Anion Displacement Research Articles

Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms

AbstractIn contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion‐preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden–Popper HIF Sr3Zr2O7, which is the first ternary lead‐free zirconate ferroelectric, is reported and room‐temperature polarization switching is demonstrated. This compound undergoes a first‐order ferroelectric‐to‐paraelectric transition, involving an unusual change in the “sense” of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first‐principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper “antiferroelectricity” is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next‐generation thermoelectrics.

High-temperature formation phases and crystal structure of hot-pressed thermoelectric compounds with chalcopyrite-type structure

In this study, we introduced the temperature-dependent formation phases and crystallographic parameters of hot-pressed silver gallium telluride AgGaTe2 and copper gallium telluride CuGaTe2 with chalcopyrite structure from 300 to 800 K. These two compounds are potential thermoelectric materials in the intermediate temperature range; however, the temperature-dependent formation phases and crystallographic parameters of hot-pressed samples have not yet been analyzed in detail. The crystal structure analysis based on synchrotron X-ray diffraction (SXRD) measurements clarifies that impurity phases such as Te and Ag2Te in the AgGaTe2 matrix and Te and CuTe in the CuGaTe2 matrix appear at some temperature regions above 300 K. The existence of such impurity phases could be correlated with the increases of the electrical resistivity and Seebeck coefficient of the samples after multiple measurement cycles of the temperature-dependent transport properties from 300 to 800 K. The tetragonal lattice parameters a and c, tetragonal lattice volume, thermal expansion coefficients, tetragonal distortion, anion displacement parameter, and isotropic displacement parameter of the hot-pressed AgGaTe2 and CuGaTe2 were also analyzed. These crystallographic parameters are expected to substantially affect the thermoelectric properties of AgGaTe2 and CuGaTe2. Our results provide prospect of the long-term high-temperature stability and clues of the detailed analysis on the transport properties of hot-pressed AgGaTe2 and CuGaTe2, which should aid their development for thermoelectric applications.

Room temperature ferroelectricity in donor-acceptor co-doped TiO2 ceramics using doping-engineering

Conventional ferroelectric oxides are of generally non-centrosymmetric crystals resulting from paraelectric-ferroelectric transitions. Here, we present a new donor-acceptor ion co-doping route to room-temperature ferroelectricity in rutile TiO2, where ionic pair of co-doping ions plays very important roles in generating local broken symmetry and polarization. The (Nb + Al) co-doped TiO2 sintered with a two-step method exhibits ferroelectricity and piezoelectric response, and it is believed that Nb5+-Al3+ pairs along TiO2 [001] direction play important role in the ferroelectricity. A large lattice distortion can be induced in the vicinity of the ionic pair, which results in local broken-symmetry. The relative displacements of cations and anions around the ionic pair, induced by the lattice distortion, cause the local polarization. The results and mechanisms presented in this work provide a new approach to the design of new ferroelectrics via doping engineering.

Prediction on the physical properties of CuInS2 with various anion positions

The effects of various anion displacements (u) on electronic structures, elastic constants, Debye temperature and the minimum thermal conductivity of CuInS2 are studied by first-principles calculation. The lattice constant couples with the anion displacement, however, they are not consistent with the relation proposed by Abrahams and Bernstein. When the anion displacement varies from 0.2 to 0.3, the Cu-S bond length is elongated, while the In-S bond length is shortened, which cause the increase of band gap with anion displacement. The anisotropies of sound velocities and lattice thermal conductivities are also discussed. The results imply that the lattice thermal conductivity along [110] direction is the smallest and decreases with u. These research findings shed light on improving the thermoelectric properties by manipulating u and the direction of propagation.

