Influence of octahedral tilting and composition on electrical properties of the Li0.2 − xNaxLa0.6TiO3 (0 ≤ x ≤ 0.2) series

The Rietveld analysis of ND patterns of polycrystalline Li_0.2 − xNa_xLa_0.6TiO₃ (0 ≤ x < 0.2) samples, recorded between 300 and 1075 K, shows an orthorhombic–tetragonal transformation, in which the octahedral tilting along the b axis is eliminated at ~ 773 K, but the vacancy ordering along the c axis remains. In Li rich samples, conductivity (10^− 3 Ω^− 1 cm^− 1 at 300 K) departs from the Arrhenius behaviour, decreasing activation energies from 0.37 to 0.14 eV when octahedral tilting is eliminated. Successive Maxwell–Wagner blocking processes, detected in the real part of dielectric constant plots, have been ascribed to the Li blocking at interior domains, grain-boundary and electrode–electrolyte interfaces. The substitution of Li⁺ by Na⁺ decreases the amount of vacant A-sites, decreasing several orders of magnitude the conductivity when the amount of vacancies approaches the vacancy percolation threshold (n_p = 0.27). Below the percolation threshold, Li ions only display local mobility, remaining confined into small domains of perovskites.     

Electron tomography on nanopores embedded in epitaxial GaSb thin films

This work reports on the morphology of nanopores and their spatial position in group III-Sb based multilayer heterostructures grown by molecular beam epitaxy on Si(0 0 1) substrates. By using electron tomography based on dark-field scanning transmission electron microscopy, quantitative information in real space is obtained about individual nanopores unintentionally embedded in GaSb layers. For this purpose adequate needle-shaped samples have to be specifically prepared from the compact material system by focused ion beam. The three-dimensional reconstruction of the probed volume allows the determination of the spatial arrangement of the pores and the analysis of the detailed shape, i.e. the crystallographic facets. Based on these results, the nanopore's geometric shape is discussed with respect to the minimization of surface tension. The formation process can be explained by an agglomeration of vacancies which are generated during the heterostructure growth.     

Measurement of K shell radiative transition probabilities and K, L2 and L3 shell/subshell fluorescence yields of some elements in the atomic number range 30 ≤ Z ≤ 40

K shell radiative transition probabilities and K, L₂ and L₃ shell/subshell fluorescence yields were measured using the K_β/K_α intensity ratio for seven elements in the atomic number range 30≤Z≤40 (Zn, As, Se, Rb, Sr, Y and Zr). The targets were irradiated with γ photons of energy 59.5 keV from Am-241. The obtained values were compared with the theoretical values. It was observed that the present values agree with previous theoretical results. The L₂ and L₃ subshell fluorescence yields were the first measured for the present elements.     

Magnetic properties related to structure and complete composition analyses of nanocrystalline La1−xMn1−yO3 powders

Structural and magnetic properties were studied on La_1−xMnO_3±δ nanocrystalline powders exhibiting different La/Mn ratios. These compounds were prepared using a gel combustion method based on a cation solution soaking by acrylamide polymerization. Structural properties were studied both by transmission electron microscopy and X-ray diffraction (XRD). Complete chemical composition analyses were performed by induced coupled plasma spectroscopy and by iodometric titration. Proportions of parasitic phases in samples, as La₂O₃ or Mn₃O₄, and actual compositions of La_1−xMnO_3±δ phases were then determined from refinements of XRD data and sample chemical compositions. As a result, perovskite structure is not any more stable for La/Mn<0.9 as it decomposes into a mixing of La_0.9MnO₃ and of Mn₃O₄ phases, in agreement with results on thermodynamic equilibrium in the La-Mn-O phase diagram. For La/Mn>0.9, a high oxygen excess is observed and leads to consider the creation of vacancies on both lanthanum and manganese sites, whose concentrations are evaluated. Magnetic properties agree well with the proposed structures and sample compositions since for La/Mn<0.9, for which a La_0.9MnO₃ phase is always found, the Curie temperature remains constant and equal to 295 K (the highest temperature never observed before in such series of compositions), while for La/Mn>0.9, there is a formation of Mn vacancies giving rise to a lowering of Curie temperature resulting of a frustration of ferromagnetic interactions.     

