from surface to bulk where they occupy vacancies created by Li removal. The composition of the compound moves toward MO(2). Understanding such unconventional Li extraction is important because Li-Mn-Ni-O compounds, irrespective of whether they contain Co, can, after O loss, store 200 mAhg(-1) of charge compared with 140 mAhg(-1) for LiCoO(2).
The reaction of methylviologen iodide with crystalline V2O5 in the molar ratio of 1 to 3.8 at 100 degrees C in water led to the formation of (MV)0.25V2O5 in quantitative yield. The structure of this organic-inorganic multilayered hybrid compound was determined by single-crystal X-ray crystallography. Strong van der Waals interactions were found between the electron-deficient aromatic organic molecules and the inorganic layers. In the solid state, the compound is a semiconductor due to small polaron hopping and shows novel reversible alkali-ion intercalation/deintercalation via electrochemistry.
A green chemistry process for environmental purification is realized on a novel visible-light-active photocatalyst (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3. This mixed valent solid-solution perovskite with a modulated electronic structure possesses a strong oxidative potential for efficient photocatalytic decomposition of acetaldehyde into CO2 at ambient temperature.
The widespread adoption and deployment of fuel cells as an alternative energy technology have been hampered by a number of formidable technical challenges, including the cost and long-term stability of electrocatalyst and membrane materials. We present a microfluidic fuel cell that overcomes many of these obstacles while achieving power densities in excess of 250 mW/cm(2). The poisoning and sluggish reaction rate associated with CO-contaminated H(2) and methanol, respectively, are averted by employing the promising, high-energy density fuel borohydride. The high-overpotential reaction of oxygen gas at the cathode is supplanted by the high-voltage reduction of cerium ammonium nitrate. Expensive, ineffective membrane materials are replaced with laminar flow and a nonselective, porous convection barrier to separate the fuel and oxidant streams. The result is a Nafion-free, room-temperature fuel cell that has the highest power density per unit mass of Pt catalyst employed for a non-H(2) fuel cell, and exceeds the power density of a typical H(2) fuel cell by 50%.
Dynamic nuclear polarization (DNP) permits increasing the NMR signal of nuclei by pumping the electronic spin transitions of paramagnetic centers nearby. This method is emerging as a powerful tool to increase the inherent sensitivity of NMR in structural biology aiming at detection of macromolecules. In aqueous solution, additional technical issues associated with the penetration of microwaves in water and heating effects aggravate the performance of the experiment. To examine the feasibility of low-field (9.7 GHz/0.35 T) DNP in high resolution NMR, we have constructed the prototype of a two-field shuttle DNP spectrometer that polarizes nuclei at 9.7 GHz/0.35 T and detects the NMR spectrum at 14 T. We report our first (1)H and (13)C DNP enhancements with this spectrometer. Effective enhancements up to 15 were observed for small molecules at (1)H 600 MHz/14 T as compared to the Boltzmann signal. The results provide a proof of principle for the feasibility of a shuttle DNP experiment and open up perspectives for the application potential of this method in solution NMR.
The surface stress response during the electrooxidation of CO at Pt{111}, Ru{0001}, and Ru(theta=0.37)/Pt{111} textured electrodes was studied in 0.1 M HClO(4) electrolytes. The surface stress signal resolves for the first time the adsorption of OH(-) at the CO-covered Ru{0001} surface prior to significant CO oxidation, a phenomenon that is not discernible in the voltammetry. The surface stress signal shows that significant tensile surface stress occurs upon oxidation of the adsorbed CO and occurs at nearly the same potential on Ru{0001} and Ru/Pt{111} surfaces. These observations demonstrate that the mechanism of bifunctionality is the OH(ads) provided to the Pt surface sites via Ru sites.
