Ion Occupancy Research Articles

Effects of A1 site occupation on dielectric and ferroelectric properties of Sr4CaRTi3Nb7O30 (R = Ce, Eu) tungsten bronze ceramics

Sr4CaRTi3Nb7O30 (R = Ce, Eu) tungsten bronze ceramics are prepared by a standard solid state reaction method. The effects of A1 site occupation on the dielectric and ferroelectric properties of Sr4CaRTi3Nb7O30 (R = Ce, Eu) tetragonal tungsten bronzes are investigated. The Sr4CaCeTi3Nb7O30 shows a normal transition behavior due to the closer size ion occupation in A1 sites, which could suppress the distortion of B2 octahedra effectively. Sr4CaEuTi3Nb7O30 ceramic exhibits two dielectric anomalies, which might be related to the fact that the large radius difference between Ca2+ and Eu3+ could lead to the uneven distribution of Ca2+ and Eu3+ in A1 sites and form two slightly different kinds of compositions with different transition temperatures in the structure. Our results indicate that the ionic radius difference in A1 sites plays an important role in determining the dielectric and ferroelectric natures of the filled tungsten bronze ceramics. Polarization—electric field (P—E) curves are evaluated at room temperature and both of them show hysteresis loops. Sr4CaCeTi3Nb7O30 shows a fat hysteresis loop, indicating the long-range ferroelectric order in the ceramic. The current density—electric field (J—E) curves are measured at room temperature with a largest leakage current density of ∼ 10−6 A/cm2, indicating that their leakage currents are rather low.

The site occupation and valence of Mn ions in the crystal lattice of Sr4Al14O25 and its deep red emission for high color-rendering white light-emitting diodes

The synthesis and component of red phosphor, Sr4Al14O25: Mn, were optimized for application in white light-emitting diodes. The microstructure and morphology were investigated by the X-ray diffraction and scanning electron microscopy. Different valences of Mn ions in Sr4Al14O25 were discriminated using the electron paramagnetic resonance and X-ray absorption near-edge structure spectroscopy techniques. The bond-valence theory was used to analyze the effective valences of Sr2+ and Al3+ in Sr4Al14O25. As a result, the strong covalence of Al3+ in the AlO4 tetrahedron other than in the AlO6 octahedron is disclosed. The deep red emission is attributed to Mn4+ occupying the center of AlO6 octahedron. The mechanism of energy transfer is mainly through dipole–dipole interaction, revealed by the analyses of critical distance and concentration quench. A high color rendering white LED prototype with color-rendering index up to Ra 93.23 packaged by using the red phosphor demonstrates its applicability.

Thermal variation of MgZn nanoferrites for magnetic hyperthermia

Mg1− x Zn x Fe2O4 (x = 0.2, 0.4, 0.5, 0.6, 0.8) nanoparticles were prepared for the characterization of the crystallographic and magnetic properties. The Rietveld refinement for x-ray diffraction was used to confirm that the Zn ion occupied on B-site for dopings over 0.5 doping. The lattice constant (a 0) was increased from 8.3969 to 8.4100 ± 0.0001 A with increasing Zn concentration. Mossbauer spectra of all samples were taken at room temperature and showed Fe3+ and Fe2+ valence states. The thermal properties of all samples were measured at 50 kHz and 25 mT. The heating temperature was increased up to 124 °C until 0.5 doping of Zn ions, however it was decreased down to 69 °C over 0.5 doping of Zn ions. These results can be explained by the fact that the saturation magnetization was increased by Fe2+ ion, but, the heating temperature was decreased due to occupation of Zn ions on B-site for dopings above 0.5.

