Spatial Distribution Of Cations Research Articles

Spatial distribution of cations through Voronoi polyhedrons and their exchange between polyhedrons in sodium silicate liquids

We study the spatial distribution of cations (Si and Na) through Voronoi polyhedrons around O and their exchange between polyhedrons in molecular dynamics models of sodium-silicate liquids. The result shows that most Na atoms reside in polyhedrons with specific configurations of Si and Na numbers. The fraction of polyhedrons with the same Si and Na index varies strongly with SiO2 content of the liquid. The average volume for polyhedrons with Si = 0, 1 and Na = 1, 2, 3 is found to vary from 31.78 to 35.09 Å3, and is smaller than 27.27 Å3 for the ones with Na = 0. The observed very fast diffusivity of sodium is caused by high rate of Na exchange between polyhedrons. The Na exchange is realized by two ways: 1) hopping between nearest neighbor polyhedrons and 2) collective displacement of a group of Na along a chain of polyhedrons. Within the time scale of the simulation the structure is strongly heterogeneous with a separate diffusion pathway where the majority of Na move, and a sodium-free silica domain. By analyzing the flow of Na and density of Na, Si, O in a fixed cubic lattice we find the exhibition of dynamics and chemical heterogeneity in the liquid.

Structure and magnetic behavior of sol-gel grown spinel NixMn3-xO4 nanoparticles: Effect of Ni fraction and induction of superparamagnetism at room temperature

Spinel NixMn3-xO4 nanostructures have attracted a huge research interest due to their applications in catalysis, electrocatalysis, high-performance supercapacitors, lithium-ion batteries, among others. However, those applications strongly depend on the Ni:Mn ratio, valence states and spatial distribution of cations in the spinel lattice. Using a low-temperature sol-gel process, we synthesized NixMn3-xO4 nanoparticles of 14–26 nm average sizes with different Ni contents. Effects of Ni incorporation on the morphology, structure and magnetic properties of the nanostructures were studied by SEM, XRD, XPS, Raman spectroscopy and VSM. While a high Ni content in the spinel nanostructures causes lattice shrinkage, lattice expansion at lower Ni content occurs due to higher occupancy of Mn2+ ions in tetrahedral sites. Structural and magnetic behaviors of the nanostructures were explained by considering the crystal field stabilization energies of the cations. We demonstrate that superparamagnetic behavior can be induced in the NixMn3-xO4 nanostructures by incorporating Ni at x>1.0.

LiCaFeF6: A zero-strain cathode material for use in Li-ion batteries

A new zero-strain LiCaFeF6 cathode material for reversible insertion and extraction of lithium ions is presented. LiCaFeF6 is synthesized by a solid-state reaction and processed to a conductive electrode composite via high-energy ball-milling. In the first cycle, a discharge capacity of 112 mAh g−1 is achieved in the voltage range from 2.0 V to 4.5 V. The electrochemically active redox couple is Fe3+/Fe2+ as confirmed by Mössbauer spectroscopy and X-ray absorption spectroscopy. The compound has a trigonal colquiriite-type crystal structure (space group P3¯1c). By means of in situ and ex situ XRD as well as X-ray absorption fine structure spectroscopy a reversible response to Li uptake/release is found. For an uptake of 0.8 mol Li per formula unit only minimal changes occur in the lattice parameters causing a total change in unit cell volume of less than 0.5%. The spatial distribution of cations in the crystal structure as well as the linkage between their corresponding fluorine octahedra is responsible for this very small structural response. With its zero-strain behaviour this material is expected to exhibit only negligible mechanical degradation. It may be used as a cathode material in future lithium-ion batteries with strongly improved safety and cycle life.