Topochemical reduction of (layered) perovskite iron oxides with metal hydrides has so far yielded stoichiometric compositions with ordered oxygen defects with iron solely in FeO(4) square planar coordination. Using this method, we have successfully obtained a new oxygen-deficient perovskite, (Sr(1-x)Ba(x))FeO(2) (0.4 ≤ x ≤ 1.0), revealing that square planar coordination can coexist with other 3-6-fold coordination geometries. This BaFeO(2) structure is analogous to the LaNiO(2.5) structure in that one-dimensional octahedral chains are linked by planar units, but differs in that one of the octahedral chains contains a significant amount of oxygen vacancies and that all the iron ions are exclusively divalent in the high-spin state. Mössbauer spectroscopy demonstrates, despite the presence of partial oxygen occupations and structural disorders, that the planar-coordinate Fe(2+) ions are bonded highly covalently, which accounts for the formation of the unique structure. At the same time, a rigid 3D Fe-O-Fe framework contributes to structural stabilization. Powder neutron diffraction measurements revealed a G-type magnetic order with a drastic decrease of the Néel temperature compared to that of SrFeO(2), presumably due to the effect of oxygen disorder/defects. We also performed La substitution at the Ba site and found that the oxygen vacancies act as a flexible sink to accommodate heterovalent doping without changing the Fe oxidation and spin state, demonstrating the robustness of this new structure against cation substitution.
Na(0.40(2))MnO(2) belongs to a family of mixed Mn(3+) and Mn(4+) porous oxides that contains both octahedral and square pyramidal Mn-O units. Neutron and synchrotron radiation studies identify the presence of both sodium ordering (T(Na) ≈ 310 K) and Mn charge and orbital ordering. Below T(Na), the centrosymmetric Pbam structure adopts an (ab 4c) supercell of Pnnm symmetry that accommodates a coupled commensurate modulation down the c-axis channels of both Na position and occupancy with Mn valence.
Electroosmotic pumps are arguably the simplest of all pumps, consisting merely of two flow-through electrodes separated by a porous membrane. Most use platinum electrodes and operate at high voltages, electrolyzing water. Because evolved gas bubbles adhere and block parts of the electrodes and the membrane, steady pumping rates are difficult to sustain. Here we show that when the platinum electrodes are replaced by consumed Ag/Ag(2)O electrodes, the pumps operate well below 1.23 V, the thermodynamic threshold for electrolysis of water at 25 °C, where neither H(2) nor O(2) is produced. The pumping of water is efficient: 13 000 water molecules are pumped per reacted electron and 4.8 mL of water are pumped per joule at a flow rate of 0.13 mL min(-1) V(-1) cm(-2), and a flow rate per unit of power is 290 mL min(-1) W(-1). The water is driven by protons produced in the anode reaction 2Ag(s) + H(2)O → Ag(2)O(s) + 2H(+) + 2e(-), traveling through the porous membrane, consumed by hydroxide ions generated in the cathode reaction Ag(2)O(s) + 2 H(2)O + 2e(-) → 2Ag(s) + 2 OH(-). A pump of 2 mm thickness and 0.3 cm(2) cross-sectional area produces flow of 5-30 μL min(-1) when operating at 0.2-0.8 V and 0.04-0.2 mA. Its flow rate can be either voltage or current controlled. The flow rate suffices for the delivery of drugs, such as a meal-associated boli of insulin.
The complex metal oxide SrCo0.5Ru0.5O3-δ possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co(3+) ion is purely high spin at room-temperature, which is unique for a Co(3+) in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund's-rule exchange due to a relatively large mean Co-O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co(3+) absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high, intermediate, and low spin states as functions of the metal-oxygen bond length for a Co(3+) ion in an octahedral coordination.
The unusual luminescence behavior of the two-coordinate gold(I) carbene complex, [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone), is reported. Upon freezing in a liquid N(2) bath, the colorless, nonluminescent solutions of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) become intensely luminescent. Strikingly, the colors of the emission differ in different solvents and appear only after the solvent has frozen. Solid [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) is also luminescent, and the luminescence is attributed to the formation of extended chains of gold(I) centers that are connected through aurophilic attractions. Crystallographic studies of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) and [Au[C(NHMe)(2)](2)](BF(4)), which is also luminescent, reveal that both involve extended chains of cations and that the anions are hydrogen bonded to the cations through cation N-H groups. However, these chains differ in the Au...Au separations in each and in the carbene ligand orientations. In contrast, [Au[C(NMe(2))(NHMe)](2)](PF(6)) forms a colorless, nonluminescent solid, and in that solid there are no Au...Au interactions, a factor which supports the contention that aggregated species are responsible for the luminescence of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) in the solid state and in frozen solutions.