Factors affecting the association of single- and double-stranded RNAs with montmorillonite nanoclays

Montmorillonite (MMT) nanoclays exist as single and stacked sheet-like structures with large surface areas that can form stable associations with many naturally occurring biomolecules, including nucleic acids. They have been utilized successfully as vehicles for delivery of both drugs and genes into cells. Most previous studies have focused on interactions of MMT with DNA. In the current study, we have investigated the binding of small RNAs similar to those used for RNA interference (RNAi) therapy to two major forms of the clay, Na-MMT and Ca-MMT. Association of both forms of MMT with several double-stranded RNAs (dsRNAs), including 25mers, 54mers and cloverleaf-shaped transfer RNAs, was weak and increased only slightly after addition of Mg2+ ions to the binding reactions. By contrast, ssRNA 25mers and 54mers bound poorly to Na-MMT but interacted strongly with Ca-MMT. The weak binding of ssRNAs to Na-MMT could be strongly enhanced by addition of Mg2+ ions. The strength of MMT-ssRNA interactions was also examined using inorganic anion competition and displacement assays, as well as electrophoretic mobility shift assays (EMSAs). The aggregate results point to a cation-bridging mechanism for binding of ssRNAs, but not dsRNAs, in the presence of divalent metal cations.

Low energy ion‐solid interactions and chemistry effects in a series of pyrochlores

AbstractThe effect of chemistry on low energy recoil events was investigated at 10 K for each type of atom in pyrochlores, using molecular dynamics simulation. Contour plots of the threshold displacement energy (Ed) in Gd2Zr2O7 have been produced along more than 80 directions for each individual species. The Ed surface for each type of atom in Gd2Zr2O7 is highly anisotropic; Ed of Zr exhibits the largest degree of anisotropy, while that of O8b exhibits the smallest. The recommended values of Ed in Gd2Zr2O7 based on the observed minima are 56, 94 and 25 eV, respectively, for Gd, Zr, and O. The influence of cation radius on Ed in pyrochlores A2B2O7 (with A‐site ranging from Lu3+ to La3+ and B‐site ranging from Ti4+ to Ce4+) was also investigated along three directions [100], [110], and [111]. The Ed in pyrochlores strongly depended on the atom type, atom mass, knock‐on direction, and lattice position. The defects produced after low energy displacement events included cation antisite defects, cation Frenkel pairs, anion Frenkel pairs, various vacancies, and interstitials. Ce doping in pyrochlores may affect the radiation response, because it resulted in drastic changes in cation and anion displacement energies and formation of an unusual type of anti‐site defect. This work demonstrates links between Ed and amorphization resistance.

Electronic and optical properties of CuGaS2 nanowires: a study of first principle calculations

In this work we study the electronic and optical properties of fully relaxed CuGaS2 nanowires using the pseudo-potential density functional method. In our calculations we have investigated nanowires with two shapes of hexagonal and triangular with their corresponding diameters in the order of 8 to 15 A in (1–10) growth direction. For CuGaS2 bulk, the geometrical parameters such as anion displacement and equilibrium lattice constant agree well with other theoretical and experimental results. We have shown that for the CuGaS2 nanowires, there is an important contraction of the Cu-S and Ga-S bond lengths in the wires of 2.1% and 1.24% of the bulk value. In addition, in this manuscript the electronic properties such as band structures and atom-projected density of states have been examined. Our results show that while the nanowire diameter increases, the band gap decreases. From partial density of states we found that the greatest valence bands involve atoms which are placed at the surface. The optical constants, the dielectric function, reflectivity, refractive index and absorption of the nanowires have been analyzed. The results show that compared to the CuGaS2 bulk, the corresponding peaks of dielectric functions of CuGaS2 nanowires are blue-shifted. The calculations reveal that the dielectric functions of the nanowires augment while the nanowires’ size increases. It is also found that the peaks related to optical parameters of nanowires are affected by the diameter of the nanowire.

Luminescence sensing of weakly-hydrated anions in aqueous solution by self-assembled europium(iii) complexes.

Self-assembled europium(iii) complexes produced dispersed nanoparticles with diameters of about 4-8 nm, which exhibited long-lived luminescence in a 20 wt% EtOH-H2O solution. The complexes could sense weakly hydrated anions, such as perchlorate, by a change in luminescence intensity using an anion displacement method.