Hydrothermal assisted synthesis and photoluminescence of (Y1-xEux)2WO6 red phosphors

Phase-pure (Y1-xEux)2WO6 (x = 0.2–0.11) red phosphors (average crystallite size ∼66 nm) in a monoclinic structure have been calcined at 1300 °C from their precursors autoclaved from the mixed solutions of rare-earth nitrate and sodium tungstate dihydrate (Na2WO4·2H2O) at 180 °C and pH∼10, instead of the commonly adopted pH values of 3–7, without the use of any surfactant. The materials were characterized in detail by the combined techniques of XRD, FT-IR, FE-SEM, TEM, EDS, and optical spectroscopy to reveal (1) the phase structure, morphology, and spatial cation distribution of the hydrothermal product, (2) the course of phase and morphology evolution that leads to the targeted tungstate phosphor, and (3) the effects of Eu3+ content on photoluminescence properties, including excitation/emission, asymmetry factor of luminescence (5D0→7F2/5D0→7F1 intensity ratio), fluorescence lifetime, and CIE chromaticity coordinates. Through energy transfer from the [WO6]6− ligands to Eu3+, the (Y1-xEux)2WO6 phosphors exhibit sharp 5D0→7FJ (J = 0–4) emissions upon UV excitation into the O2--W6+ charge transfer band peaked at 327 nm, with the red emission at 611 nm being the most prominent (5D0→7F2 transition of Eu3+). The optimal Eu3+ content was determined to be ∼9 at.% (x = 0.09), and concentration quenching of luminescence was analyzed to be owing to exchange interaction. The emission color moves closer to the red region in the CIE chromaticity diagram with increasing Eu3+ doping, while fluorescence lifetime of the 611 nm emission, analyzed to be around 1.05 ± 0.05 ms, shows weak dependence on the Eu3+ content.

New insights into methane-oxygen ion chemistry

External electric fields may reduce emissions and improve combustion efficiency by active control of combustion processes. In-depth, quantitative understanding of ion chemistry in flames enables predictive models to describe the effect of external electric fields on combustion plasma. This study presents detailed cation profile measurements in low-pressure, burner-stabilized, methane/oxygen/argon flames. A quadrupole molecular beam mass spectrometer (MBMS) coupled to a low-pressure (P=30Torr) combustion chamber was utilized to measure ion signals as a function of height above the burner. Lean, stoichiometric and rich flames were examined to evaluate the dependence of ion chemistry on flame stoichiometry. Additionally, for the first time, cataloging of flame cations is performed using a high mass resolution time-of-flight mass spectrometer (TOF-MS) to distinguish ions with the same nominal mass. In the lean and stoichiometric flames, the dominant ions were H3O+, CH3O2+, C2H7O+, C2H3O+ and CH5O+, whereas large signals were measured for H3O+, C3H3+ and C2H3O+ in the rich flame. The spatial distribution of cations was compared with results from numerical simulations constrained by thermocouple-measured flame temperatures. Across all flames, the predicted H3O+ decay rate was noticeably faster than observed experimentally. Sensitivity analysis showed that the mole fraction of H3O+ is most sensitive to the rate of chemi-ionization CH+O↔CHO++E−. To our knowledge, this work represents the first detailed measurements of positive ions in canonical low-pressure methane flames.

Protein-DNA and ion-DNA interactions revealed through contrast variation SAXS.

Understanding how DNA carries out its biological roles requires knowledge of its interactions with biological partners. Since DNA is a polyanionic polymer, electrostatic interactions contribute significantly. These interactions are mediated by positively charged protein residues or charge compensating cations. Direct detection of these partners and/or their effect on DNA conformation poses challenges, especially for monitoring conformational dynamics in real time. Small-angle x-ray scattering (SAXS) is uniquely sensitive to both the conformation and local environment (i.e. protein partner and associated ions) of the DNA. The primary challenge of studying multi-component systems with SAXS lies in resolving how each component contributes to the measured scattering. Here, we review two contrast variation (CV) strategies that enable targeted studies of the structures of DNA or its associated partners. First, solution contrast variation enables measurement of DNA conformation within a protein-DNA complex by masking out the protein contribution to the scattering profile. We review a specific example, in which the real-time unwrapping of DNA from a nucleosome core particle is measured during salt-induced disassembly. The second method, heavy atom isomorphous replacement, reports the spatial distribution of the cation cloud around duplex DNA by exploiting changes in the scattering strength of cations with varying atomic numbers. We demonstrate the application of this approach to provide the spatial distribution of monovalent cations (Na+, K+, Rb+, Cs+) around a standard 25-base pair DNA. The CV strategies presented here are valuable tools for understanding DNA interactions with its biological partners.