The α-Subunit Regulates Stability of the Metal Ion at the Ligand-associated Metal Ion-binding Site in β3 Integrins

The aspartate in the prototypical integrin-binding motif Arg-Gly-Asp binds the integrin βA domain of the β-subunit through a divalent cation at the metal ion-dependent adhesion site (MIDAS). An auxiliary metal ion at a ligand-associated metal ion-binding site (LIMBS) stabilizes the metal ion at MIDAS. LIMBS contacts distinct residues in the α-subunits of the two β3 integrins αIIbβ3 and αVβ3, but a potential role of this interaction on stability of the metal ion at LIMBS in β3 integrins has not been explored. Equilibrium molecular dynamics simulations of fully hydrated β3 integrin ectodomains revealed strikingly different conformations of LIMBS in unliganded αIIbβ3 versus αVβ3, the result of stronger interactions of LIMBS with αV, which reduce stability of the LIMBS metal ion in αVβ3. Replacing the αIIb-LIMBS interface residue Phe(191) in αIIb (equivalent to Trp(179) in αV) with Trp strengthened this interface and destabilized the metal ion at LIMBS in αIIbβ3; a Trp(179) to Phe mutation in αV produced the opposite but weaker effect. Consistently, an F191/W substitution in cellular αIIbβ3 and a W179/F substitution in αVβ3 reduced and increased, respectively, the apparent affinity of Mn(2+) to the integrin. These findings offer an explanation for the variable occupancy of the metal ion at LIMBS in αVβ3 structures in the absence of ligand and provide new insights into the mechanisms of integrin regulation.

Effects of site occupancy and valence state of Fe ions on ferromagnetism in Fe-doped In2O3 diluted magnetic semiconductor

The effects of oxygen vacancies, valence state of Fe ions, and site occupancy of Fe ions on ferromagnetism in Fe-doped In2O3 were investigated both experimentally and theoretically. The Fe3+ ions prefer to occupy the 8b sites and do not contribute to the room temperature (RT) ferromagnetism. It is found that the presence of Fe2+ ions in In2O3 can be induced through the creation of oxygen vacancies. The Fe2+ ions tend to occupy the 24d sites, and the origin of RT ferromagnetism can be related to the strong Fe:4s and Fe:3d hybridization. Our findings not only give a clear picture on the origin of ferromagnetism of Fe-doped In2O3 but also provide a way to tune the magnetic property of Fe-doped In2O3 through the control of valence states of dopant and the control of sites for dopant occupation.

Characterization of Fe Substitution into La-Hexaaluminate Systems and the Effect on N2O Catalytic Decomposition

Compared to Ba-hexaaluminates with β-Al2O3 structure, La-hexaaluminates with magnetoplumbite (MP) structure have more substituted Al sites in the preferentially exposed mirror plane, which should favor N2O decomposition. In this regard, LaFexAl12–xO19 catalysts with x = 1 and 5 were herein prepared using the coprecipitation method. 57Fe Mössbauer spectroscopy and X-ray diffraction structure refinements revealed that Fe3+ ions originating in Fe2O3 species mainly entered tetrahedral Al(2) and trigonal bipyramidal Al(5) sites. Meanwhile, Fe3+ ions in octahedral sites of perovskite-type LaFeO3 intermediates preferentially accommodated in octahedral Al(3) sites in the mirror plane of La-hexaaluminates. Correlation of normalized rates (moles of N2O per hour per square meter) of the catalysts at 500 °C with the occupancy of Fe ions in different Al crystallographic sites of the MP phase indicated that Fe3+ ions in the Al(3) and Al(5) sites were highly active for N2O decomposition. In contrast, Fe2+ derived from a Fe3+ → Fe2+ reduction under high-temperature calcination played a negative role.

Protein Backbone Mutagenesis Reveals a Novel Link between Ion Occupancy and C-type Inactivation in K+ Channels

The selectivity filter of K+ channels contains four sequential ion binding sites built primarily from backbone carbonyl oxygen atoms. In addition to ion discrimination, the selectivity filter participates in C-type inactivation, which is a conformation change at the selectivity filter that converts it from a conductive to a non-conductive state. C-type inactivation is modulated by permeant ions but the mechanism underlying this effect is not known. To investigate the link between ion occupancy and C-type inactivation, we used amide to ester substitutions in the protein backbone to alter ion binding at specific sites in the selectivity filter and determined the effect on C-type inactivation. We found that introducing an ester linkage at the 1' site in the selectivity filter of the KcsA channel did not affect inactivation but introducing ester linkages at the 2' and the 3' sites severely reduced inactivation. We determined the structure of the 2' ester mutant of the KcsA channel and found that the 2' ester substitution did not alter the structure of the selectivity filter but did eliminate ion binding specifically to the S2 site. We introduced an equivalent ester substitution at the 2' position in the selectivity filter of the voltage-gated K+ channel KvAP and found that this substitution dramatically slowed inactivation, similar to the effect observed in the KcsA channel. Our results suggest that ion occupancy at the S2 site is necessary for C-type inactivation in K+ channels.