The three-dimensional structures of emeraldine base polyaniline (PANI) and (polyaniline)(0.5)V(2)O(5) x 1.0 H(2)O have been determined by total X-ray scattering experiments. Atomic pair distribution functions (PDF) were measured to obtain experimental observables against which structural models were tested and refined. The PDF approach is necessary because of the limited structural coherence in these nanostructured materials. Polyaniline possesses a well-defined local atomic arrangement that can be described in terms of an 84-atom orthorhombic unit cell. The nanocomposite (PANI)(0.5)V(2)O(5) x 1.0 H(2)O too is locally well ordered and may be described in terms of a small number of structure-sensible parameters. The PDF approach allows the construction of structure models of PANI and (PANI)(0.5)V(2)O(5) x 1.0 H(2)O on the basis of which important materials' properties can be explained predicted and possibly improved.
Electrochemical oxidation of carbonate esters at the LixNi0.5Mn1.5O4-? / electrolyte interface results in Ni/Mn dissolution and surface film formation, which negatively affect the electrochemical performance of Li-ion batteries. Ex situ X-ray absorption (XRF/XANES), Raman and fluorescence spectroscopy, along with imaging of LixNi0.5Mn1.5O4-? positive and graphite negative electrodes from tested Li-ion batteries, reveal the formation of a variety of MnII/III/NiII complexes with ?-diketone ligands. These metal complexes, which are generated upon anodic oxidation of ethyl and diethyl carbonates at LixNi0.5Mn1.5O4-?, form a surface film that partially dissolves in the electrolyte. The dissolved MnIII complexes are reduced to their MnII analogs, which are incorporated in the solid electrolyte interphase (SEI) surface layer at the graphite negative electrode. This work elucidates possible reaction pathways and evaluates their implications for Li+ transport kinetics in Li-ion batteries.
(Left) Sr 8 Fe 6 Mo 2 O 24 cubic supercell of SFMO. Atom (color): Fe/Mo (gray), Sr (turquoise), O (red). (Middle) Isotropic distribution of Fe/Mo as studied in this work in the cubic SFMO supercell. Atom (color): Fe (purple), Mo (green). Sr and O atoms not shown for ease of viewing. (Right) Schematic of the pseudocubic cell (gray) arising from the Pnma cell (black) as studied in this work. The isotropic Fe/Mo distribution is used also in the calculations involving the pseudocubic cell. 
A log − log plot of the Rietveld re fi nement of powder neutron di ff raction data for Sr 2 Fe 1.5 Mo 0.5 O 6 − δ (SFMO) at 298 K collected with a center wavelength of 1.066 Å. Open circles are observed data, the solid red line is the calculated pattern, and the blue line is the di ff erence. Tick marks indicate the allowed Bragg re fl ections for SFMO. The small “ v ” marks two visible re fl ections from the vanadium sample canister ( fi tted as a second phase). R p = 2.46%, R wp = 3.76%, χ 2 = 12.48. 
Atomic Parameters for SFMO from the Rietveld Refinement of Powder Neutron Diffraction Data
Calculated (DFT+U) formation energies of an oxygen vacancy ( E form O * ) along Mo − O − Fe and Fe − O − Fe bonds from the 
(Top) Projected densities of states (PDOS) of Sr 2 Fe 1.5 Mo 0.5 O 6−δ (δ = 0.0 and δ = 0.125, with an Fe−O*−Fe vacancy) at the DFT+U level of theory with (U-J) Fe = 4 eV. Sums of α-and β-spin PDOS on Fe(d), Mo(d) and O(p) orbitals are shown. (Bottom) Zoom-in near the Fermi level. The band energies are shifted such that the Fermi energy (E F ) is set to zero for the stoichiometric material (δ = 0). Then the energy scale for the nonstoichiometric material is adjusted to align its O 2s states with the δ = 0 case (solid vertical line). Dashed lines indicate the relevant Fermi energies. Mo PDOS are not shown for ease of viewing, since they are negligible near the Fermi level.
