Study of the Microstructure of Amorphous Silica Nanostructures Using High-Resolution Electron Microscopy, Electron Energy Loss Spectroscopy, X-ray Powder Diffraction, and Electron Pair Distribution Function

Silica has many industrial (i.e., glass formers) and scientific applications. The understanding and prediction of the interesting properties of such materials are dependent on the knowledge of detailed atomic structures. In this work, amorphous silica subjected to an accelerated alkali silica reaction (ASR) was recorded at different time intervals so as to follow the evolution of the structure by means of high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and electron pair distribution function (e-PDF), combined with X-ray powder diffraction (XRPD). An increase in the size of the amorphous silica nanostructures and nanopores was observed by HRTEM, which was accompanied by the possible formation of Si–OH surface species. All of the studied samples were found to be amorphous, as observed by HRTEM, a fact that was also confirmed by XRPD and e-PDF analysis. A broad diffuse peak observed in the XRPD pattern showed a shift toward higher angles following the higher reaction times of the ASR-treated material. A comparison of the EELS spectra revealed varying spectral features in the peak edges with different reaction times due to the interaction evolution between oxygen and the silicon and OH ions. Solid-state nuclear magnetic resonance (NMR) was also used to elucidate the silica nanostructures. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Synthesis, structure and magnetic properties of the new oxygen-deficient perovskite Na₃MnV₂O_7.5

The new ternary vanadate Na3MnV2O7.5 has been synthesized by solid state reaction route. Its crystal structure was determined from both powder X-ray diffraction data and high-resolution transmission electron microscopy (HRTEM) analysis. Na3MnV2O7.5 (P2/c, a = 7.4004(4) Å, b = 5.6065(3) Å, c = 17.0876(11) Å, β = 90.0114(2) °, and Z = 4) crystallizes with a new type of oxygen-deficient perovskite ABO3-δ (δ = 1/2) with the sodium atoms occupying the A site and the vanadium and manganese atoms perfectly ordered in the B site. In Na3MnV2O7.5 the manganese and the vanadium atoms are octahedral and tetrahedral coordinated, respectively. The MnO6 and VO4 polyhedra share edges and form a perovskite 3D-framework with high disorder of the oxygen atoms. HRTEM confirmed this structural model. Na3MnV2O7.5 was characterized by magnetic susceptibility and specific heat measurements. It is antiferromagnetic with no magnetic ordering down to 2 K

First Principles Study on the Features of CaxSr2−xTa2O7 (x = 0, 1) as a Photocatalytic Material

With hydrogen as one of the energetic vectors craved for use in the future, the successful de-carbonization of the energy sector will require an increase in hydrogen production from renewable resources. Materials that are able to catalyze the water-splitting reaction through sunlight absorption have been widely studied as an adequate solution for green hydrogen generation. Among the proposed tantalum-based oxide materials, Sr2Ta2O7 displays moderate photocatalytic activity. Aiming to improve the photocatalytic properties by means of compositional modifications, this work presents a DFT study of the Sr substitution with Ca. The structural, energetic, and electronic features of the phases of CaxSr2−xTa2O7 (0 < x < 1) have been examined. The computational results utilizing the SCAN functional show that there is a slight decrement in the band gap value (from 3.65 eV for x = 0 to 3.50 eV for x = 1) concomitant to a minor distortion of the crystal structure

First Principles Study on the Features of Ca_xSr_2−xTa₂O₇ (x = 0, 1) as a Photocatalytic Material

With hydrogen as one of the energetic vectors craved for use in the future, the successful de-carbonization of the energy sector will require an increase in hydrogen production from renewable resources. Materials that are able to catalyze the water-splitting reaction through sunlight absorption have been widely studied as an adequate solution for green hydrogen generation. Among the proposed tantalum-based oxide materials, Sr2Ta2O7 displays moderate photocatalytic activity. Aiming to improve the photocatalytic properties by means of compositional modifications, this work presents a DFT study of the Sr substitution with Ca. The structural, energetic, and electronic features of the phases of CaxSr2−xTa2O7 (0 < x < 1) have been examined. The computational results utilizing the SCAN functional show that there is a slight decrement in the band gap value (from 3.65 eV for x = 0 to 3.50 eV for x = 1) concomitant to a minor distortion of the crystal structure

Structure evolution with Sr content of the perovskite-like materials La2-xSrxCoTiO6 (0 ≤x≤ 0.5)

The oxide series La2-x Sr x CoTiO6 (0 ≤ x ≤ 1.0) belong to the perovskite family with general formula ABO3. The evolution of the room-temperature structure as a function of the Sr content was studied using complementary techniques by applying the symmetry-adapted modes formalism (AMPLIMODES). In the compositional range presented in this article (0 ≤ x ≤ 0.5), the compounds adopt distorted perovskite structures of monoclinic (space group P21/n) or orthorhombic (space group Pnma) symmetry, both with octahedral tilting scheme (a - a - c +) (out of phase along two perovskite main directions and in phase along the third direction). The main difference between these structures is the existence of rock-salt order of B ions in the monoclinic symmetry, which is lost for x ≥ 0.30. As the Sr content increases, a better matching of the A - O and B - O distances occurs. This is produced by an elongation of the A - O distance as La3+ is replaced by the larger ion Sr2+, and the shortening of the B - O distance due to the oxidation of Co2+ to Co3+ induced by the aliovalent substitution. As a result, the cuboctahedral A-site cavity becomes less and less distorted; the A ion tends to occupy its ideal positions, increasing its coordination and giving rise to a more symmetrical structure. In the whole compositional range, the symmetry-adapted atomic displacements (modes) responsible for the out-of-phase tilting of the BO6 octahedra remain active but those associated with the in-phase tilting become negligible, anticipating for x ≥ 0.6 a transition to a new structure with tilting scheme either (a 0 a 0 c -) (space group I4/mcm) or (a - a - a 0) (space group Imma) or (a - a - a -) (space group R3c). © 2014.We thank the Spanish Ministerio de Ciencia e Innovación and Comunidad de Madrid for funding the projects MAT2010-19837-C06-01 and S-2009/PPQ-1626, respectively. Financial support from Universidad CEU San Pablo is also acknowledged. We acknowledge the Spanish Ministerio de Ciencia e Innovación and Consejo Superior de Investigaciones Científicas for financial support.Peer Reviewe