Ti surface doping of LiNi0.5Mn1.5O4-δ positive electrodes for lithium ion batteries.

The particle surface of LiNi0.5Mn1.5O4-δ (LNMO), a Li-ion battery cathode material, has been modified by Ti cation doping through a hydrolysis-condensation reaction followed by annealing in oxygen. The effect of different annealing temperatures (500-850 °C) on the Ti distribution and electrochemical performance of the surface modified LNMO was investigated. Ti cations diffuse from the preformed amorphous 'TiO x ' layer into the LNMO surface during annealing at 500 °C. This results in a 2-4 nm thick Ti-rich spinel surface having lower Mn and Ni content compared to the core of the LNMO particles, which was observed with scanning transmission electron microscopy coupled with compositional EDX mapping. An increase in the annealing temperature promotes the formation of a Ti bulk doped LiNi(0.5-w)Mn(1.5+w)-t Ti t O4 phase and Ti-rich LiNi0.5Mn1.5-y Ti y O4 segregates above 750 °C. Fourier-transform infrared spectrometry indicates increasing Ni-Mn ordering with annealing temperature, for both bare and surface modified LNMO. Ti surface modified LNMO annealed at 500 °C shows a superior cyclic stability, coulombic efficiency and rate performance compared to bare LNMO annealed at 500 °C when cycled at 3.4-4.9 V vs. Li/Li+. The improvements are probably due to suppressed Ni and Mn dissolution with Ti surface doping.

Grain-Boundary Engineering for Aging and Slow-Crack-Growth Resistant Zirconia.

Ceramic materials are prone to slow crack growth, resulting in strength degradation over time. Although yttria-stabilized zirconia (Y-TZP) ceramics have higher crack resistance than other dental ceramics, their aging susceptibility threatens their long-term performance in aqueous environments such as the oral cavity. Unfortunately, increasing the aging resistance of Y-TZP ceramics normally reduces their crack resistance. Our recently conducted systematic study of doping 3Y-TZP with various trivalent cations revealed that lanthanum oxide (La2O3) and aluminum oxide (Al2O3) have the most potent effect to retard the aging kinetics of 3Y-TZP. In this study, the crack-propagation behavior of La2O3 and Al2O3 co-doped 3Y-TZP ceramics was investigated by double-torsion methods. The grain boundaries were examined using scanning transmission electron microscopy and energy-dispersive spectroscopy (STEM-EDS). Correlating these analytic data with hydrothermal aging studies using different doping systems, a strategy to strongly bind the segregated dopant cations with the oxygen vacancies at the zirconia-grain boundary was found to improve effectively the aging resistance of Y-TZP ceramics without affecting the resistance to crack propagation.

{110}-Layered B-cation ordering in the anion-deficient perovskite Pb2.4Ba2.6Fe2Sc2TiO13 with the crystallographic shear structure.

A novel anion-deficient perovskite-based compound, Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13), was synthesized via the citrate-based route. This compound is an n = 5 member of the AnBnO(3n-2) homologous series with unit-cell parameters related to the perovskite subcell a(p)≈ 4.0 Å as a(p)√2 ×a(p)× 5a(p)√2. The crystal structure of Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) consists of quasi-2D perovskite blocks with a thickness of three octahedral layers separated by the 1/2[110](1[combining macron]01)(p) crystallographic shear (CS) planes, which are parallel to the {110} plane of the perovskite subcell. The CS planes transform the corner-sharing octahedra into chains of edge-sharing distorted tetragonal pyramids. Using a combination of neutron powder diffraction, (57)Fe Mössbauer spectroscopy and atomic resolution electron energy-loss spectroscopy we demonstrate that the B-cations in Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) are ordered along the {110} perovskite layers with Fe(3+) in distorted tetragonal pyramids along the CS planes, Ti(4+) preferentially in the central octahedra of the perovskite blocks and Sc(3+) in the outer octahedra of the perovskite blocks. Magnetic susceptibility and Mössbauer spectroscopy indicate a broadened magnetic transition around T(N)∼ 45 K and the onset of local magnetic fields at low temperatures. The magnetic order is probably reminiscent of that in other AnBnO(3n-2) homologues, where G-type AFM order within the perovskite blocks has been observed.