We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr(2)Fe(1.5)Mo(0.5)O(6-δ) (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group Pnma. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr(2)Fe(1.5)Mo(0.5)O(5.90(2)) (δ = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for its impressive electronic conductivity.
Seven cadmium- and zinc-containing Zintl phases, A9Zn(4+x)Pn9 and A9Cd(4+x)Pn9 (0 < or = x < or = 0.5), A = Ca, Sr, Yb, Eu; Pn = Sb, Bi, have been synthesized, and their structures have been determined by single-crystal X-ray diffraction. All compounds are isostructural and crystallize in the centrosymmetric orthorhombic space group Pbam (no. 55, Z = 2), and their structures feature tetrahedra of the pnicogens, centered by the transition metal. The tetrahedra are not isolated but are connected through corner sharing to form ribbons, which are separated by the divalent cations. The occurrence of a small phase width and its variation across this family of compounds has been systematically studied by variable temperature crystallography, resistivity, and magnetic susceptibility measurements, and these results have been reconciled with electronic structure calculations performed using the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method. These analyses of the crystal and electronic structure indicate that the polyanionic subnetwork requires 19 additional electrons, whereas only 18 electrons are provided by the cations. Such apparent "electron deficiency" necessitates the presence of an interstitial atom in order for an optimal bonding to be achieved; however, an interplay between the sizes of the cations and anions and the total valence electron concentration (governed by the stoichiometry breadth) is suggested as a possible mechanism for achieving structure stability. The structural relationship between these and some known structures with two-dimensional layers are discussed as well.
We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.
The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.
The local environments and short-range ordering of LiNi 0.5Mn0.5O2, a potential Li-ion battery positive electrode material, were investigated by using a combination of X-ray and neutron diffraction and isotopic substitution (NDIS) techniques, 6Li Magic Angle Spinning (MAS) NMR spectroscopy, and for the first time, X-ray and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Three samples were studied: 6Li(NiMn) 0.5O2,7Li(NiMn)0.5O2, and 7Li(NiMn)0.5O2 enriched with 62Ni (denoted as 7LiZERONi0.5Mn0.5O 2), so that the resulting scattering length of Ni atoms is null. LiNi0.5Mn0.5O2 adopts the LiCoO2 structure (space group R3m) and comprises separate lithium layers, transition metal layers (Ni, Mn), and oxygen layers. NMR experiments and Rietveld refinements show that there is approximately 10% of Ni/Li site exchange between the Li and transition metal layers. PDF analysis of the neutron data revealed considerable local distortions in the layers that were not captured in the Rietveld refinements performed using the Bragg diffraction data and the LiCoO2 structure, resulting in different M-O bond lengths of 1.93 and 2.07 Å for Mn-O and Ni/Li-O, respectively. Large clusters of 2400-3456 atoms were built to investigate cation ordering. The RMC method was then used to improve the fit between the calculated model and experimental PDF data. Both NMR and RMC results were consistent with a nonrandom distribution of Ni, Mn, and Li cations in the transition metal layers; both the Ni and Li atoms are, on average, close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Constraints from both experimental methods showed the presence of short-range order in the transition metal layers comprising LiMn6 and LiMn5Ni clusters combined with Ni and Mn contacts resembling those found in the so-called "flower structure" or structures derived from ordered honeycomb arrays.
