Anja Olafsen Sjåstad

Preparation of Nb-substituted titanates by a novel sol-gel assisted solid state reaction.

Single-phase layered Nb-substituted titanates, Na(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) and Cs(0.7)Ti(1.8-x)Nb(x)O(4) (x = 0-0.03), were for the first time synthesized by a novel sol-gel assisted solid state reaction (SASSR) route. Conventional solid state reactions as well as sol-gel synthesis did not succeed in producing phase pure Nb-substituted titanates. In the SASSR synthesis route we combine the advantages of traditional sol-gel technique (i.e., homogeneous products formed at low temperatures) and solid state reaction (i.e., formation of stable, crystalline phases) for preparing single-phase niobium-substituted layered titanates. The obtained products were characterized by X-ray powder diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and thermogravimetric analysis. Results indicate that the Ti(IV) in the host layer of the samples could be partially replaced by Nb(V) without structural deterioration. After proton-exchange, more water molecules were intercalated into the interlayer of H(0.7)Ti(1.8-x)Nb(x)O(4) x nH(2)O with increasing niobium content, whereas the interlayer distance of H(2)Ti(3-x)Nb(x)O(7) (x = 0-0.06) was unchanged.

Structural and magnetic aspects of La4(Co1-xNix)3O10+δ (0 ≤ x ≤ 1)

The Ruddlesden–Popper (RP3) type oxides, La4Co3O10+δ and La4Ni3O10+δ, form a complete solid solution. Powder X-ray and neutron diffraction data show that La4(Co1−xNix)3O10+δ is isostructural to the monoclinic La4Co3O10+δ structure (P21/a) described for all compositions without any further structural distortions as suggested in the literature. A slight elongation of the Co/Ni–O bonds facing the rock salt interlayer occurs for Ni-rich compositions. The magnetic properties of the solid solution series are mapped in the temperature range from 4 to 300 K, and the results are presented in a magnetic phase diagram. Three regimes with antiferromagnetic order (AF) exist at low temperatures, TN < 10 − 30 K. For x = 0.00, the AF is ascribed to Co(II), whereas a broader AF regime around x = 0.50 is ascribed mainly to Ni(II). Pauli paramagnetism is observed close to metallic La4Ni3O10+δ, x > 0.80. The possibility to tune the oxidation state of the transition metal atoms is demonstrated for La4Co3O10+δ, and exemplified by weakening of a temperature-induced spin transition at around 480 K.

Electronic and Magnetic Structures of Hole Doped Trilayer La4–xSrxNi3O8 from First-Principles Calculations

The magnetic and electronic properties of trilayer La4Ni3O8, similar to hole-doped cuprates, are investigated by performing full-potential linearized augmented plane wave method-based spin-polarized calculations with LDA and GGA functionals including Hubbard U parameters to account for strong correlation effects. On the basis of these calculations, we found that La4Ni3O8 is a C-type anti-ferromagnetic (C-AFM) Mott insulator in agreement with previous experimental and theoretical observations. Our calculations suggest that the two crystallographically nonequivalent nickel atoms Ni1 and Ni2 are found to be in high-spin state with an average valency of +1.33. Intermediate band-gap states are originated from dz2 electrons of both types of Ni ions after including the strong correlation effects. To understand the role of hole doping on electronic structure, phase stability, and magnetic properties of La4Ni3O8, similar calculations were performed for La4-xSrxNi3O8 as a function of x, using the supercell approach. We found that the hole doping brings insulator-to-metal transition without changing the C-AFM ordering, though the magnetic moment is enhanced at both Ni sites. Moreover, these Ni atoms are always in an average valence state irrespective of hole doping or volume change. So the electronic properties of hole-doped La4Ni3O8 cannot be compared with hole-doped cuprates that are high-TC superconductors.

From Colloidal Monodisperse Nickel Nanoparticles to Well-Defined Ni/Al2O3 Model Catalysts

In the past few decades, advances in colloidal nanoparticle synthesis have created new possibilities for the preparation of supported model catalysts. However, effective removal of surfactants is a prerequisite to evaluate the catalytic properties of these catalysts in any reaction of interest. Here we report on the colloidal preparation of surfactant-free Ni/Al2O3 model catalysts. Monodisperse Ni nanoparticles (NPs) with mean particle size ranging from 4 to 9 nm were synthesized via thermal decomposition of a zerovalent precursor in the presence of oleic acid. Five weight percent Ni/Al2O3 catalysts were produced by direct deposition of the presynthesized NPs on an alumina support, followed by thermal activation (oxidation-reduction cycle) for complete surfactant removal and surface cleaning. Structural and morphological characteristics of the nanoscale catalysts are described in detail following the propagation of the bulk and surface Ni species at the different treatment stages. Powder X-ray diffraction, electron microscopy, and temperature-programmed reduction experiments as well as infrared spectroscopy of CO adsorption and magnetic measurements were conducted. The applied thermal treatments are proven to be fully adequate for complete surfactant removal while preserving the metal particle size and the size distribution at the level attained by the colloidal synthesis. Compared with standard impregnated Ni/Al2O3 catalysts, the current model materials display narrowed Ni particle size distributions and increased reducibility with a higher fraction of the metallic nickel atoms exposed at the catalyst surface.