Counterion Effects on Electrolyte Interactions with Gold Nanoparticles

Electrolyte interactions with nanoparticles (NPs) at the solid/liquid interfaces are highly complicated as the charged species can be directly adsorbed onto the NP surfaces, confined in the diffusion layer immediately surrounding the NPs, and dispersed in bulk solutions. Existing studies on electrolyte interactions with NPs are based primarily on the electrical double layer theory that focuses mainly on electrolyte interactions with NPs with fixed pre-existing charges. Demonstrated herein is a comprehensive study of counterion effects during the electrolyte bindings to gold nanoparticles (AuNPs), including halide-induced AuNP aggregation and fusion, quantitative cation and anion coadsorption, selective cation and anion displacement on AuNPs, and surface-enhanced Raman spectroscopic features of the ionic species adsorbed onto AuNP surfaces. In contradiction to previous reports that electrolyte effects are anion-specific, we demonstrated that cations can play a dominant role in the halide-induced AuNP aggre...

First-principles study of pyroelectricity in GaN and ZnO

First-principles calculations are made for the primary pyroelectric coefficients of wurtzite GaN and ZnO. The pyroelectricity is attributed to the quasiharmonic thermal shifts of internal strains (internal displacements of cations and anions carrying their Born effective charges). The primary (zero-external-strain) pyroelectricity dominates at low temperatures, while the secondary pyroelectricity (the correction from external thermal strains) becomes comparable with the primary pyroelectricity at high temperatures. Contributions from the acoustic and the optical phonon modes to the primary pyroelectric coefficient are only moderately well described by the corresponding Debye function and Einstein function respectively.

Atomic motion of resonantly vibrating quartz crystal visualized by time-resolved X-ray diffraction

Transient atomic displacements during a resonant thickness-shear vibration of AT-cut α-quartz are revealed by time-resolved X-ray diffraction under an alternating electric field. The lattice strain resonantly amplified by the alternating electric field is ∼104 times larger than that induced by a static electric field. The resonantly amplified lattice strain is achieved by fast displacements of oxygen anions and collateral resilient deformation of Si−O−Si angles bridging rigid SiO4 tetrahedra, which efficiently transduce electric energy into elastic energy.

Phase transformations inLn2O3materials irradiated with swift heavy ions

Phase transformations induced in the cubic C-type lanthanide sesquioxides, ${\mathrm{Ln}}_{2}{\mathrm{O}}_{3}$ (Ln = Sm, Gd, Ho, Tm, and Lu), by dense electronic excitation are investigated. The structural modifications resulting from exposure to beams of 185 MeV Xe and 2246 MeV Au ions are characterized using synchrotron x-ray diffraction and Raman spectroscopy. The formation of a B-type polymorph, an X-type nonequilibrium phase, and an amorphous phase are observed. The specific phase formed and the transformation rate show dependence on the material composition, as well as the ion beam mass and energy. Atomistic mechanisms for these transformations are determined, indicating that formation of the B-type phase results from the production of anti-Frenkel defects and the aggregation of anion vacancies into planar clusters, whereas formation of the X-type and amorphous phases requires extensive displacement of both anions and cations. The observed variations in phase behavior with changing lanthanide ionic radius and deposited electronic energy density are related to the energetics of these transformation mechanisms.

A density functional (PBE, PBEsol, HSE06) study of the structural, electronic and optical properties of the ternary compounds AgAlX2 (X = S, Se, Te)

First principles density functional theory (DFT) calculations for bulk structural, electronic and optical properties of ternary compounds AgAlX2 (X = S, Se, Te) were performed with two flavours of generalised gradient approximations (PBE and PBEsol) and the hybrid functional HSE06. Using cohesive energy as a stability criterion, we found that the chalcopyrite structure is the favoured phase for these materials. PBEsol gives structural properties closer to the experimental values when compared to the results of PBE. Tetragonal distortion and anion displacement were calculated and we found them to be the cause of the crystal field splitting. Reduction of the bandgap and band splitting around the Γ in the Brillouin zone was noted when spin-orbit coupling was included in our study especially in the case of AgAlTe2. The HSE06 bandgap and frequency dependent dielectric function were in very good agreement with experimental results. We have also shown that the maximum absorption peak lies in the ultraviolet range irrespective of the functional used. The refractive index is also discussed.