Two homologous and isostructural compounds Na(5)M(2+x)Sn(10-x) (M = Zn, Hg) were obtained by direct reaction of the elements at high temperature. The crystal structures of these novel phases were determined from single-crystal X-ray diffraction data and represent a new structure type in tin chemistry. They crystallize in the space group Pbcn (No. 60, Z = 4) with a = 12.772(1), b = 10.804(1), and c = 12.777(1) A, V = 1763.1(2) A(3) for Na(5)Zn(2.28)Sn(9.72(2)) (I) and a = 12.958(1), b = 10.984(1), and c = 12.960(1) A, V = 1844.5(2) A(3) for Na(5)Hg(2.39)Sn(9.61(1)) (II). The structures consist of an anionic 3D open framework of tetrahedrally coordinated Sn and M atoms interwoven with a cationic 2D array of interconnected {NaNa(4)} tetrahedra. The framework can be partitioned into fragments of realgar-like units {Sn(8-x)M(x)}(2x-) and twice as many {Sn-M}(2-) dimers. Formally, the compounds are charge-balanced Zintl phases for x = 0.5. As the structure refinements lead to x = 0.28 and 0.39 for I and II, respectively, both structures are electron-rich and expected to be metallic. Theoretical investigations at the density functional theory level reveal a deep minimum at the Fermi level for x = 0.5. According to rigid band analyses, the electronic structure of the phases with the experimentally observed compositions corresponds to heavily doped semiconductors, thereby meeting an important requirement of thermoelectric materials.
[(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4) (1) which possesses both the [FeIII(CA)3]3- (CA= chloranilate) and hydroxooxoiron(III) ions has had its structure determined by single-crystal X-ray diffraction. The 2-300 K magnetic susceptibility of 1 provides the magnetic parameters, g = 2.07, J/kB = -165 K (115 cm-1), theta = -1 K, and the spin impurity, rho = 0.05, which indicates a strong antiferromagnetic interaction between iron(III) ions via the oxo anion.
Vertically aligned single-crystal ZnO nanorods have been successfully fabricated on semiconducting GaN, Al0.5Ga0.5N, and AlN substrates through a vapor-liquid-solid process. Near-perfect alignment was observed for all substrates without lateral growth. Room-temperature photoluminescence measurements revealed a strong luminescence peak at approximately 378 nm. This work demonstrates the possibility of growing heterojunction arrays of ZnO nanorods on AlxGa1-xN, which has a tunable band gap from 3.44 to 6.20 eV by changing the Al composition from 0 to 1, and opens a new channel for building vertically aligned heterojunction device arrays with tunable optical properties and the realization of a new class of nanoheterojunction devices.
We found that CODH is a fascinating enzyme for the electrochemical conversion of CO2 to CO. It could reduce CO2 to CO at -0.57 V vs NHE with approximately 100% current efficiency in 0.1 M phosphate buffer (pH 6.3). Nature's unique structure of C-cluster in CODH would be responsible for the low overpotential and the selective and fast conversion of CO2. The turnover number per C-cluster is 700 h-1, and the pH optimum is 6.3.
The incommensurate modulated crystal structure of the new misfit-layer calcium cobalt oxide (Ca0.85OH)2alphaCoO2 was investigated using a superspace-group formalism with synchrotron X-ray diffraction data. The compound is a kind of composite crystal that consists of two interpenetrating subsystems, [CoO2]infinity layers containing triangular lattices formed by edge-sharing CoO6 octahedra, separated from each other by [2Ca0.85OH]infinity double-layered rock-salt-type slabs. Both the subsystems are monoclinic lattices with the unit cell parameters, a1 = 2.8180(4) A, b = 4.8938(6) A, c = 8.810(1) A, alpha0 = 95.75(3) degrees , and alpha(=|q|=a1/a2) = 0.57822(8), viz., a2 = 4.8736 A, with Z = 2. A possible superspace group is C2/m(alpha10)s0-C21/m(alpha(-1)10) for the respective subsystems. The atomic positions deviate from the average positions of the fundamental structure due to the incommensurable periodic interaction between the subsystems. A significant structural modulation was found in the [2Ca0.85OH] subsystem, whereas the modulation in the [CoO2] subsystem is less than in [2Ca0.85OH], due to the tight bonding of the close-packed CoO6 octahedra. The degree of modulation in the CoO2 layers, i.e., the potential modulation, is almost the same as those of other compounds of the misfit-layer cobalt oxides. Flattened CoO6 octahedra indicate hole doping into the CoO2 layers. The [2Ca0.85OH] blocks act as the charge reservoir layers, and the defect Ca ions are presumably the source of the holes.
