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NbAs is a Weyl semimetal and belongs to the group of topological phases that exhibit distinct quantum and topological attributes. Topological phases have a fundamentally different response to external perturbations, such as magnetic fields. To obtain insights into the response of such phases to pressure, we conducted a comprehensive study on the pressure-induced electronic and structural transitions in NbAs. We used micro-X-ray diffraction (XRD) and micro-X-ray spectroscopy (XAS) techniques to elucidate the changes at different atomic and electronic length scales (local, medium, and bulk) as combined with theoretical calculations. High-pressure XRD measurements revealed a rather common compression behavior up to ~12 GPa that could be fitted to an equation of state formalism with a bulk modulus of K 0 = 179.6 GPa. Complementary Nb K-edge XAS data unveiled anomalies at pressure intervals of ~12–15 and ~25–26 GPa in agreement with previous literature data from XRD studies. We attribute these anomalies to a previously reported topological Lifshitz transition and the tetragonal-to-hexagonal phase transition, respectively. Analysis of EXAFS results revealed slight changes in the mean next-nearest neighbor distance Nb–As(1) (~2.6 Å) at ~15 GPa, while the second nearest neighboring bond Nb–Nb(1) (~3.4 Å) shows a pronounced anomaly. This indicates that the electronic changes across the Lifshitz transition are accommodated first in the medium-range atomic structure and then at the local range and bulk. The variances of these bonds show anomalous but progressive evolutions close to the tetragonal-to-hexagonal transition at ~25 GPa, which allowed us to derive the evolution of vibration properties in this material. We suggest a prominent displacive character of the I 4 1 m d → P 6 ¯ m 2 transition facilitated by phonon modes. [ABSTRACT FROM AUTHOR]

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above ~ 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) is a powerful technique to investigate the short-range order around selected atomic species in condensed matter. The theoretical framework and previous applications to undercooled elemental liquid metals are briefly reviewed. Specific results on undercooled liquid Ni obtained using a peak fitting approach validated on the spectra of solid Ni are presented. This method provides a clear evidence that a signature from close packed triangular configurations of nearest neighbors survives in the liquid state and is clearly detectable below k ≈ 5 Å−1, stimulating the improvement of data-analysis methods that account properly for the ensemble average, such as Reverse Monte Carlo.

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO2 heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO2 nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO2 photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO2 due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO2 nanoparticles. [ABSTRACT FROM AUTHOR]

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO₂ heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO₂ nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO₂ photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO₂ due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO₂ nanoparticles.