First exploration of Na-ion migration pathways in the NASICON structure Na3V2(PO4)3

Ion occupation and migration pathways are investigated to explore the ion-migration mechanism of Na3V2(PO4)3 with the help of first principles calculations. Na3V2(PO4)3 with a NASICON framework generates high performances as a cathode material in sodium-ion batteries.

Aluminum ion occupancy in the structure of synthetic saponites: Effect on crystallinity

Two series of saponites with fixed (Si+Al)/Mg and Si/Mg ratios, respectively, were synthesized by using hydrothermal methods. The obtained products were characterized by XRD, XRF, 27 Al, and 29 Si MAS NMR, SEM, and TEM. XRD patterns showed that well-ordered saponites were obtained in the initial Si/Al ratio range of 5.43-7.89. Beyond this Si/Al ratio range, poorly crystallized saponites were obtained with small crystallized particles, which can be seen from TEM images. When intercalating saponite with surfactant, the intercalated products displayed strong and well-ordered (00 l ) reflections, indicating that layered saponite has been successfully synthesized in the present study. 27 Al MAS NMR spectra demonstrated that well-crystallized synthetic saponites had a higher Al(IV)/Al(VI) ratio than the poorly crystallized samples, which is an important factor affecting the crystallinity of synthetic saponite. A one-to-one substitution (i.e., 1 Al 3 + → 1 Mg 2+ ) actually occurred in the octahedral sheet and this substitution had a negative effect on the crystallinity of the synthetic saponites. After grafting the synthetic saponites with silane, the decreased intensity of the 29Si NMR signal at -86 ppm and the increased intensity of Q 3 Si(0Al) and Q 3 Si(1Al) signals strongly suggested that the signal at ca. -86 ppm corresponded to Q 2 Si at the layer edges of saponite.

Molecular Basis for Sodium Versus Calcium Binding in the Sodium-Calcium Exchanger

Na+/Ca2+ exchangers (NCX) and potassium-dependent Na+/Ca2+ exchangers (NCKX) are two related families of transporters involved in Ca2+ signaling that function by extruding cytosolic Ca2+ (and K+ in NCKX) in exchange for extracellular Na+. Previous studies have established that this exchange process is electrogenic and with a 3Na+:1Ca2+ stoichiometry, and have identified specific acidic amino acids believed to be crucial for ion binding and translocation. Recently the crystal structure of the NCX from Methanococcus jannaschii was determined at 1.9 A resolution, revealing an intriguing transmembrane topology consisting of inverted structural repeats, and the presence of four putative ion binding sites formed by highly conserved residues. Notwithstanding these groundbreaking insights, based on the structure alone several ion occupancy states can be hypothesized that would be compatible with the experimental exchange stoichiometry. Here, we use extensive molecular simulations, free energy calculations and mutational analysis to investigate the occupancy and specificity of the ion binding sites in NCX_Mj. The results permit us to identify a 3Na+ bound configuration that is both compatible with the X-ray structure and ranked as the most probable from a thermodynamic standpoint. In addition, two putative Ca2+ bound configurations are identified that reconcile anomalous scattering data due to Ca2+ with an alternative X-ray structure with Cd2+ bound. On the basis of these ion configurations, we investigate the mechanism of alternating-access in this exchanger, and explore the molecular events that trigger the conformational transition between inward- and outward-facing states.