Crystal Structure of Sinhalite MgAlBO4 under High Pressure

We report on high-pressure angle-dispersive X-ray diffraction data up to 27 GPa for natural MgAlBO4 sinhalite mineral and ab initio total energy calculations. The experimental bulk modulus of sinhalite is B0 = 171(3) GPa with a first-pressure derivative of B0′ = 4.2(3). A comparison with other olivine-type compounds shows that the value for B0 is 27% larger than that of Mg2SiO4 forsterite and 29% smaller than that of Al2BeO4 chrysoberyl. These differences are interpreted, on the basis of our ab initio calculations, in terms of the relative incompressibility of Al–O bonds in AlO6 octahedra (with a calculated bulk modulus of 250(1) GPa) as compared to Mg–O bonds in MgO6 octahedra (with a calculated bulk modulus of 130(1) GPa). The spatial cation distribution in the Pbnm orthorhombic unit cell and different polyhedral compressibilities entails a strong anisotropic compression comparable to that of forsterite. The axial compressibilities are 1.06(2) × 10–3, 2.17(2) × 10–3, and 1.30(3) × 10–3 GPa–1 for a, b, and c axes, respectively. The crystal chemistry of sinhalite under compression is compared to that of other olivine-like compounds. Compressibility trends and possible high-pressure phases are discussed.

Growth of Ce-doped LGSO fiber-shaped crystals by the micro pulling down technique

Ce-doped Lu2xGd2−2xSiO5 (LGSO:Ce) fiber-shaped crystals (x = 0.5) were grown by the micro-pulling down (µ-PD) technique. To optimize the activator concentration for achieving the best scintillation parameters, the cerium concentration in the melt was varied in the range from 0.01 to 1.5 at.%. Distributions of Gd3+ and Ce3+ in LGSO:Ce crystals grown by the µ-PD and Czochralski (Cz) techniques were compared. The spatial distribution of cations across the LGSO:Ce scintillation crystals grown by the µ-PD technique is studied using wide-field microscopy under simultaneous excitation of two types of Ce-related centers and confocal microscopy under the selective excitation of Ce3+ in CeO6 crystallographic sites. It is revealed that the fiber-shaped crystals contain a single crystal core surrounded by crystalline material with a higher density of inclusions and cracks that are predominantly directed along the crystal axis. The formation of inclusions and cracks is interpreted by a nonuniform radial distribution of LGSO:CE cations.

Geometric magnetic frustration in RE2O2S oxysulfides (RE = Sm, Eu and Gd)

RE2O2S oxysulfides (with RE=Sm, Eu and Gd) were prepared and characterized regarding their structural and magnetic properties. The compounds crystallized in the trigonal symmetry (space group P-3m/D33d), with the lattice parameter varying linearly with the ionic radius of the RE cation. All these oxysulfides are magnetically frustrated and only the gadolinium sample showed magnetic order down to 3K. The magnetic frustration is attributed to the spatial distribution of cations over the lattice, where the RE’s magnetic moments occupy the sites forming a triangular plane lattice, perpendicular to the direction. This geometric magnetic frustration was firstly recognized for these oxysulfides.

High spatially resolved cation concentration profile at the grain boundaries of Sc-doped BaZrO3

Abstract Recent studies have demonstrated that a higher dopant concentration in the grain boundary region (which includes boundary core and neighboring layers) of acceptor-doped oxides results in a remarkable decrease of grain boundary electrical resistance. In the present work, the spatial distribution of cations in the proton conducting scandium doped barium zirconate is investigated by transmission electron microscopy with high lateral resolution. Cation profiles with a resolution of 0.5 nm obtained using electron energy-loss spectroscopy indicate segregation of a noticeable part of scandium into the grain boundary core. This direct observation is consistent with the measured grain boundary electrical behavior and is interpreted in terms of the space charge model.