Relaxor ferroelectricity and magnetoelectric coupling in ZnO-Co nanocomposite thin films: beyond multiferroic composites.

ZnO-Co nanocomposite thin films are synthesized by combination of pulsed laser deposition of ZnO and Co ion implantation. Both superparamagnetism and relaxor ferroelectricity as well as magnetoelectric coupling in the nanocomposites have been demonstrated. The unexpected relaxor ferroelectricity is believed to be the result of the local lattice distortion induced by the incorporation of the Co nanoparticles. Magnetoelectric coupling can be attributed to the interaction between the electric dipole moments and the magnetic moments, which are both induced by the incorporation of Co. The introduced ZnO-Co nanocomposite thin films are different from conventional strain-mediated multiferroic composites.

Polar and magnetic layered A-site and rock salt B-site-ordered NaLnFeWO6 (Ln = La, Nd) perovskites.

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T(N) ≈ 25 K for NaLaFeWO6 and at ∼21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = ((1/2) 0 (1/2)) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

Structural and magnetic phase transitions in the A(n)B(n)O(3n-2) anion-deficient perovskites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16.

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.

Magnetic and structural studies of the multifunctional material SrFe(0.75)Mo(0.25)O(3-δ).

SrFe0.75Mo0.25O3-δ has been recently discovered as an extremely efficient electrode for intermediate temperature solid oxide fuel cells (IT-SOFCs). We have performed structural and magnetic studies to fully characterize this multifunctional material. We have observed by powder neutron diffraction (PND) and transmission electron microscopy (TEM) that its crystal symmetry is better explained with a tetragonal symmetry (I4/mcm space group) than with the previously reported orthorhombic symmetry (Pnma space group). The temperature dependent magnetic properties indicate an exceptionally high magnetic ordering temperature (TN ∼ 750 K), well above room temperature. The ordered magnetic structure at low temperature was determined by PND to be an antiferromagnetic coupling of the Fe cations. Mössbauer spectroscopy corroborated the PND results. A detailed study, with X-ray absorption spectroscopy (XAS), in agreement with the Mössbauer results, confirmed the formal oxidation states of the cations to be mixed valence Fe(3+/4+) and Mo(6+).

Unexpected optical response of single ZnO nanowires probed using controllable electrical contacts.

Relying on combined electron-beam lithography and lift-off methods Au/Ti bilayer electrical contacts were attached to individual ZnO nanowires (NWs) that were grown by a vapor phase deposition method. Reliable Schottky-type as well as ohmic contacts were obtained depending on whether or not an ion milling process was used. The response of the ZnO NWs to ultraviolet light was found to be sensitive to the type of contacts. The intrinsic electronic properties of the ZnO NWs were studied in a field-effect transistor configuration. The transfer characteristics, including gate threshold voltage, hysteresis and operational mode, were demonstrated to unexpectedly respond to visible light. The origin of this effect could be accounted for by the presence of point defects in the ZnO NWs.

Crystal structure and phase transitions in Sr3WO6.

