Sverre M. Selbach

Strain-controlled oxygen vacancy formation and ordering in CaMnO3

We use first-principles calculations to investigate the stability of biaxially strained Pnma perovskite CaMnO

Direct observation of ferroelectric field effect and vacancy-controlled screening at the BiFeO3/LaxSr1−xMnO3 interface

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Van der Waals density functional study of the energetics of alkali metal intercalation in graphite

towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain-relaxation mechanism. Our first-principles calculations of oxygen vacancy defect formation energy indeed show a strong strain dependence: We find that tensile strain lowers the formation energy, consistent with the established chemical expansion concept that oxygen deficiency increases the molar volume in oxides. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

Alginate-controlled formation of nanoscale calcium carbonate and hydroxyapatite mineral phase within hydrogel networks.

The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.

Hydrothermal synthesis and characterization of KNbO3nanorods

We report on the energetics of intercalation of lithium, sodium and potassium in graphite by density functional theory using recently developed van der Waals (vdW) density functionals. First stage intercalation compounds are well described by conventional functionals like GGA, but van der Waals functionals are crucial for higher stage intercalation compounds and graphite, where van der Waals interactions are important. The vdW-optPBE functional gave the best agreement with reported structure and energetics for graphite and LiC6 and was further applied for intercalation of Na and K. The enthalpy of formation of LiC6 and KC8 were found to be −16.4 and −27.5 kJ mol−1 respectively. NaC6 and NaC8 were unstable with positive enthalpies of formation (+20.8 and +19.9 kJ mol−1). The energetics of stacking of graphene and intercalant layers was investigated from first to fifth stage intercalation compounds. Higher stage compounds of Li and K were stable, but with less negative enthalpy of formation with increasing stage order. The higher stage Na compounds possessed positive enthalpy of formation, but lower in magnitude than the energy difference of 0.6 kJ mol−1 between graphite with AB and AA stacking. The abnormal behaviour of the lower stage Na intercalation compounds was rationalized by the lower energy involved in the formation of the chemical bond between carbon Na relative to the corresponding bond with Li or K. The chemical bond between alkali metal and carbon is characterized by charge transfer from the alkali-metal to carbon resulting in ionized alkali-metals. The intercalation induces only a subtle increase in the in-plane C–C bond lengths, with longer C–C bonds in the vicinity of the alkali metals but without breaking the hexagonal symmetry.

Defect Engineering in Plasmonic Metal Oxide Nanocrystals

A one-step method was used to make nanostructured composites from alginate and calcium carbonate or calcium phosphate. Nanometer-scale mineral phase was successfully formed within the gel network of alginate gel beads, and the composites were characterized. It was found that calcite was the dominating polymorph in the calcium carbonate mineralized beads, while stoichiometric hydroxyapatite was formed in the calcium phosphate mineralized beads. A combination of electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and powder X-ray diffraction showed that alginate played an active role in controlling mineral size, morphology and polymorphy. For the calcium phosphate mineralized beads, alginate was shown to modulate stoichiometric hydroxyapatite with low crystallinity at room temperature, which may have important applications in tissue engineering. The results presented in this work demonstrate important aspects of alginate-controlled crystallization, which contributes to the understanding of composite material design.

Diffusion of alkali metals in the first stage graphite intercalation compounds by vdW-DFT calculations

Single-crystalline KNbO3nanorods were prepared from Nb2O5 by hydrothermal synthesis at 180 °C in a KOH solution using sodium dodecyl sulfate surfactant. The morphology of the KNbO3 product was strongly influenced by the addition of the surfactant, the concentration of the reactants, the reaction time and temperature, demonstrating that KN nanorods were only obtained in a narrow window of synthesis parameters. At the optimized conditions the orthorhombic KNbO3nanorods were 100–300 nm in diameter, up to 5 µm in length and grew along the [001] direction. The proposed growth mechanism is based on self-assembly of cube-shaped or facetted KNbO3nanocrystals into mesocrystals, which further ripen into nanorods. High temperature X-ray diffraction, calorimetry and Raman spectroscopy evidenced that the orthorhombic to tetragonal and tetragonal to cubic phase transition of KNbO3nanorods occurred at significantly lower temperatures than for bulk KNbO3, reflecting finite size or disorder effects.