Electrocatalysis of Fuel Cell Relevant Reactions in Protic Ionic Liquids

Protic ionic liquids (PILs) are a sub-class of ionic liquids that have attracted attention recently as effective proton carriers for medium-temperature fuel cells because PILs have favorable properties such as high proton conductivity, low vapour pressure and excellent chemical and thermal stability.1,2 PIL-based fuel cells (PILFCs) can conduct protons without deliberate humidification, unlike the polymer electrolytes used in conventional proton exchange membrane fuel cells (PEMFCs), meaning that water management in PILFCs is simpler than in PEMFCs. Moreover, the rate of the oxygen reduction reaction (ORR) in PILFCs can exceed that of the rate of ORR in PEMFCs as PILFCs are able to operate above 100 °C. Despite recent research in the development of PIL-based fuel cells, electrocatalysis in this media has not yet been studied in any great detail. In particular, detailed study on the effect of electrode-adsorbate interactions during electrocatalysis of the ORR at Pt in PILs has been lacking. This contribution investigates the role of ion adsorption on the electrocatalysis of ORR, CO oxidation and hydrazine oxidation in a series of ammonium-based PILs. Surface oxides that form during trace water oxidation play a key role during electrocatalysis in PILs. Pt surface oxides hinder the ORR, and this reaction only occurs at potentials where the Pt surface is in its reduced state. By contrast, the rate of hydrazine oxidation in PILs increases as the extent of residual Pt oxide increases. Moreover, CO oxidation coincides with oxidation of the Pt surface in each PIL, suggesting that the reaction proceeds via a Langmuir-Hinshewood mechanism involving adsorbed CO and surface oxides. The “CO displacement” technique was used to determine whether the cations or anions of the PILs were adsorbed on the Pt surface, and this insight was used to explain the trends observed during electrocatalysis of the ORR in PILs. For example, the onset potential of the ORR in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO], see Figure 1A for structure) was 0.7 V vs RHE at 45 °C, and in diethylmethylammonium bis (trifluoromethanesulfonyl)imide ([dema][Tf2N]) was -0.2 V vs RHE at 45 °C. It is important to note that the cations of these PILs are the same, and their physicochemical properties such as viscosity and conductivity are similar. Figure 1 shows the i-t curve when CO was dosed at various potentials into [dema][TfO] and [dema][Tf2N]. In [dema][TfO], cation displacement was observed negative of 0.25 V vs RHE and anion ([TfO]-) displacement was seen positive of 0.28 V vs RHE (Figure 1A). In contrast, anion displacement was observed at all potentials studied in [dema][Tf2N] (Figure 1B), indicating the strong interaction between Pt surfaces and the [Tf2N]- anion. This result indicates that the binding of the [Tf2N]- anions to the Pt surface causes the high overpotential of the ORR in [dema][Tf2N]. Extension of the “surface electrocatalysis” approach described here to well-defined surfaces will reveal whether ion adsorption from PILs exhibits structural sensitivity and will yield new insights into electrocatalysis in PILs. Ultimately, this approach may lead to the rational design of electrocatalysts for use in PIL electrolytes

Effect of Ligand Structure on Olefin Polymerization by a Metallocene/Borate Catalyst: A Computational Study

We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes: Cp2ZrMe2 (Cp), rac-SiMe2-bis(1-(2-Me-(4-PhInd))ZrMe2 (4-PhInd), and rac-SiMe2-bis(1-(2-Me-(4,5-BenzInd))ZrMe2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically active ion pair, anion displacement, chain propagation, and chain termination steps, are reported for ethene homopolymerization, alongside with investigation of ethene–propene copolymerization reactions. Initially, all catalysts form inner-sphere ion pairs ([L2ZrMe]+–[B(C6F5)4]−) with a direct Zr–F interaction, which is weak enough to be displaced by the incoming monomer. In comparison to Cp, the bulky and electron-rich 4-PhInd and 4,5-BenzInd show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to Cp. Th...