The crystal structures of the beta and gamma polymorphs of Sr(3)WO(6) and the gamma<-->beta phase transition have been investigated using electron diffraction, synchrotron X-ray powder diffraction, and neutron powder diffraction. The gamma-Sr(3)WO(6) polymorph is stable above T(c) approximately 470 K and adopts a monoclinically distorted double perovskite A(2)BB'O(6) = Sr(2)SrWO(6) structure (space group Cc, a = 10.2363(1)A, b = 17.9007(1)A, c = 11.9717(1)A, beta = 125.585(1)(o) at T = 1373 K, Z = 12, corresponding to a = a(p) + 1/2b(p) - 1/2c(p), b = 3/2b(p) + 3/2c(p), c = -b(p) + c(p), a(p),b(p), c(p), lattice vectors of the parent Fm3m double perovskite structure). Upon cooling it undergoes a continuous phase transition into the triclinically distorted beta-Sr(3)WO(6) phase (space group C1, a = 10.09497(3)A, b = 17.64748(5)A, c = 11.81400(3)A, alpha = 89.5470(2)(o), beta = 125.4529(2)(o), gamma = 90.2889(2)(o) at T = 300 K). Both crystal structures of Sr(3)WO(6) belong to a family of double perovskites with broken corner sharing connectivity of the octahedral framework. A remarkable feature of the gamma-Sr(3)WO(6) structure is a non-cooperative rotation of the WO(6) octahedra. One third of the WO(6) octahedra are rotated by approximately 45 degrees about either the b(p) or the c(p) axis of the parent double perovskite structure. As a result, the WO(6) octahedra do not share corners but instead share edges with the coordination polyhedra of the Sr cations at the B positions increasing their coordination number from 6 to 7 or 8. The crystal structure of the beta-phase is very close to the structure of the gamma-phase; decreasing symmetry upon the gamma-->beta transformation occurs because of unequal octahedral rotation angles about the b(p) and c(p) axes and increasing distortions of the WO(6) octahedra.

Ternary magnesium rhodium boride Mg2Rh1-xB6+2x with a modified Y2ReB6-type crystal structure.

The new ternary magnesium rhodium boride Mg2Rh1-xB6+2x has been prepared by the reaction of the mixture of Mg powder, RhB, and crystalline boron in a Ta container sealed under argon. The crystal structure of Mg2Rh0.75(1)B6.50(4) is determined using single-crystal X-ray diffraction, electron diffraction, and high-resolution electron microscopy (space group Pbam, a=8.795(2) A, b=11.060(2) A, c=3.5279(5) A, Z=4, 630 reflections, RF=0.045). It represents a modified Y2ReB6 structure type with an unusual replacement of part of the Rh atoms by boron pairs located in the pentagonal channels parallel to the c axis. The pairs interconnect the neighboring planar boron nets into the 3D framework. The variation of the lattice parameters reveals a homogeneity range Mg2Rh1-xB6+2x. The random distribution of the Rh atoms and boron pairs and the stabilizing effect of the boron pairs on the Y2ReB6 type structure motif are discussed using electronic band structure calculations and chemical bonding analysis with the electron localization function (ELF).

Radionuclide release and transport from nuclear underground tests performed at Mururoa and Fangataufa--predictions under uncertainty.

In the context of a study by the International Geomechanical Commission (IGC) and the International Atomic Energy Agency (IAEA) on the effects of nuclear tests at the atolls of Mururoa and Fangataufa, release to the biosphere is estimated for 35 radionuclides originating from 147 nuclear underground tests. Based on a qualitatively characterised hydrogeological situation of atolls and relatively scarce site-specific data, a model chain was developed to conservatively estimate the radionuclide fluxes via groundwater, from their sources, the explosion cavities, towards the biosphere, the ocean or lagoon. Finite element hydro-thermal modelling was used to describe water flow. Parameters were calibrated by a very few measured pre-test temperature profiles in bore holes. The impact of the tests on groundwater flow and mechanical impact on rock was considered. Estimates were made to quantify spatial extensions and temporal evolution of impact by using measurements on refilling rate of the cavities. Tests were categorised according to their specific yield and location although detailed data were missing. A base case parameter set was defined for the hydraulic conditions and for the initial radionuclide inventory of individual tests. Models were used to describe the concentration of radionuclides in the cavities as a function of time. Radionuclide transport from the cavities to the biosphere was represented by two different approaches: a double porosity model for the fractured volcanic rock and a single porosity model for the overlaying, highly porous carbonates. Results consist of conservative estimates on radionuclide release into the environment, or concentration in the lagoon or ocean water. Their sensitivity was investigated using different models and parameters. A few measured data (concentrations in a few cavities, in the deep carbonates and in the lagoons for selected radionuclides, such as 3H, 14C, 36Cl, 90Sr, 129I, 137Cs239 240Pu and 241Am) were available for a comparison with the calculations. In view of the lack and uncertainty of site-specific data, the agreement is of acceptable quality.