Using protein backbone mutagenesis to dissect the link between ion occupancy and C-type inactivation in K + channels

K(+) channels distinguish K(+) from Na(+) in the selectivity filter, which consists of four ion-binding sites (S1-S4, extracellular to intracellular) that are built mainly using the carbonyl oxygens from the protein backbone. In addition to ionic discrimination, the selectivity filter regulates the flow of ions across the membrane in a gating process referred to as C-type inactivation. A characteristic of C-type inactivation is a dependence on the permeant ion, but the mechanism by which permeant ions modulate C-type inactivation is not known. To investigate, we used amide-to-ester substitutions in the protein backbone of the selectivity filter to alter ion binding at specific sites and determined the effects on inactivation. The amide-to-ester substitutions in the protein backbone were introduced using protein semisynthesis or in vivo nonsense suppression approaches. We show that an ester substitution at the S1 site in the KcsA channel does not affect inactivation whereas ester substitutions at the S2 and S3 sites dramatically reduce inactivation. We determined the structure of the KcsA S2 ester mutant and found that the ester substitution eliminates K(+) binding at the S2 site. We also show that an ester substitution at the S2 site in the KvAP channel has a similar effect of slowing inactivation. Our results link C-type inactivation to ion occupancy at the S2 site. Furthermore, they suggest that the differences in inactivation of K(+) channels in K(+) compared with Rb(+) are due to different ion occupancies at the S2 site.

Effects of molybdenum concentration and valence state on the structural and magnetic properties of BaFe11.6Mo x Zn0.4−x O19 hexaferrites

Barium hexaferrites BaFe11.6Mo x Zn0.4−x O19 (x= 0.1, 0.2, 0.4) were prepared by precipitation of the precursors using wet chemical mixture method and then sintering the dried powders at 1100 °C. The properties of the prepared samples were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer, and Mössbauer spectroscopy. XRD patterns revealed that all prepared samples had BaFe12O19 hexaferrite structure as a majority phase. SEM images demonstated that the samples consisted mainly of hexagonal platelet-like grains with diameters ranging from 100 to 500 nm. Mössbauer spectra revealed that Zn2+ ions occupy 4f 1 sites leading to the splitting of the 12k component. However Mo6+ ions occupy 2b sites while Mo4+ prefer 4f 1 and 12k sites. For the sample with x = 0.4,Mo6+ and Mo4+ ions were found to have preference for 2b and 12k sites, respectively, and to induce the development of Fe2+ ions in the hexaferrite, leading to noticeable changes in the magnetic properties of the system. The observed magnetic properties were found to be consistent with the preferential site occupation of metal ions, and the hyperfine fields derived from Mössbauer spectra of these samples.

Effect of Electrostatic and Size on Dopant Occupancy in Lithium Niobate Single Crystal

We proposed a simple and an effective method to predict the site occupancy and threshold concentration of metal ions in lithium niobate (LiNbO3, LN) single crystal. The ionic energy parameter E(i), defined by the ionic electronegativity and ionic radius, was proposed to describe the electrostatic and size effects of cations on the structural stability of LN. The dopant location can be easily identified by comparing the E(i) deviation of dopant from those of host cations Li(+) and Nb(5+), and the dopant prefers to occupy the lattice site with the smaller deviation of E(i). Our calculated occupancies agree well with those experimental results, which demonstrate the predictive power of our present method. We in this work predicted the preferred occupancies of 60 metal ions in LN single crystal. Further, the threshold concentrations of some frequently used dopants were calculated on the basis of the assumption that all doped LN crystals can endure the same variation of E(i).

Preparation and electrochemical properties of spinel LiFexCuyMn1.2O4 by ultrasonic spray pyrolysis

Abstract Nanocrystalline LiFe x Cu y Mn 1.2 O 4 ( x and y =0.2, 0.4 and 0.6) particles were prepared by the ultrasonic spray pyrolysis method using nitrate salts at 800 °C in air atmosphere. Particle properties were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Also, cyclic voltammetry and galvanostatic tests were performed to investigate the effects of the double substituent and doping amounts on electrochemical behavior. Results show that the aggregation of nanocrystallites around 90 nm size formed submicron spherical cathode particles. Transition metal ratios in particles exhibited a perfect fit with desired amounts. Although the change of iron and copper amounts do not show significant differences in the particle size and shape morphology, they modify the 4 V and 3 V potential plateaus of spinel LiMn 2 O 4 . The discharge capacities of LiFe 0.2 Cu 0.6 Mn 1.2 O 4 particles are 39 and 23 mAh g −1 for 4 and 2.6 V potential regions, respectively. 4 V discharge capacity disappeared with increasing of iron and decreasing of copper contents due to random occupation of iron and copper ions in the spinel lattice.