A Continuous Symmetry Measure of [4Fe−4S]+ Core Distortions and Analysis of Supramolecular Synthons in Crystal Structures of (Et4N)3[Fe4S4Cl4]·Et4NCl at 100 and 295 K

Distortions of the [4Fe-4S](+) cores of synthetic models from T(d) symmetry are analyzed in terms of the continuous symmetry measures (CSM, S(T(d))), and these are related to lattice effects in terms of the supramolecular synthon terminology common to crystal engineering of small molecule structures. The small tetragonal compression to D(2d) from idealized T(d) symmetry observed at low temperature is attributed to environmental factors. New members of the isomorphous series of compositional variations of double salts of the air-sensitive reduced cluster (Et(4)N)(3)[Fe(4)S(4)Cl(4)].Et(4)NCl (1) are prepared in modest yield by treatment of FeCl(2) with NaHS, or (Et(4)N)(2)[FeCl(4)] with Et(4)NSH and a base. The crystals are isomorphous with the corresponding HS(-) ligated cluster. Crystal data: tetragonal, P42(1)c, Z = 2, a = b = 12.2550 (4), c = 16.278 (1) A at 100 K, and a = b = 12.385 (1), c = 16.344 (2) A at 295 K. The crystallographically imposed S(4) symmetry obtained with sterically unincumbering ligands affords a better view of the intrinsic geometry of the core structure. The cocrystallization of the halide ion affords the opportunity to compare three types of weak C-H...X hydrogen bonds, or hydrogen bridges, between tetraalkylammonium cations and anions within the same crystal lattice. The C...Cl(-) distances (3.590 and 3.634 at 100 K increase to 3.616 and 3.655 A, respectively, at 295 K) are virtually temperature independent, indicative of hard hydrogen bridges, whereas the C...Cl-Fe distances are 3.702-3.718 A at 100 K but are 3.753-3.764 A at room temperature, suggesting a softer hydrogen bridge. A similar trend applies to the two sets of C...mu(3)-S distances (3.766-3.788 A and 3.594-3.604 A at 100 K and 3.821-3.848 A and 3.614-3.676 A at room temperature). The longer hydrogen bridges are more linear (170 degrees ) than the shorter ones (134 degrees ). The core distortions are correlated with spatial distribution of cations around the clusters.

A polarizable ion model for the structure of molten AgI

The results are reported of the molecular dynamics simulations of the coherent static structure factor of molten AgI at 923 K using a polarizable ion model. This model is based on a rigid ion potential, to which the many body interactions due to the anions induced polarization are added. The calculated structure factor is in better agreement with recent neutron diffraction data than that obtained by using simple rigid ion pair potentials. The Voronoi-Delaunay method has been applied to study the relationship between voids in the spatial distribution of cations and the prepeak of the structure factor.

Diffuse electron scattering of a region in the system Li 2SnO 3-CoO

The occurrence of short range order in a region of the system Li 2SnO 3-CoO was identified via electron diffraction experiments. In the region studied, the resultant products have a NaCl-type structure in which oxygen anions and the Li 1+, Co 2+, and Sn 4+ cations form a distorted close packed network. Two types of electron diffraction patterns were obtained, one of them belongs to an ordered superstructure of rock salt type and the other one exhibits diffuse scattering intensity. Short-range order arises due to occurrence of a particular spatial cation distribution at microscopic level. These microscopic regions are responsible for the diffuse scattering in the electron diffraction patterns, which exhibit striking geometrical forms depending on crystallographic orientation. We describe the most probable cation distribution of the cationic species Li 1+, Co 2+, and Sn 4+, on octahedral clusters to give rise to the observed diffuse scattering.