To improve the energy/power density of energy storage materials, numerous efforts have focused on the exploration of new structure prototypes, in particular metal-organic fameworks, Prussian blue analogues, open-framework oxides, and polyanion salts. Here we report a novel pyrochlore phase that appears to be useful as a high-capacity cathode for Li and Na batteries. It is an iron fluoride polymorph characterized by an intersecting tunnel structure, providing the space for accommodation and transport of Li and Na ions. It is prepared using hydrolyzable ionic liquids, which serve as reaction educts and structure-directing agents not only as far as the chemical structure is concerned but also in terms of morphology (shape, defect structure, electrode network structure). A capacity higher than 220 mA h g(-1) (for Li and Na storage) and a lifetime of at least 300 cycles (for Li storage) are demonstrated.
The layered oxysulfides Sr2MnO2Cu2m-0.5Sm+1 (m = 1-3) consist of alternating perovskite-type Sr2MnO2 layers and copper sulfide layers. The copper ions can be replaced electrochemically and reversibly by Li. The lithiated materials were studied by Li MAS NMR, and Li resonances were observed with shifts that could be rationalized based on the number of sulfide layers. The materials were cycled versus Li and showed enhanced capacity retention in comparison to pure Cu2S; the good electrochemical performance was ascribed to the presence of the layered framework structure and rapid Li+ and Cu+ conductivity in the sulfide layers.
The onset of pressure-induced hydration and volume expansion is lowered to 0.6 GPa via the increased flexibility of the host lattice using isomorphous substitution of Al by larger Ga in a sodium aluminosilicate natrolite.
Wiring systems powered by high-efficient superconductors have long been a dream of scientists, but researchers have faced practical challenges such as finding flexible materials. Here we report superconductivity in Nb2PdxS5-delta fibers with transition temperature up to 7.43 K, which have typical diameters of 0.3~3 micro-m. Superconductivity occurs in a wide range of Pd (0.6<x<1) and S (0<delta<0.61) contents, suggesting that the superconductivity in this system is very robust. Long fibers with suitable size provide a new route to high-power transmission cables and electronic devices.
Reduction of La(1-x)Ca(x)MnO(3) (0.6 ≤ x ≤ 1) perovskite phases with sodium hydride yields materials of composition La(1-x)Ca(x)MnO(2+δ). The calcium-rich phases (x = 0.9, 1) adopt (La(0.9)Ca(0.1))(0.5)Mn(0.5)O disordered rocksalt structures. However local structure analysis using reverse Monte Carlo refinement of models against pair distribution functions obtained from neutron total scattering data reveals lanthanum-rich La(1-x)Ca(x)MnO(2+δ) (x = 0.6, 0.67, 0.7) phases adopt disordered structures consisting of an intergrowth of sheets of MnO(6) octahedra and sheets of MnO(4) tetrahedra. X-ray absorption data confirm the presence of Mn(I) centers in La(1-x)Ca(x)MnO(2+δ) phases with x < 1. Low-temperature neutron diffraction data reveal La(1-x)Ca(x)MnO(2+δ) (x = 0.6, 0.67, 0.7) phases become antiferromagnetically ordered at low temperature.
The new Mn14Al56+xGe3-x (x = 0-0.6) compounds of a new structure type have been synthesized and characterized by physical property measurements and electronic structure calculations. In contrast to their well-known silicon analogues, their unique structure (P) exhibits unprecedented partially destroyed Mackay icosahedra that retain the icosahedral symmetry only in half of the individual polyhedra. The electronic band-structure analysis indicates that the chemical bonding in the structure is still optimized despite the destruction of the Mackay icosahedra and that a further valence electron concentration (VEC) optimization is achieved by the partial occupation of aluminum on a germanium site. The electronic band-structure calculation results were in agreement with the poor metallicity observed for the samples. While the Mn14Al56Ge3 is metallic, the resistivity of Mn14Al56.6Ge2.4 shows a minimum around 20 K and a maximum around 100 K. Both of the samples are Pauli-paramagnetic with an additional small Curie component.
