[en] Arsenic and antimony hexafluoride salts have played an important role in the history of both solution and solid-state NMR spectroscopy. Here, solid polycrystalline KAsF₆ and KSbF₆ have been studied via high-resolution variable temperature ¹⁹F, ⁷⁵As, ¹²¹Sb, and ¹²³Sb solid-state NMR spectroscopy at high magnetic field (B₀ = 21.14 T). Both KAsF₆ and KSbF₆ undergo solid-solid phase transitions at approximately 375 and 301 K, respectively. We use variable temperature NMR experiments to explore the effects of crystal structure changes on NMR parameters. C_Q(⁷⁵As) values for KAsF₆ at 293, 323, and 348 K are -2.87 ± 0.05 MHz, -2.58 ± 0.05 MHz, and -2.30 ± 0.05 MHz, respectively, the sign determined via DFT calculations. In the higher temperature cubic phase, C_Q(⁷⁵As) = 0 Hz, consistent with the crystal symmetry at the arsenic nucleus in this phase. In contrast, C_Q values for ¹²¹Sb and ¹²³Sb in the cubic phase of KSbF₆ are nonzero. E.g., at 293 K, C_Q(¹²¹Sb) = 6.42 ± 0.10 MHz, and C_Q(¹²³Sb) = 8.22 ± 0.10 MHz. In the higher temperature tetragonal phase (343 K) of KSbF₆, these values are 3.11 ± 0.20 MHz and 4.06 ± 0.20 MHz, respectively. CASTEP calculations performed on the cubic and tetragonal structures support this trend. Isotropic indirect spin-spin coupling constants are ¹J(⁷⁵As,¹⁹F) = -926 ± 10 Hz (293 K) and -926 ± 3 Hz (348 K), and ¹J(¹²¹Sb,¹⁹F) = -1884 ± 3 Hz (293 K), and -1889 ± 3 Hz (343 K). Arsenic-75 and antimony-121,123 chemical shift values show little variation over the studied temperature ranges. (author)

[en] Natural contamination of nitrate, fluoride, arsenic and dissolved salts in ground water sources is the main health menace at present in different parts of Pakistan. The metalloids especially arsenic, fluoride and nitrate pose severe health hazards to human being. The present research work investigated the removal techniques for arsenic, fluoride and nitrate from drinking water by adsorption process. Ion exchange resins, activated carbon and activated alumina were used for removal of selected contaminants. These adsorbents were evaluated by comparing their removal efficiency as well as requisite operator skills. The result of activated alumina was found good as compared to activated carbon, mix bed resins and ion exchange resins (IRA-400) for maximum removal of arsenic, nitrate and fluoride. The removal efficiency of arsenic, fluoride and nitrate were found 96%, 99%, 98% respectively in case of activated alumina. The advantage of adsorption process is easy to use and relatively cheaper as compared to other treatment methodologies. (author)

[en] The quasi one-dimensional organic conductor (TMTTF)_2AsF_6 shows the charge ordering transition at T_C_O=101 K to a state of the ferroelectric Mott insulator which is still well conducting. We present and interpret the experimental data on the gigantic dielectric response in the vicinity of T_C_O, concentrating on the frequency dependence of the inverse 1/ε of the complex permittivity ε=ε′+iε′′. Surprisingly for a ferroelectric, we could closely approach the 2nd order phase transition and to deeply reach the critical dynamics of the polarization. We could analyze the critical slowing-down when approaching T_C_O from both sides and to extract the anomalous power law for the frequency dependence of the order parameter viscosity. Moreover, below T_C_O we could extract a sharp absorption feature coming from a motion of domain walls which shows up at a frequency well below the relaxation rate

[en] Reactions of tellurium with metal pentafluorides MF${}_5$ (M=As, Sb, Nb, Ta) in liquid SO${}_2$ under supercritical conditions at 160 °C yield (Te${}_7$)[MF${}_6$]${}_2$ containing the polymeric Zintl cation (Te${}_7$${}^{2+}$)${}_n$ consisting of chair-shaped Te${}_6$ rings connected via single Te atoms. Crystal structure analyses show all four compounds to crystallize isotopically in the (Te${}_7$)[AsF${}_6$]${}_2$ structure type. Monitoring the reactions via Raman spectra reveal that (Te${}_4$)[MF${}_6$]${}_2$ is formed as an intermediate, which reacts with Te finally to (Te${}_7$)[MF${}_6$]${}_2$. Crystal structures of (Te${}_4$)[MF${}_6$]${}_2$ (M=Nb, Ta) were determined. Microwave assisted solvothermal synthesis give analogous results in very short reaction time. All compounds are also accessible via electrochemical oxidation of Te in presence of [MF${}_6$]${}^{-<!--\; ???\; -->}$ as electrolyte. (Te${}_7$)[AsF${}_6$]${}_2$ is a semiconductor with σ = 10${}^{-<!--\; ???\; -->7}$ S/cm at 300 K and an energy for the thermally activated electron process of 0.91 eV. Theoretical calculations on the self-consistent hybrid DFT level confirm that the fundamental band gap is independent from the kind of M. The frontier orbitals are predominantly composed of Te states. Raman bands at 196, 178, 153, and 138 cm${}^{-<!--\; ???\; -->1}$ are specific for (Te${}_7$${}^{2+}$)${}_n$. Phonon calculations of (Te${}_7$)[TaF${}_6$]${}_2$ confirm three intensive Raman bands at 210, 174 and 163 cm${}^{-<!--\; ???\; -->1}$. The corresponding normal modes are mainly localized on (Te${}_7$)${}_n$. (© 2023 The Authors. Zeitschrift für anorganische und allgemeine Chemie published by Wiley-VCH GmbH)