Local coordination and spatial distribution of cations in mixed-alkali borate glasses

A comprehensive NMR study is presented to elucidate the local coordination and spatial distribution of the alkali-metal cations in mixed alkali borate glasses [(M2O)x(Na2O)1−x]0.3(B2O3)0.7 (M = Li, K and x = 0.2, 0.4, 0.6, 0.8, 1.0). 11B MAS-NMR results indicate that the network structure in these glasses is constant and remains unaffected by the cation substitution process. 11B{23Na} rotational echo double resonance (REDOR) results reveal that the anionic BO4/2− groups and the neutral BO3/2 units interact equally strongly with sodium indicating the absence of cation clustering. The corresponding dipolar second moments extracted from the 11B{23Na} REDOR curves scale linearly with Na content, consistent with random mixing of the alkali ions. More quantitative information is available on the basis of 23Na–23Na and 23Na–6,7Li dipole–dipole couplings measured by spin-echo double resonance (SEDOR) spectroscopy. The experimental results are compared with various cation distribution scenarios and found to be most consistent with a model in which the different types of ions are statistically mixed within a homogeneous distribution of the entire cation population. These results are consistent with the view that the mixed alkali effect is due to the site mismatch between unlike cations A and B. Close inspection of the 23Na chemical shift trends reveals that the cation sites in mixed alkali glasses are modified in a universal manner: the sites of the larger cation are compressed while those of the smaller ion are expanded. This effect creates a secondary type of site mismatch impeding ion transport between regular A sites and A′ sites in the vicinity of a substituent cation.

11B{ 23Na} Rotational echo double resonance NMR: a new approach for studying the spatial cation distribution in sodium borate glasses

11B{ 23Na} Rotational echo double resonance (REDOR) NMR has been used to probe the spatial relationship between the network former (boron) and the network modifier (sodium) species in sodium borate glasses. Under conditions of fast magic angle spinning (MAS), site resolved REDOR data have been obtained, revealing that the trigonal (BO 3/2) and tetrahedral (BO 4/2) units interact more or less equally strongly with the sodium ions. These results are consistent with a homogeneous distribution of sodium within the voids of the borate network and argue strongly against the presence of clustered ionic arrangements.

Solid-State NMR Structural Studies of the Ternary Molybdenum Cluster Chalcogenides CdxMo6Se8 (x = 1, 2)

The structures of the ternary Chevrel phases CdMo{sub 6}Se{sub 8} and Cd{sub 2}Mo{sub 6}Se{sub 8} have been studied by several complementary {sup 111}Cd NMR spectroscopic techniques. Specifically, cadmium mobility and bonding properties are probed by temperature and frequency dependent measurements of static line shapes, magic angle spinning (MAS) NMR spectra, and spin-lattice relaxation rates. Furthermore, models for the spatial cadmium distribution are tested on the basis of {sup 111}Cd-{sup 111}Cd dipole-dipole interactions, measured by spin-echo decay spectroscopy on isotopically labeled materials (97% {sup 111}Cd). CdMo{sub 6}Se{sub 8} undergoes a phase transition near 130 K; the cadmium ions are static on the NMR time scale over the whole temperature range in both phases. The spatial cation distribution is close to homogeneous, and it specifically excludes the presence of Cd-Cd dimers. For the high-temperature phase, the {sup 111}Cd spectra indicate a large degree of static disorder. In addition, the large {sup 111}Cd chemical shift temperature coefficient and fast spin-lattice relaxation reveal strong interactions between the intercalated cadmium atoms and the conduction band wave functions of the Mo{sub 6}Se{sub 8} matrix. This behavior is typical for charge-transfer intercalation compounds in which the conduction band is only partially filled. Cd{sub 2}Mo{sub 6}Se{sub 8}, whichmore » crystallizes in the rhombohedral R{bar 3} structure, shows the typical NMR signature of a rigid, semiconducting compound. The intercalated metal species show no apparent interaction with conduction electron wave functions. {sup 111}Cd MAS and spin-echo decay data suggest a disordered atomic distribution of Cd{sup 2+} ions with a minimum Cd-Cd internuclear distance of 258 pm.« less