We investigated the formation mechanism of thermoelectric [Ca(2)CoO(3)](0.62)[CoO(2)] (CCO) on beta-Co(OH)(2) templates with maintained orientations by identifying the intermediate phases and specifying the relationship between their crystallographic orientations. We mixed beta-Co(OH)(2) templates with the complementary reactant CaCO(3) and prepared a compact by tape casting, with the developed (001) plane of the templates aligned along the casting plane. High-temperature XRD of the compact revealed that beta-Co(OH)(2) decomposed into Co(3)O(4) by 873 K, and Co(3)O(4) reacted with CaO to form CCO by 1193 K via the formation of the newly detected intermediate phase beta-Na(x)()CoO(2)-type Ca(x)()CoO(2) at 913-973 K. Pole figure measurements and SEM and TEM observations revealed that the relationship between the crystallographic planes was (001) beta-Co(OH)(2)//{111} Co(3)O(4)//(001) Ca(x)()CoO(2)//(001) CCO. The crystal structures of the four materials possess the common CoO(2) layer (or similar), which is composed of edge-sharing CoO(6) octahedra, parallel to the planes. The cross-sectional HRTEM analysis of an incompletely reacted specimen showed transient lattice images from Ca(x)()CoO(2) into CCO, in which every other CoO(2) layer of Ca(x)()CoO(2) was preserved. Thus, it was demonstrated that a textured CCO ceramic is produced through a series of in situ topotactic conversion reactions with a preserved CoO(2) layer of its template.
The role of the hydride anion in controlling the electronic properties of the transition metal oxide hydride LaSrCoO(3)H(0.7) is investigated theoretically by full potential DFT band structure calculation and experimentally by determination of the Neel temperature for three-dimensional magnetic ordering. The mechanism by which hydrogen is introduced into the solid is addressed by in situ X-ray diffraction studies of the formation of the oxide hydride, which reveal both a relationship between the microscopic growth of the observed oxide hydride order and the anisotropic broadening of the diffraction profile, and the existence of a range of intermediate compositions.
The corundum-type In(2-2x)Zn(x)Sn(x)O(3) solid solution (cor-ZITO, x ≤ 0.7) was synthesized at 1000 °C under a high pressure of 70 kbar. cor-ZITO is a high-pressure polymorph of the transparent conducting oxide bixbyite-In(2-2x)Zn(x)Sn(x)O(3) (x ≤ 0.4). Analysis of the extended X-ray absorption fine structure suggests that significant face-sharing of Zn and Sn octahedra occurs, as expected for the corundum structure type. In contrast to the ideal corundum structure, however, Zn and Sn are displaced and form oxygen bonds with lengths that are similar to those observed in high-pressure ZnSnO(3). Powder X-ray diffraction patterns of cor-ZITO showed the expected unit cell contraction with increased cosubstitution, but no evidence for ilmenite-type ordering of the substituted Zn and Sn. A qualitative second harmonic generation measurement, for the solid solution x = 0.6 and using 1064 nm radiation, showed that Zn and Sn adopt a polar LiNbO(3)-type arrangement.
The compound In(OH)BDC.0.75BDCH2 (BDC = benzenedicarboxylate), 1, has been synthesized and characterized by single-crystal X-ray diffraction. The structure comprises two distinct sublattices formed by a covalently linked In(OH)BDC lattice and ordered chains of hydrogen-bonded H2BDC molecules and can be described as a hybrid inorganic coordination polymer-organic vernier structure. Each InO6 octahedron of the octahedral chain has a length of 3.6 A along the chain axis, whereas each H2BDC molecule has a length of 9.6 A along the guest column axis. Therefore, a unit of eight InO6 octahedra of the octahedral chain is just in registry with three H2BDC molecules of the guest column giving a repeat unit of 28.76 A along the channel axis direction.
