Submicron-structured Li3V2(PO4)3/C has been synthesized by carbon-thermal reduction method with poly(acrylic acid) and d-(+)-glucose as carbon sources. The pure monoclinic structure of Li3V2(PO4)3/C ...has been confirmed by XRPD and Rietveld refinement, scanning electron microscopy, and transmission electron microscopy. The specific capacity of Li3V2(PO4)3/C as cathode has been evaluated, between 3.0 and 4.3 V vs Li+/Li, in the two different electrolytes: 1 M solution of LiPF6 in EC:DMC (1:1) at room temperature and in EC:DMC:DEC (1:1:1) at low temperature (0 and −20 °C). Because of the submicron-size dimensions of Li3V2(PO4)3 and of the good carbon coating, the material discharge capacities as high as 103, 98 and 81 mAh g−1 at 40, 60 and 100C rates, retains 93% of the initial discharge capacity after 1000 cycles at 100C. The performances even at low temperature are also good with values of the order of 84, 40, 23, 19 mAh g−1 at 0 °C and 69, 24, 19, 15 mAh g−1 at −20 °C at 3, 7.5, 15, 30 C-rates, respectively.
•Li3V2(PO4)3/C obtained by using poly(acrylic acid) and glucose as carbon sources.•Poly(acrylic acid) is a good dispersing agent to obtain sub-micron particles.•High specific discharge capacity has been obtained at high C-rates.•High cycle stability and long cycle life displayed over 1000 cycles at 100C.•Excellent performances at low temperatures (0, −20 °C) and high C-rates (30C).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
An X-ray spectroscopy and theoretical study of the chemical state of several sulfur bearing minerals and a synthetic sodium sulfite sample was performed. X-ray absorption and high-resolution Kα X-ray ...emission spectra were recorded and compared to ab initio quantum chemical calculations. A consistent interpretation of the chemical shift in the Kα emission spectra is obtained based on three different theoretical approaches (density functional theory, multiple scattering theory, and atomic multiplet theory). An analysis of the theoretical sulfur orbital population and valence bond is in agreement with the fluorescence energy position of the Kα lines even within the sulfide (S2−) series. It is shown that the Kα energy shifts can be used for a quantitative determination of the proportion of different sulfur species in heterogeneous samples.
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IJS, KILJ, NUK, PNG, UL, UM
Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. ...Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe
3+
/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870–0.994 O3(O
2−
) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of
a
H
2
O (0.014–0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of
a
H
2
O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between −2.52 and −1.32 log units below the fayalite–magnetite–quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine–orthopyroxene–spinel equilibria (
f
O
2
between −1.98 and −0.30 log units). A comparative evaluation of the two methods suggests that when amphibole is present in mantle peridotites, the application of oxy-amphibole equilibrium is preferred, because ol–opx–sp oxy-calibrations are not “sensitive” enough in recording the effects (if any) of amphibole in the peridotite matrix. Amphibole acts as the main H acceptor among the peridotite minerals and may prevent fluid circulation and buffer oxygen fugacity. The important conclusion of this study is that amphibole within the lithospheric mantle does not always means high water activity and oxidizing conditions.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The structural environment of rare earth elements (REE) in synthetic silicate glasses of Na2Si2O5 composition has been characterized through extended X-ray absorption fine structure spectroscopy ...(EXAFS) at the respective rare-earth LIII-edges, in order to obtain oxidation state, coordination number and <REE–O> bond distances. In these glasses, synthesised in air and at room pressure, the studied lanthanide ions are in the trivalent state. The EXAFS-determined <REE–O> distances decrease with an increase in atomic number of the rare-earth ions. The lanthanides are all octahedrally coordinated, with a well-defined first coordination sphere around rare-earth ions, and with no indication of a longer-range order. To ascertain the possible presence of REE–REE clustering, several EXAFS fits have also been performed starting from different model structures including REE also in the second coordination shell. No evidence has been found in these glasses of REE–O–REE bridges.