Influence of Jahn–Teller Cu2+ (3d9) ion on structural and magnetic properties of Al–Cr co-substituted CuFe2O4

Four specimens of two compositions of the spinel oxide system CuAlxCrxFe2−2xO4 with x = 0.2 and 0.6 have been synthesized by double sintering ceramic technique with different thermal history i.e. quenched and slow-cooled. X-ray diffractometry has confirmed that tetragonal distortion is present in slow-cooled sample, while it is absent in quenched sample. The lower value of saturation moment compared to Neel’s moment for each composition is explained by invoking random canting of spins model. Hyperfine interaction parameters: nuclear hyperfine field, quadrupole splitting (shifts), isomer shift and Fe3+ ion distribution are deduced through computer simulation of Mossbauer spectra. 57Fe Mossbauer spectra recorded at room temperature show the effect of magnetic dilution of inter-sublattice (A–B) interaction (frustration) more in case of quenched-sample compared to slow-cooled one. FTIR spectra also corroborate the influence of Jahn–Teller (J–T) distortion of octahedral void arising due to larger Cu2+ ion occupancy at octahedral sites. The force constants are also estimated from FTIR spectra.

Optical characteristics of Er3+ ion in Er/Yb:LiNbO3 crystal: Comparison with the dissimilar effect of anti-photorefractive ions Zn2+, In3+ and Zr4+

The different influences of Zn2+, In3+ and Zr4+ ions on the optical characteristics of Er3+ ion in Er/Yb:LiNbO3 crystals were discussed. An enhanced 1.54μm emission was observed for Zr/Er/Yb:LiNbO3 crystal, but the Zn2+ tri-doping resulted in a decreased one, and the intensity of 1.54μm emission remained about same in In/Er/Yb:LiNbO3 crystal. The populations of the green emitting 4S3/2/2H11/2 states were achieved through the three-, two- and two-phonon processes in Zn/Er/Yb:LiNbO3, In/Er/Yb:LiNbO3 and Zr/Er/Yb:LiNbO3 crystals, respectively. Zn2+ and In3+ ions affected the optical characteristics of Er3+ ion via modifying the Er3+ ion occupancy in Er/Yb:LiNbO3 crystal. The formation of ErLi2+–ErNb2− ion pairs caused by the Zn2+ and In3+ ions could increase the rate of cross relaxation process. The OH− absorption spectra showed that the incorporation of Zr4+ ions increased OH− content, which increased the probability of the nonradiative relaxation process of 4I11/2→4I13/2 (Er) in Zr/Er/Yb:LiNbO3 crystal. The J–O intensity parameters Ωt (t=2, 4 and 6), the radiative lifetime (τrad) and fluorescence branching ratio (β) in Zr/Er/Yb:LiNbO3 crystal were predicted by Judd–Ofelt theory. Füchtbauer–Ladenburg and McCumber methods were carried out to calculate the emission cross-sections at 1.54μm emission. The gain cross-section, estimated as a function of the population inversion ratio, allowed us to evaluate a potential laser performance of Zr/Er/Yb:LiNbO3 crystal.