Thermal evolution of the crystal structure of proton conducting BaCe0.8Y0.2O3−δ from high-resolution neutron diffraction in dry and humid atmosphere

Defects may tend to make crystals interesting but they do not always improve performance. In doped metal oxide nanocrystals with localized surface plasmon resonance (LSPR), aliovalent dopants and oxygen vacancies act as centers for ionized impurity scattering of electrons. Such electronic damping leads to lossy, broadband LSPR with low quality factors, limiting applications that require near-field concentration of light. However, the appropriate dopant can mitigate ionized impurity scattering. Herein, we report the synthesis and characterization of a novel doped metal oxide nanocrystal material, cerium-doped indium oxide (Ce:In2O3). Ce:In2O3 nanocrystals display tunable mid-infrared LSPR with exceptionally narrow line widths and the highest quality factors observed for nanocrystals in this spectral region. Drude model fits to the spectra indicate that a drastic reduction in ionized impurity scattering is responsible for the enhanced quality factors, and high electronic mobilities reaching 33 cm(2)V(-1) s(-1) are measured optically, well above the optical mobility for tin-doped indium oxide (ITO) nanocrystals. We investigate the microscopic mechanisms underlying this enhanced mobility with density functional theory calculations, which suggest that scattering is reduced because cerium orbitals do not hybridize with the In orbitals that dominate the bottom of the conduction band. Ce doping may also reduce the equilibrium oxygen vacancy concentration, further enhancing mobility. From the absorption spectra of single Ce:In2O3 nanocrystals, we determine the dielectric function and by simulation predict strong near-field enhancement of mid-IR light, especially around the vertices of our synthesized nanocubes.

Interstitial oxygen as a source of p-type conductivity in hexagonal manganites

Diffusion of alkali metal cations in the first stage graphite intercalation compounds (GIC) LiC6, NaC6, NaC8 and KC8 has been investigated with density functional theory (DFT) calculations using the optPBE-vdW van der Waals functional. The formation energies of alkali vacancies, interstitials and Frenkel defects were calculated and vacancies were found to be the dominating point defects. The diffusion coefficients of the alkali metals in GIC were evaluated by a hopping model of point defects where the energy barriers for vacancy diffusion were derived from transition state theory. For LiC6, NaC6, NaC8 and KC8, respectively, the diffusion coefficients were found to be 1.5 × 10−15, 2.8 × 10−12, 7.8 × 10−13 and 2.0 × 10−10 m2 s−1 at room temperature, which is within the range of available experimental data. For LiC6 and NaC6 a curved vacancy migration path is the most energetically favourable, while a straight pathway was inferred for NaC8 and KC8. The diffusion coefficients for alkali metal vacancy diffusion in first stage GICs scales with the graphene interlayer spacing: LiC6 ≪ NaC8 < NaC6 ≪ KC8.

Structural phases driven by oxygen vacancies at the La0.7Sr0.3MnO3/SrTiO3 hetero-interface

The crystal structure of the proton conducting perovskite BaCe(0.8)Y(0.2)O(3-δ) (BCY20) has been studied via high-resolution in situ neutron diffraction performed in controlled dry and humid (heavy water) oxygen flow. Two phase transitions, cubic Pm3[combining macron]m→R3[combining macron]c (775 °C)→Imma (250 °C) were observed on cooling from 1000 °C in dry O(2). A significant shift of the phase stability fields was observed on cooling in wet oxygen (pD(2)O ≈ 0.2 atm) with the R3[combining macron]c structure stabilised at 900 °C, and the R3[combining macron]c→Imma transition occurring at 675 °C. On cooling below 400 °C a monoclinic, I2/m, phase started to appear. The structural dependence on hydration level is primarily due to the de-stabilisation of the correlated, octahedra tilts as a consequence of structural relaxation around the oxygen vacancies present in the non-hydrated phase. The tendency of hydrated BaCe(0.8)Y(0.2)O(3-δ) to show octahedral tilting is also found to be enhanced, indicating that the deuteronic (protonic) defects influence the crystal structure, possibly via hydrogen bonding. Stabilisation of the monoclinic I2/m phase is attributed to the structural effect of deuterons that is inferred to increase on cooling as deuterons localise to a greater extent. Changing from wet oxidising (O(2) + D2O(g)) to wet reducing (5% H2 in Ar + D2O(g)) atmosphere did not influence the structure or the phase stability, indicating that Ce(4+) was not reduced under the present conditions. Based on the observed cell volume expansion protonic defects are present in the material at 900 °C at a D(2)O partial pressure of ∼0.2 atm. The origin of the chemical expansion is explained by the effective size of the oxygen vacancy being significantly smaller than the [OD] defect. Rietveld analysis has been used to locate possible sites for the deuterons in the high temperature, R3[combining macron]c and Imma, phases that are most relevant for proton transport.