► We determined the local structure around REE in sodium disilicate glasses. ► REE3+ ions have octahedral geometry consistent with the analogous crystalline oxides. ► REE show no clear clustering tendency in sodium disilicate glasses. ► The <REE–O> distances vary in agreement with the lanthanide contraction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Iron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic ...composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydrothermal vessels at 800°C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units. Comparison of the pre-edge peak data with those of Fe model compounds of known oxidation state and coordination number allowed determination of the Fe oxidation state and coordination number in all glasses analyzed. Within each group of samples, Fe tends to oxidize with increasing oxygen fugacity as expected. However, alkali content is shown to have a strong effect on the Fe3+/(Fe3++Fe2+) ratio at constant oxygen fugacity: this ratio varies from 0.25 to 0.55 (±0.05) for the least peralkaline series, and from 0.45 to 0.80 (±0.05) for the most peralkaline series. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination in the most peralkaline glasses. Extrapolation of pre-edge peak data suggests the presence of both fourfold and fivefold coordination for trivalent Fe in the other two series. Divalent Fe is suggested to be mainly in fivefold coordination in all the three glass series. The presence of minor amounts of sixfold- and fourfold-coordinated Fe cannot be ruled out by XANES data alone. XANES data suggest that the amount of alkalis also affects the Fe3+ coordination environment resulting in a decrease in the average coordination numbers. Extended X-ray absorption fine structure (EXAFS) data of the most oxidized and peralkaline sample indicate that Fe3+ is in tetrahedral coordination with =1.85 Å (±0.02). This value compares well with literature data for 4Fe3+ in crystalline phases (e.g., in tetra-ferriphlogopite or rodolicoite) or in silicate glasses (e.g., phonolite glasses) supporting the XANES-determined coordination number obtained for the most peralkaline glasses. Calculated NBO/T ratios decrease slightly with Fe oxidation because of the higher fraction of network forming Fe, thus increasing the polymerization of the tetrahedral network.
The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy ...(XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the Kβ emission to the cation and the local symmetry is discussed.
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The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) ...spectroscopy in order to provide quantitative data on <Fe-O> distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean <Fe-O> distances and Fe coordination numbers for the three tektite samples are constant within the estimated error (<Fe-O > =2.00 Å ± 0.02 Å, CN = 4.0 ± 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 × 1.92 Å and 2 × 2.08 Å). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5.
Comparison of the tektites XANES spectra with those of a suite of different impact glasses clearly shows that tektites display a relatively narrow range of Fe oxidation state and coordination numbers, whereas impact glasses data span a much wider range of Fe oxidation states (from divalent to trivalent) and coordination numbers (from tetra-coordinated to esa-coordinated). These data suggest that the tektite production process is very similar for all the known strewn fields, whereas impact glasses can experience a wide variety of different temperature–pressure–oxygen fugacity conditions, leading to different Fe local structure in the resulting glasses. These data could be of aid in discriminating between tektite-like impact glasses and impact glasses sensu strictu.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Europium LIII-edge XAS spectra were recorded for silicate glasses of different compositions, quenched from melts equilibrated at different oxygen fugacity (fO2). The Eu XANES spectra vary ...systematically with glass composition and with fO2 (-log fO2 approximately 0 to approximately 11.9) indicating changes in the Eu oxidation state. The intensity of the main peaks on the absorption edges were quantified and used to determine the Eu2+/(Eu2++Eu3+) ratio. All the Eu-bearing glasses synthesized in air show the prevalent presence of Eu3+ but also, unexpectedly, the presence of a small amount of Eu2+ in the basaltic glasses and up to 20% of Eu2+ in the haplogranitic sample. Moreover, XANES analyses of the samples synthesized at reducing conditions (from FMQ to IW-2) show that europium in haplogranitic glasses is always more reduced than in basaltic glasses. No relationship has been found between Eu valence and alkali content in the studied glasses. The structural environment of Eu in the glasses was determined by EXAFS analyses, demonstrating the different Eu behavior as function of the fO2. In fact, in air, Eu3+ both for basaltic and haplogranitic compositions, is bonded to six O atoms in a regular octahedron (CN=6±0.5) with similar distances of about 2.30±0.02 Å. On the other hand, the almost purely divalent samples have Eu2+ in a higher coordination (CN=9±1) and longer distances (2.68±0.02 Å). This work clearly demonstrates that, in addition to oxygen fugacity, melt composition also plays a strong role in affecting Eu oxidation state. Moreover, for the first time, experimentally derived structural data of Eu2+ in silicate glasses of geological interest are presented.
The understanding of redox equilibria as well as the knowledge of the elemental distribution in magmatic melts are of fundamental importance to constrain the genesis of magmas. In particular, the ...partitioning of trace elements (e.g., Eu) has demonstrated to be a useful tool for estimating the redox conditions in Earth and planetary materials. However, for a more complete comprehension of Eu in silicate melts, information regarding the effects of temperature (T), redox conditions, compositions, and the possible interference of other multivalent elements is still lacking. Here we provide new data on the oxidation states of two commonly coexistent multivalent elements (Eu and Fe) in melts, acquired by "in situ" dispersive X-ray absorption spectroscopy experiments at high temperatures and at different oxygen fugacity conditions. This work, for the first time, shows the possibility to monitor in real-time the behavior and valence variations of two elements under varying environmental conditions (like T and redox state).
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