Search results for "medium"
showing 10 items of 3746 documents
XAFS studies of local atomic structure of niobium tellurides
1997
Abstract We present XAFS (K-edge) studies of the local atomic structure around the Nb and Te ions in niobium tellurides (NbTe 2 , Nb 3 Te 4 and NbTe 4 ) in comparison with ZrTe 2 . Using a multi-shell best-fit analysis procedure, we have reconstructed local environments of the Nb and Te ions and compared them with the existing structural models. We found that at the Nb K-edge the XAFS data of NbTe 4 are extremely sensitive to metal clustering and to the subsequent departure from the average crystallographic positions. Moreover, at the Te K-edge the XAFS data allowed us to distinguished the different type of TeTe bonds, for instance: the dimer (TeTe: 2.9 A) and the squares (TeTe: 3.3 A) i…
Computer simulations of electron paramagnetic resonance spectra of P2O5Li2WO4Li2O glasses
1992
Abstract P2O5Li2WO4Li2O glasses have been synthesized and studied over a wide range of compositions. The glasses were characterized mainly by thermodifferential analysis and electron paramagnetic resonance (EPR). The EPR spectra are characterized by the presence of two signals with very different intensities, associated with the Wv (major signal) and Mov (minor signal) paramagnetic centres. The signals have been satisfactorily reproduced by computer simulation, assuming anisotropy in the g values as well as the hyperfine parameters and linewidths. EPR parameters have been extracted and indicate metallic ions located in environments close to square pyramidal.
Electrochemical Detection of High Oxidation States of Chromium( IV and V ) in Chromium‐Doped Cassiterite and Tin‐Sphene Ceramic Pigmenting Systems
2006
Solid-state electrochemistry is applied to detect the presence of chromium centres with high oxidation states in chromium-doped cassiterite and tin-sphene ceramic pigmenting systems. Voltammetric data indicate that CrV and CrIV centres with different coordinative arrangements exist in the studied materials, and yield proton-assisted reduction processes at +0.95, +0.42 AgCl/Ag in contact with 0.50 M H2SO4. Electrochemical measurements are correlated with spectroscopic data. The CrIV and CrV metal ions display a significant light-driven electrocatalytic effect on the oxidation of 1,4-dihydrobenzoquinone and Fe(CN)64– ions that involves the formation of surface-confined adducts. (© Wiley-VCH V…
Structurally simple complexes of CO2
2015
The ability to bind CO2 through the formation of low-energy, easily-broken, bonds could prove invaluable in a variety of chemical contexts. For example, weak bonds to CO2 would greatly decrease the cost of the energy-intensive sorbent-regeneration step common to most carbon capture technologies. Furthermore, exploration of this field could lead to the discovery of novel CO2 chemistry. Reduction of complexed carbon dioxide might generate chemical feedstocks for the preparation of value-added products, particularly transportation fuels or fuel precursors. Implementation on a large scale could help to drastically reduce CO2 concentrations in the atmosphere. However, literature examples of weak…
Incommensurate nature of the multilayered molecular ferromagnetic metals based on bis(ethylenedithio)tetrathiafulvalene and bimetallic oxalate comple…
2004
The salt [ET]x[MnRh(ox)(3)].CH(2)Cl(2) (x = 2.526(1)) has been obtained and characterized. This paramagnetic metal is essentially isostructural to the ferromagnetic metal [ET]y[MnCr(ox)(3)].CH(2)Cl(2) (y approximately equal to 3) and provides a definite answer on the origin of the structural disorder present on such systems. As in the ferromagnetic analogue, this material shows high electrical conductivity at room temperature (13 S.cm(-1)) and metallic behavior.
Chemical modification of porous calcium hydroxyapatite surfaces by grafting phenylphosphonic and phenylphosphite acids
2006
Abstract It is well known that the incorporation of organic molecules can provide an effective route to modify the surface properties of apatite ceramics. The present study shows that phosphonates can react with calcium ions to control the formation of the hydroxyapatite structures. The grafting of C6H5PO(OH)2 (PPOH) and C6H5PO(OH)H (PPH) was used to increase the specific surface area as well as the porosity of hydroxyapatite materials. The higher specific surface area for HAp treated by PPOH is around 267 m2/g compared to the pure HAp (140 m2/g). The phenyl (C6H5) groups are removed by thermal treatment around 500 °C. After treatment at 800 °C, organoapatites (PP-HAp) are converted to stoi…
Understanding the microscopic origin of gold nanoparticle anisotropic growth from molecular dynamics simulations.
2013
We use molecular dynamics simulations in order to understand the microscopic origin of the asymmetric growth mechanism in gold nanorods. We provide the first atomistic model of different surfaces on gold nanoparticles in a growing electrolyte solution, and we describe the interaction of the metal with the surfactants, namely, cetyltrimethylammonium bromide (CTAB) and the ions. An innovative aspect is the inclusion of the role of the surfactants, which are explicitly modeled. We find that on all the investigated surfaces, namely, (111), (110), and (100), CTAB forms a layer of distorted cylindrical micelles where channels among micelles provide direct ion access to the surface. In particular,…
Design of molecular materials combining magnetic, electrical and optical properties †
2000
The possibilities offered by hybrid functional materials formed by two molecular networks in the context of crystal engineering are illustrated with two different examples: (i) hybrid magnets constructed from combination of an extended ferromagnetic or ferrimagnetic inorganic network, with a molecular paramagnetic metal complex acting as template. (ii) Hybrid organic–inorganic compounds combining an organic π-electron donor network that furnishes the pathway for electronic conductivity, with inorganic metal complexes that act as structural and/or magnetic components. These examples illustrate how this hybrid approach allows the design of molecular materials combining non-conventional magnet…
An Inorganic Double Helix Sheathing Alkali Metal Cations: ANb2P2S12(A=K, Rb, Cs), A Series of Thiophosphates Close to the Metal-Nonmetal Boundary-Cha…
2004
The new quaternary niobium thiophosphates ANb 2 P 2 S 1 2 (A=K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A 2 S, P 2 S 5 , and S at 600-700°C in evacuated silica tubes. They crystallize as "stuffed" variants of the tetragonal TaPS 6 structure type in the tetragonal space group I42d with eight formula units per unit cell and lattice constants a=15.923(2) and c=13.238(3) A for CsNb 2 P 2 S 1 2 , a= 15.887(3) and c=13.132(3) A for RbNb 2 P 2 S 1 2 , and a=15.850(2) and c= 13.119(3) A for KNb 2 P 2 S 1 2 . Their structures are based on double helices formed from interpenetrating, noninteracting spiral chains of binuclear [Nb 2 S 1 2…