Alkali ion distribution in mixed-alkali chalcogenide glasses: 23Na-{ 7Li} spin echo double resonance NMR studies of the system [(Na 2S) 1- y(Li 2S) y] 0.5(GeS 2) 0.5

The relative arrangement of sodium and lithium ions in mixed alkali thiogermanate glasses of composition [(Na 2S) 1- y (Li 2S) y ] 0.5(GeS 2) 0.5 ( y = 0.20, 0.40, 0.50, 0.60, 0.80) has been studied by 23Na-{ 7Li} spin echo double resonance (SEDOR) spectroscopy. The results indicate that lithium and sodium cations are intimately mixed and not part of microscopically or macroscopically segregated phases. In a more quantitative analysis, the experimental results are compared with various Li Na cation distribution models. While a complete quantitative description would require independent knowledge of the overall spatial cation distribution in these glasses, the SEDOR data are found to be consistent with a distribution of the alkali ions that is close to homogeneous and not clustered. On the other hand, if clustering did occur, the SEDOR data would give proof of preferred interactions among like cations (‘like cation clustering’). There is no evidence for the cation pairing models previously invoked to describe the structure of mixed alkali glasses.

Cation ordering scenarios in mixed alkali silicate glasses. Experimental constraints from 23Na{6Li} spin echo double resonance NMR

AbstractThe spatial arrangement of sodium and lithium ions in mixed‐alkali silicate glasses of composition [(Li2O)1‐y(Na2O)y,]0.4[SiO2]0.6 (y = 0.25, 0.50 and 0.75) is explored by 23Na{6Li} spin echo double resonance (SEDOR) NMR spectroscopy on isotopically labelled materials. The results are discussed within the context of two basic models for the spatial cation distribution, both of which are calculated with the constraint of resulting in the same cation density as experimentally measured for the glasses: The first scenario considers a homogeneous distribution of the cations by arranging them at regular distances on a cubic lattice. For this base model, a statistical mutual arrangement of lithium and sodium is in fairly good agreement with the experimental data. The second scenario models a clustered distribution by fractionally occupying the cation sites of the Na2O lattice in a random fashion. If this base model obtains, the experimental NMR results indicate preferential interactions among like cations. Approximate agreement with the experimental SEDOR decay curves can be obtained by assuming a flat distribution function of MNa‐Li2 values, ranging from zero to M20, the average second moments calculated for a statistical mutual distribution of Li and Na ions. This model also gives satisfactory fits to previously published 23Na{7Li} SEDOR data on compositional analogous glasses with the natural isotope distribution.

Cation Distribution in Mixed-Alkali Silicate Glasses. NMR Studies by 23Na−{7Li} and 23Na−{6Li} Spin Echo Double Resonance

The relative arrangement of sodium and lithium ions in mixed-alkali silicate glases of composition [(Li2O)1-y(Na2O)y]x[SiO2]1-x (y = 0.25, 0.50, and 0.75; x = 0.40 and 0.50) is studied by 23Na−{7Li} spin echo double-resonance (SEDOR) NMR spectroscopy, supplemented by additional 23Na−{6Li} experiments on isotopically labeled materials. The experimental results are compared with various cation distribution scenarios and found to be quantitatively most consistent with a model in which lithium and sodium ions occupy random positions on a cubic lattice mimicking a uniform (homogeneous) spatial cation distribution. The experimental data give no evidence for preferred interactions among unlike cations or cation-pairing models previously invoked to describe the structure of mixed-alkali glasses. To the contrary, if the overall cation distribution were to diverge from homogeneous, the data would be most consistent with preferred interactions among like cations.

The spatial distribution of cations in nematocytes of Hydra vulgaris

The spatial distribution of cations in nematocytes of Hydra vulgaris