Thermal and Structural Aspects of the Hydride-Conducting Oxyhydride La2LiHO3 Obtained via a Halide Flux Method

Oxyhydrides, in which oxide and hydride anions share the same anionic lattice, are relatively rare compounds. La2LiHO3 belongs to this family. We report the synthesis of La2LiHO3 by means of an alkali halide flux method, which allows the production of larger quantities of material relative to the usually adopted synthesis routes. Powder X-ray and neutron diffraction studies show that La2LiHO3 adopts an n = 1 Ruddlesden-Popper (RP)-type structure with an orthorhombic distortion (Immm) due to hydride and oxide anion ordering. No sign of polymorphism is observed. La2LiHO3 is seen to decompose in an oxygen atmosphere at ∼450 °C into La2LiO3.5. We show that the high mobility of hydride anions close to the decomposition temperature is likely the main factor in inducing the oxidation. The crystal structure of La2LiO3.5 is also determined and takes an n = 1 RP-type structure with an orthorhombic distortion (Fmmm). This newly reported large-scale synthesis approach, combined with the proven high thermal stability, is a key factor for potential practical applications of this oxyhydride in real devices.

New Insights into Hydride Bonding, Dynamics, and Migration in La2LiHO3 Oxyhydride

Hydride anion-conducting oxyhydrides have recently emerged as a brand new class of ionic conductors. Here we shed a first light onto their local vibrations, bonding mechanisms, and anion migration properties using the powerful combination of high-resolution inelastic neutron scattering and a set of rigorously experimentally validated density functional theory calculations. By means of charge-density analysis we establish the bonding to be strongly anisotropic; ionic in the perovskite layer and covalent in the rock salt layer. Climbing nudged elastic band calculations allow us to predict the hydride migration paths, which crucially we are able to link to the observed exotic ionic-covalent hybrid bonding nature. In particular, hydride migration in the rock salt layer is seen to be greatly hindered by the presence of covalent bonding, forcing in-plane hydride migration in the perovskite layer to be the dominant transport mechanism. On the basis of this microscopic insight into the transport and bonding, we are able to propose future candidates for materials that are likely to show enhanced hydride conductivity.

Colossal positive magneto-resistance in oxygen deficient Ca 4 Mn 3 O 10

Huge changes in resistance induced by an external magnetic field, i.e., a colossal magnetoresistance effect, is a long-lived theme of the research due to both fundamental science and device applications. Careful analysis of oxygen-deficient Ca4Mn3O10, synthesized by the citric acid method, establishes a direct correlation between magnetic and electric properties. An electric resistive anomaly at the magnetic ordering temperature of ~115 K supports this correlation. We currently report on a colossal positive magnetoresistance up to ~1300% below the metal-to-insulator transition temperature for oxygen-deficient Ca4Mn3O10. The positive magnetoresistance effect in this system may be attributed to the field-induced carrier localization.

Colossal Positive Magnetoresistance in Oxygen-Deficient Ca4Mn3O10

Huge changes in resistance induced by an external magnetic field, i.e., a colossal magnetoresistance effect, is a long-lived theme of the research due to both fundamental science and device applications. Careful analysis of oxygen-deficient Ca₄Mn₃O₁₀, synthesized by the citric acid method, establishes a direct correlation between magnetic and electric properties. An electric resistive anomaly at the magnetic ordering temperature of ~115 K supports this correlation. We currently report on a colossal positive magnetoresistance up to ~1300% below the metal-to-insulator transition temperature for oxygen-deficient Ca₄Mn₃O₁₀. The positive magnetoresistance effect in this system may be attributed to the field-induced carrier localization.

Engineering Functions into Platinum and Platinum–Rhodium Nanoparticles in a One‐Step Microwave Irradiation Synthesis

Abstract Platinum (Pt) and platinum–rhodium (PtRh) nanoparticles (NPs) are active catalysts for a range of important industrial reactions, and their response has been shown to be affected by size, morphology, composition, and architectural configuration. We report herein the engineering of these functionalities into NPs by suitably modifying our single‐step fabrication process by using microwave irradiation dielectric heating. NPs with different morphologies are acquired by manipulating the reaction kinetics with the concentration of the capping agent while keeping the reaction time constant. Pt@Rh core@shell octopod‐cube, Pt‐truncated‐cube, and cube and small‐sphere NPs having “near‐monodisperse” distributions and average sizes in the range of 4 to 18 nm are obtained. By increasing the microwave time the composition of Pt@Rh can be tuned, and NPs with a Rh‐rich shell and a tunable Pt100−xRhx (x≤41 at %) core are fabricated. Finally, alloy bimetallic PtRh NPs with controlled composition are designed by simultaneous tuning of the relative molar ratio of the metal precursors and the microwave irradiation time.

Stacking faults type disorder in layered double hydroxides

Layered double hydroxides (LDH) are a broad group of widely studied materials. The layered character of these materials and their high flexibility for accommodating different metals and anions make them technologically interesting in a range of areas including catalysis, photocatalysis, gas sorption and separation, medicine, pigments, thermal barriers, polymer fillers, and fire retardants. The general formula for an LDH compound is [M1−xIIMxIII(OH)2][An−]x/n·mH2O, where MII is a divalent metal cation which can be substituted by an MIII trivalent cation, and An− is a charge compensating anion located between positively charged layers. Here we present a comprehensive study on possible structural disorder in LDH. We show how X-ray powder diffraction (XRPD) can be used to reveal important features of the LDH crystal structure such as stacking faults, random interlayer shifts, anion−molecule orientation, crystal water content, distribution of interlayer distances, and also LDH slab thickness. All calculations were performed using the Discus package, which gives a better flexibility in defining stacking fault sequences, simulating and refining XRPD patterns, relative to other commonly used programs such as DIFFaX, DIFFaX+, and FAULTS. Finally, we show how the modelling can be applied to two LDH samples: Ni0.67Cr0.33(OH)2(CO3)0.16·mH2O (3D structure) and Mg0.67Al0.33(OH)2(NO3)0.33 (2D layered structure). The presented examples show how XRPD can be successfully used for both highly crystalline and very disordered materials. In summary, we present a novel way of modelling the structure function, F(Q), to provide a source of structural information for poorly crystalline and 2D flake-type samples of LDH.