Oxygen Vacancy Formation and Migration within the Antiphase Boundaries in Lanthanum Scandate-Based Oxides: Computational Study.

The atomic structure of antiphase boundaries in Sr-doped lanthanum scandate (La1-xSrxScO3-δ) perovskite, promising as the proton conductor, was modelled by means of DFT method. Two structural types of interfaces formed by structural octahedral coupling were constructed: edge- and face-shared. The energetic stability of these two interfaces was investigated. The mechanisms of oxygen vacancy formation and migration in both types of interfaces were modelled. It was shown that both interfaces are structurally stable and facilitate oxygen ionic migration. Oxygen vacancy formation energy in interfaces is lower than that in the regular structure, which favours the oxygen vacancy segregation within such interfaces. The calculated energy profile suggests that both types of interfaces are advantageous for oxygen ion migration in the material.

The safest time to fly: pandemic response in the era of Fox News.

We document a causal effect of the conservative Fox News Channel in the USA on physical distancing during COVID-19 pandemic. We measure county-level mobility covering all US states and District of Columbia produced by GPS pings to 15-17 million smartphones and zip-code-level mobility using Facebook location data. Using the historical position of Fox News Channel in the cable lineup as the source of exogenous variation, we show that increased exposure to Fox News led to a smaller reduction in distance traveled and a smaller increase in the probability of staying home after the national emergency declaration in the USA. Our results show that slanted media can have a harmful effect on containment efforts during a pandemic by affecting people's behavior.

Correlation between structure, surface defect chemistry and 18O/16O exchange for La2Mo2O9 and La2(MoO4)3.

The La2Mo2O9 and La2(MoO4)3 powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10-11 atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10-3 atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (ra) and incorporation (ri) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.

Effect of grain boundaries in La0.84Sr0.16CoO3-δ on oxygen diffusivity and surface exchange kinetics.

The single crystal and polycrystalline specimens of La0.84Sr0.16CoO3-δ oxide were synthesized and characterized by X-ray powder diffraction analysis, energy dispersive X-ray microanalysis, the electron backscatter diffraction technique, and X-ray photoelectron spectroscopy. A thin slab was prepared from the grown single crystal with its surface corresponding to the (110) plane. The kinetics of the oxygen exchange between the gas phase and a single crystal and a polycrystalline specimen was studied by means of 16O/18O oxygen isotope exchange at T = 750-850 °C and PO2 = 5.3 × 10-3-2.2 × 10-2 atm. Temperature dependencies of the oxygen heterogeneous exchange rate, the oxygen dissociative adsorption and incorporation rates, and oxygen diffusion coefficients were obtained. The relationship between the crystallographic orientation of oxides and the kinetic parameters of oxides has been established. Correlations between the surface state and the rates of individual stages of oxygen exchange as well as oxygen diffusion pathways in the single crystal compared with those in the polycrystalline specimen are considered.

Oxygen surface exchange and diffusion in mayenite single crystal.

Oxygen surface exchange and diffusion in Ca12Al14O33±Î´ single crystal were studied by a unique in situ method based on isotope equilibration in the gas phase. Although the interphase exchange rate and oxygen diffusion coefficient demonstrate good agreement with available data, only the employed method is efficient to isolate the contributions of various types of exchange; thus, for the first time, it is possible to estimate the surface heterogeneity of mayenite. The obtained results disprove conclusions previously developed in the literature; the temperature region of 750 °C to 850 °C is not the intermediate region where two types of oxygen diffusion coexist. Complex discussion of the accumulated information on the temperature-dependent properties of mayenite allowed us to represent a model describing the observed dependencies based on the near-surface layer stability in the studied temperature range.

Band gap engineering and transport properties of Ba2In2O5: effect of fluorine doping and hydration.

Two types of fluorine doped barium indate solid solutions were prepared by a solid state method: Ba2-0.5xIn2O5-xFx (x = 0, 0.1, 0.2) and Ba2In2O5-0.5yFy (y = 0, 0.1, 0.25). Good agreement between the theoretical and experimental values of the pycnometric densities for the studied solid solutions confirms these two distinctly different models of solid solution formation. According to analysis of the XRD data introduction of fluorine ions into the oxygen sublattice of barium indate leads to a decrease in the a lattice parameter and unit cell volume for both types of solid solutions. The effect of fluorine doping and hydration on the band gap of barium indate was studied by means of diffuse reflectance spectroscopy. The estimated value of the band gap width Eg for undoped Ba2In2O5 is 2.94 eV and is in good agreement with literature data. Introduction of fluorine results in a slight increase of Eg and emergence of an additional absorption band in the region of 2.56-2.65 eV, near the fundamental absorption edge, which can be attributed to the -defects appearing upon fluorine doping. The increase of Eg with fluorine introduction correlates well with the decrease of the electronic transport numbers and can be explained by two competing effects: (1) lowering of the top of the valence band due to replacement of O2- ions by F-; and (2) changes in the local structure, i.e., lattice contraction and tilting of the InO(F)x framework. Hydration of the barium indate also leads to an increase of Eg, which was attributed to structural transformation from orthorhombic symmetry to tetragonal. However, with an increase in the fluorine concentration such changes of Eg become less pronounced because of the decrease of the hydration degree due to formation of -defects.