We report here the synthesis of CexZr1-xO2 and (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) core-shell nanopowders in a single step by liquid-feed flame spray pyrolysis (LF-FSP) of the metalloorganic precursors, Ce(O2CCH2CH3)(3)(OH), alumatrane [N(CH2CH2O)(3)Al], and Zr(O2CCH2CH3)(2)(OH)(2). Solutions of all three precursors in ethanol with ceramic yields of 2.5 wt% were aerosolized with O-2, combusted at temperatures above 1500 degrees C, and rapidly quenched at similar to 1000 degrees C/ms to form CexZr1-xO2 and (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) nanopowders of selected compositions, at rates of 50-100 g/h. The resulting, as-processed, materials are unaggregated nanopowders with average particle sizes (APSs) < 20 nm and corresponding specific surface areas of 30-50 m(2)/g. The as-processed powders were characterized in terms of phase, particle size, specific surface area, compositions, and morphology by XRD, BET, DLS, SEM, TEM, XPS, TGA-DTA, and FT-IR. LF-FSP provides access to binary CexZr1-xO2 nanopowders and ternary (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) nanopowders in one step. The obtained Ce0.7Zr0.3O2 powders are solid solutions with a cubic phase. In contrast, LF-FSP of mixtures of the three precursors at specific compositions [x = 0.5, 0.7 for (Ce0.7Zr0.3O2)(x)@(Al2O3)(1-x)] provide core-shell nanopowders in a single step. The most reasonable explanation is that there are differences in the rates of condensation, nucleation and miscibility between the gas phase ions that form the CexZr1-xO2 solid solutions and those that condense to delta-Al2O3 during processing. These as-produced materials are without microporosity at surface areas of >= 30 m(2)/g. Evidence is presented suggesting the formation of (Ce/Zr)(3+) species in the as-processed (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) core-shell materials. An accompanying paper indicates that these materials offer significant and novel catalytic activities for hydrocarbon oxidation and deNO(x) processes without using platinum as a co-catalyst.
The BiCu2(P1-xVx)O6 system shows the appearance of various phenomena that progressively change as a function of the average (P/V)O4 groups size. Then, from x = 0 to x ≈ 0.7, a solid solution exists with respect to the basic orthorhombic unit cell of BiCu2PO6. For greater x values (0.7 < x <0.96), structural modulations with incommensurate q vector that slightly change versus x appear. The 4-D treatment of single-crystal XRD data of the modulated phase corresponding to x = 0.87 at 100 K (orthorhombic, a = 12.379(3)Å, b = 5.2344(9) Å, c = 7.8270(14) Å, q = 0.268(3) b*, super space group: Xbmm(0γ) s00, X stands for the nonprimitive centering vector (1/2,0,1/2,1/2), R(obs)overall = 5.27%, R(obs)fundamental = 4.48%, R(obs)satellite = 6.58%) has evidenced strong positional modulated effects within the [BiCu2O2]3+ ribbons while three XO4 configurations compete along the x4 fourth dimension. There is no P/V segregation along x4 in good agreement with steric-only origins of the modulation. Finally for 0.96 < x <1, two phases coexist, i.e., BiCu2VO6 (X = 1) and a modulated phase of the previous domain. The BiCu2VO6 crystal structure shows a unit cell tripling associated with monoclinic symmetry lowering. The VO4 orientations between two ribbons proceed with respect to the interribbon distance. Then the full system shows flexible interactions between modulated Bi/M/O-based ribbons and surrounding tetrahedral groups, depending on the average XO4 size. Furthermore, between two ribbons the Cu2+ arrangement forms magnetically isolated zigzag copper two-leg ladders. Our preliminary results show a spin-gap behavior likely due to the existence of true S = 1/2 Heisenberg two-leg ladders. Modulated compositions are gapless, in good agreement with band-broadening toward a continuum in the magnetic excitation spectrum. The continuous distribution of Cu-Cu distances along the rungs and legs of the ladders should be mainly responsible for this magnetic change.
The apo crystal structure of CTX-M-9 beta-lactamase has been determined to 0.88 A at pH 8.8. This unusually clear picture of proton positions and residue interactions supports the role of Glu166 as the general base for the controversial acylation step of class A beta-lactamase catalysis. The ability to distinguish low-energy conformations sampled by the enzyme allows us to link the two conformations of Lys73 to different protonation states of Glu166.
Top-cited authors
Hongjie Dai
  • Stanford University
Stephen A. Macko
  • University of Virginia
Hailiang Wang
  • Yale University
Yongye Liang
  • Southern University of Science and Technology
Tsutomu Miyasaka
  • Toin University of Yokohama