Positron annihilation spectroscopic studies on Nd-doped ceria

We report a new result on the characterization of Nd-doped ceria, Ce_1−xNd_xO_2−x/2 (x=0.075-0.675) using positron lifetime spectroscopy (LTS) and coincidence Doppler broadening (CDB) measurements. A systematic increase in lifetime that is attributed to formation of Nd-oxygen vacancy associates is seen from x=0.075-0.4 followed by a sharper increase up to x=0.5. The change in profile of lifetime around x=0.4 suggests drastic increase in the concentration of these associates. Discontinuity in lifetime around x=0.5 is ascribed to ordering of oxygen vacancies. Coincidence Doppler broadening measurements indicate reduction in the overlap of positron wavefunction with oxygen core electrons due to trapping of positrons. Low-temperature (50-300 K) lifetime measurements indicate the presence of Rydberg-like positron states associated with Nd³⁺ sites.     

Annealing in Argon Universally Upgrades the Na-Storage Performance of Mn-Based Layered Oxide Cathodes by Creating Bulk Oxygen Vacancies

Manganese-rich layered oxide cathodes of sodium-ion batteries (SIBs) are extremely promising for large-scale energy storage owing to their high capacities and cost effectiveness, while the Jahn–Teller (J–T) distortion and low operating potential of Mn redox largely hinder their practical applications. Herein, we reveal that annealing in argon rather than conventional air is a universal strategy to comprehensively upgrade the Na-storage performance of Mn-based oxide cathodes. Bulk oxygen vacancies are introduced via this method, leading to reduced Mn valence, lowered Mn 3d-orbital energy level, and formation of the new-concept Mn domains. As a result, the energy density of the model P2-Na_0.75Mg_0.25Mn_0.75O₂ cathode increases by ≈50 % benefiting from the improved specific capacity and operating potential of Mn redox. The Mn domains can disrupt the cooperative J–T distortion, greatly promoting the cycling stability. This exciting finding opens a new avenue towards high-performance Mn-based oxide cathodes for SIBs.     

A BiVO4 Photoanode with a VOx Layer Bearing Oxygen Vacancies Offers Improved Charge Transfer and Oxygen Evolution Kinetics in Photoelectrochemical Water Splitting

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO₄) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VO_x) with enriched oxygen vacancies conformally grown on BiVO₄ photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO₄/VO_x photoanode exhibits a photocurrent density of 6.29 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VO_x that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.     

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy-rich Mesoporous Co3O4

The development of efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co₃O₄ (mCo₃O₄-350) showed excellent catalytic activity for selective oxidation of aromatic-alkanes, especially for oxidation of ethylbenzene with a conversion of 42 % and selectivity of 90 % for acetophenone at 120 °C. Notably, mCo₃O₄ presented a unique catalytic path of direct oxidation of aromatic-alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo₃O₄ activate around Co atoms, causing electronic state change from Co³⁺_(Oh)→Co²⁺_(Oh). Co²⁺_(Oh) has great attraction to ethylbenzene, and weak interaction with O₂, which provide insufficient O₂ for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo₃O₄, sharply contrasted to non-selective oxidation of ethylbenzene on commercial Co₃O₄.     

Computer simulation and study of radiation defects in graphene

Computer simulation has been used to study the properties of radiation defects in a graphene sheet. Several possible stable configurations of 3D radiation defects in graphene have been simulated. Several types of absorbed species were calculated involving a single carbon atom and a dumbbell absorbed at graphene sheet surface and a dumbbell configuration at a vacancy created by radiation knocking out a carbon atom. The calculations also included the effect of structural relaxation. The defect caused by the atom being displaced from the structure converted the sheet from metallic to semiconducting.     

Computer simulation of the vacancy defects interaction with shuffle dislocation in silicon

The interactions between the 60^° shuffle dislocation and two different types of vacancy defects in silicon are separately studied via the molecular dynamics simulation method. The Stillinger–Weber potential is used to describe the atomic interactions. The results show that the dislocation slip velocity will decrease due to the interaction with the vacancy cluster (V ₆). The simulation also reveals that the divacancy will be absorbed by the dislocation. Meanwhile, a climbing of the dislocation occurs during their interactions. However, the divacancy has little effect on the dislocation slip velocity. Based on the above results, the decrease in threading dislocation density in SiGe/Si heterostructures with the use of low-temperature Si buffer layer may be explained.