Stabilization of the Conductive Conformation of a Voltage-gated K+ (Kv) Channel: THE LID MECHANISM

Voltage-gated K(+) (Kv) channels are molecular switches that sense membrane potential and in response open to allow K(+) ions to diffuse out of the cell. In these proteins, sensor and pore belong to two distinct structural modules. We previously showed that the pore module alone is a robust yet dynamic structural unit in lipid membranes and that it senses potential and gates open to conduct K(+) with unchanged fidelity. The implication is that the voltage sensitivity of K(+) channels is not solely encoded in the sensor. Given that the coupling between sensor and pore remains elusive, we asked whether it is then possible to convert a pore module characterized by brief openings into a conductor with a prolonged lifetime in the open state. The strategy involves selected probes targeted to the filter gate of the channel aiming to modulate the probability of the channel being open assayed by single channel recordings from the sensorless pore module reconstituted in lipid bilayers. Here we show that the premature closing of the pore is bypassed by association of the filter gate with two novel open conformation stabilizers: an antidepressant and a peptide toxin known to act selectively on Kv channels. Such stabilization of the conductive conformation of the channel is faithfully mimicked by the covalent attachment of fluorescein at a cysteine residue selectively introduced near the filter gate. This modulation prolongs the occupancy of permeant ions at the gate. It is this longer embrace between ion and gate that we conjecture underlies the observed stabilization of the conductive conformation. This study provides a new way of thinking about gating.

Hydrothermal synthesis of ZnSe:Cu quantum dots and their luminescent mechanism study by first-principles

An one-pot synthesis of aqueous ZnSe:Cu nanocrystals (NCs) is realized in aqueous solution by a facile yet efficient hydrothermal technique. The dopant emission spectrum of the NCs is tunable, spanning a wide range from 438 to 543nm. Room-temperature quantum yield for the NCs prepared at the optimal conditions reaches as high as 20% without any post-treatment. The ZnSe:Cu NCs prepared in a neutral aqueous solution (pH=8) are remarkably stable and exhibit comparatively high photoluminescent quantum yield (PL QY) as high as 17%. First-principles pseudopotential calculations using plane-wave basis functions have been performed. The formation energies of copper ions occupied in the interstitial octahedron and substitutional tetrahedral Zn2+ sites have been calculated. The occupation of copper ions in the interstitial octahedral site is found to be more thermodynamics-facilitated by −0.98eV. The density of state analysis indicates that the Cu-related emission is primary dominated by the substitutional tetrahedral Cu ions, and the large dopant related emission width of ZnSe:Cu NCs originated from the corresponding Cu 3d impurity band.

Recent Progress in Garnet-Type Structure Solid Li Ion Electrolytes: Composition – Structure – Ionic Conductivity Relationship and Chemical Stability Focused

Garnet-type oxides, Li5La3M2O12 (M = Nb, Ta) are a group of materials that exhibit high electrochemical stability and high conductivity for Li ions, making them promising electrolyte materials for all-solid-state Li-ion batteries. Li ion conductivity can continue to be increased by substituting alkaline earth and Li ions for La3+ in the structure. Our recent work has shown a simple empirical relation between concentration of Li and ionic conductivity in several garnet-type compounds. The occupation of Li ions in various crystallographic sites also appears to control Li ion conductivity in the garnet-type structures. In the present paper, we report current progress in garnet-based Li ion electrolytes and also discuss the effect of chemical doping on ionic conductivity and chemical stability of Li5La3Nb2O12 in water and organic acids.

Concerted Migration Mechanism in the Li Ion Dynamics of Garnet-Type Li7La3Zr2O12

The garnet-type Li7La3Zr2O12 (LLZO) belonging to cubic symmetry (space group Ia3̅d) is considered as one of the most promising solid electrolyte materials for all-solid state lithium ion batteries. In this study, the diffusion coefficient and site occupancy of Li ions within the 3D network structure of the cubic LLZO framework have been investigated using ab initio molecular dynamics calculations. The bulk conductivity at 300 K is estimated to be about 1.06 × 10–4 S cm–1 with an energy barrier of 0.331 eV, in reasonable agreement with experimental results. The complex mechanism for self-diffusion of Li ions can be viewed as a concerted migration governed by two crucial features: (i) the restriction imposed for occupied site-to-site interatomic separation, and (ii) the unstable residence of Li ion at the 24d site, which can serve as the trigger for ion mobility and reconfiguration of surrounding Li neighbors to accommodate the initiated movement. Evidence for Li ordering is also found at low temperature for the LLZO system.