[en] A novel graphene-based nanocomposite membrane was synthesized by interfacial polymerization (IP) through chemical bonding of the graphene oxide (GO) layer to polyethersulfone surface. Detailed characterization of the composite membrane through AFM, SEM, ATR-FTIR, XRD analysis, and Raman spectroscopy indicates strong potential of the membrane in highly selective removal of the toxic contaminants like arsenic and fluoride while permeating the essential minerals like calcium and magnesium. This makes the membrane suitable for production of safe drinking water from contaminated water. The membrane applied in a flat-sheet cross-flow module succeeded in removal of more than 98% arsenic and around 80% fluoride from contaminated water while selectively retaining the useful calcium and magnesium minerals in drinking water. A sustained pure water flux of around 150 LMH (liter per square meter per hour) during operation over long hours (> 150 h) with only 3–5% drop in flux indicates antifouling character of the membrane module.

[en] A major public health concern in Mexico is the natural contamination of groundwater with fluoride and arsenic. Therefore, this work aimed to evaluate the magnitude of human health risk after determining fluoride and arsenic concentrations in groundwater samples (n = 50) from the Metropolitan area of the city of San Luis Potosi, Mexico. Fluoride levels in water were determined via a potentiometric method using an ion-selective electrode. Arsenic concentrations in water samples were determined with an Atomic Absorption technique. Subsequently, a probabilistic health risk assessment was developed (Monte Carlo Analysis). Fluoride levels in water ranged from 0.20 to 3.50 mg/L. For arsenic, the mean level found in the assessed water samples was 15.5 ± 5.50 μg/L (range: 2.50–30.0 μg/L). In addition, when the probabilistic health risk assessment was completed, a mean HI (cumulative hazardous index) of higher than 1 was detected, indicating a high NCR (non-carcinogenic risk) for children and adults. According to the results found in this study, exposure protection campaigns are imperative in the Metropolitan area of the city of San Luis Potosí, Mexico, to successfully diminish exposure to arsenic and fluoride and, as a consequence, decrease the NCR in the population living in that region of Mexico.

[en] Spatial phase separation into mesoscopic domains of non-magnetic [superconducting (SC) below T_c ≅ 25.5 K] and two kinds of magnetic phases, one showing disordered spin density wave (d-SDW) order and another associated with glassy weak magnetism (WM), are observed below ∼100 K by muon spin rotation (μSR) in LaFeAsO_1−xF_x for x = 0.057(3), which is near the boundary of these phases on the doping phase diagram. In contrast to the competing order observed in the regular SDW phase of Ba(Fe_1−xCo_x)₂As₂, the WM domain exhibits cooperative coupling of superconducting and magnetic order parameters as inferred from strong diamagnetism and the associated negative shift of μSR frequency just below T_c. (author)

[en] While the high transition temperatures suggest that the conventional BCS phononmediated mechanism may not provide the main pairing mechanism in the recently discovered RFeAsO_1−xF_x (1111-type) superconductors, there is, as yet, no consensus, despite extensive experimental and theoretical study. We report here the results of an inelastic neutron scattering investigation of an overdoped polycrystalline sample of LaFeAsO_1−xF_x with x = 0.15 (T_c = 26 K). Four excitation peaks were observed at 13.6±1.5, 24.2±0.8, 32.2±0.5, and 41.4±1.0 meV. They were identified as phonon modes based on their wavevector and temperature dependence. The peak positions agree well with first-principles calculations of phonon density of states as well as experimental data on both the insulating parent and optimally doped LaFeAsO_1−xF_x compounds. No evidence for the presence of a resonance mode was found. We found that the phonon density of states of the x = 0.15 sample remains unchanged below T_c and is similar to samples with other fluorine concentrations. This suggests that a standard electron-phonon pairing mechanism cannot explain the high transition temperatures observed in these materials.

No abstract available

[en] Low-energy spin excitations on polycrystalline LaFeAsO_1−xF_x samples have been studied by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ⁽ω) at 11 meV decreases with increasing fluorine content x across the phase transition from antiferromagnetic to superconducting phases. For superconducting samples, a peak in χ⁽ω) develops at ω = E_res below T_c, accompanied by its reduction below E_res. Fe 3d orbital contribution on the dynamical spin susceptibility and the electron scattering between Γ and M Fermi surfaces are discussed.