Water Uptake and Transport Properties of La1-xCaxScO3-α Proton-Conducting Oxides.

In this study, oxide materials La1-xCaxScO3-α (x = 0.03, 0.05 and 0.10) were synthesized by the citric-nitrate combustion method. Single-phase solid solutions were obtained in the case of calcium content x = 0.03 and 0.05, whereas a calcium-enriched impurity phase was found at x = 0.10. Water uptake and release were studied by means of thermogravimetric analysis, thermodesorption spectroscopy and dilatometry. It was shown that lower calcium content in the main phase leads to a decrease in the water uptake. Conductivity was measured by four-probe direct current (DC) and two-probe ascension current (AC) methods at different temperatures, pO2 and pH2O. The effects of phase composition, microstructure and defect structure on electrical conductivity, as well as correlation between conductivity and water uptake experiments, were discussed. The contribution of ionic conductivity of La1-xCaxScO3-α rises with decreasing temperature and increasing humidity. The domination of proton conductivity at temperatures below 500 °C under oxidizing and reducing atmospheres is exhibited. Water uptake and release as well as transport properties of La1-xCaxScO3-α are compared with the properties of similar proton electrolytes, La1-xSrxScO3-α, and the possible reasons for their differences were discussed.

Effect of proton uptake on the structure of energy levels in the band-gap of Sr-doped LaScO3: diffuse reflectance spectroscopy and coherent potential approximation calculations.

Features of the energy levels in the band-gap of La1-xSrxScO3-x/2 and the effect on those levels of proton uptake from H2 and H2O atmospheres were studied by diffuse reflectance spectroscopy and coherent potential approximation (CPA) calculations. It was shown that oxygen vacancies appearing due to acceptor doping with Sr form energy levels near the bottom of the conduction band that are strongly hybridized with the states of the nearest atoms. Excitation of electrons from the valence band to these vacancy levels gives rise to an additional absorption band which overlaps with the fundamental absorption edge. Proton incorporation from both H2 and H2O atmospheres leads to formation of proton levels below the valence band. However, during H2 uptake, electrons from hydrogen atoms occupy oxygen vacancy levels, and as a result additional absorption in the red-IR range appears due to electronic transitions from these levels to the conduction band. On the other hand, H2O uptake leads to the disappearance of oxygen vacancy levels. Experimental results obtained are similar to literature data on the optical absorption properties of some other proton-conducting perovskites, allowing the conclusion that findings from these CPA calculations on the nature of the energy levels could be extrapolated to some extent to those oxides.

Oxygen surface exchange and diffusion in Pr1.75Sr0.25Ni0.75Co0.25O4±Î´.

Oxygen surface exchange and diffusion in Pr1.75Sr0.25Ni0.75Co0.25O4±Î´ have been investigated using two methods: pulsed isotope exchange (PIE) and oxygen isotope exchange with gas phase equilibration (IE GPE). Oxygen surface exchange kinetics is considered in the framework of two-step models including two consecutive stages: dissociative adsorption of oxygen and incorporation of oxygen adatoms into the crystal lattice. The rates of oxygen heterogeneous exchange (rH) as well as the rates of dissociative adsorption (ra) and oxygen incorporation (ri) have been calculated. The applicability of the two-step model is discussed based on the concept of a novel two-step mechanism with distributed rates of dissociative adsorption and incorporation of oxygen. It is shown that the two-step model can be applicable for the description of oxygen exchange kinetics in Pr1.75Sr0.25Ni0.75Co0.25O4±Î´ only at temperatures below 750 °C. Above this temperature, only the statistical model with distributed rates can be used. At low temperatures (<750 °C), the oxygen incorporation rate is found to be smaller than the rate of oxygen dissociative adsorption. Thus, under these experimental conditions the stage of oxygen incorporation is considered to be rate-determining. When increasing the temperature, the difference between ra and ri decreases and the stages become competing. The oxygen isotope exchange kinetic profiles obtained using the IE GPE method are found to be complicated and include a surface exchange stage as well as at least two diffusion relaxation processes. The reasons for the existence of these two processes are discussed.