Rules and stereoelectronic guidelines for the anionic nucleophilic displacement of furanoside and furanose O-sulfonates.

Rules for predicting anionic SN2 displacement viability in furanose and furanoside sulfonates are presented.

Effect of organic solvents on Li+ ion solvation and transport in ionic liquid electrolytes: a molecular dynamics simulation study.

Molecular dynamics simulations of N-methyl-N-propylpyrrolidinium (pyr13) bis(trifluoromethanesulfonyl)imide (Ntf2) ionic liquid [pyr13][Ntf2] doped with [Li][Ntf2] salt and mixed with acetonitrile (AN) and ethylene carbonate (EC) organic solvents were conducted using polarizable force field. Structural and transport properties of ionic liquid electrolytes (ILEs) with 20 and 40 mol % of organic solvents have been investigated and compared to properties of neat ILEs. Addition of AN and EC solvents to ILEs resulted in the partial displacement of the Ntf2 anions from the Li(+) first coordination shell by EC and AN and shifting the Li-Ntf2 coordination from bidentate to monodentate. The presence of organic solvents in ILE has increased the ion mobility, with the largest effect observed for the Li(+) cation. The Li(+) conductivity has doubled with addition of 40 mol % of AN. The Li(+)-N(Ntf2) residence times were dramatically reduced with addition of solvents, indicating an increasing contribution from structural diffusion of the Li(+) cations.

Donor-anion interactions at the charge localization and charge ordering transitions of (TMTTF)2AsF6 probed by NEXAFS.

High-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the As M-edge, F K-edge and S L-edge of the Fabre salt (TMTTF)2AsF6 were performed from room temperature (RT) to 90 K, allowing to reach the charge localization regime below Tρ ≈ 230 K and to cross the charge ordering (CO) transition at TCO ≈ 102 K. The F K-edge and S L-edge spectra exhibit several transitions which have been indexed on the basis of first-principles DFT calculations. Upon cooling from RT significant energy shifts up to +0.8 eV and -0.4 eV were observed in transitions exhibited by the F 1s and S 2p spectra respectively, while the As 3p doublet does not show a significant shift. Opposite energy shifts found in the F 1s and S 2p spectra reflect substantial thermal changes in the electronic environment of F atoms of the anion and S atoms of TMTTF. The changes found around the charge localization crossover suggest an increase of the participation of the S d orbitals in the empty states of TMTTF as well as an increase of the strength of donoranion interactions. A new F 1s pre-edge signal detected upon entry into the CO phase is a clear fingerprint of the symmetry breaking occurring at TCO. We propose that this new transition is caused by a substantial mixing between the HOMO of the AsF6(-) anion and the unoccupied part of the TMTTF HOMO conduction band. Analysis of the whole spectra also suggests that the loss of the inversion symmetry associated with the CO is due to an anion displacement increasing the strength of SF interactions. Our data show unambiguously that anions are not, as previously assumed, innocent spectators during the electronic modifications experienced by the Fabre salts upon cooling. In particular the interpretation of the spectra pointing out a thermally dependent mixing of anion wave functions with those of the TMTTF chains demonstrates for the first time the importance of anion-donor interactions.

Crystal structure and dielectric property of supramolecular macrocyclic [(NDPA)·(18-crown-6)]2+·(DMA)+·3ClO4− assemblies

Abstract One novel organic–inorganic hybrid supramolecular assemblies [(NDPA)·(18-crown-6)]2+·(DMA)+·3ClO4− (1), has been successfully constructed through the prominent strategies of crystal engineering (NDPA = N,N-dimethyl-1,4-phenylenediamine, DMA = dimethylamine), and characterized by IR, powder XRD and single crystal X-ray diffraction. In the structure, the supramolecular organic cations and inorganic ClO4− anions are arranged alternately and linked by N O⋯H hydrogen bonds. It is worthy to note that the ClO4− are linked to form one-dimensional inorganic chain through strong N H⋯O hydrogen bonds along b-axis. There is no distinct dielectric anomaly in the temperature-dependent and frequency-dependent dielectric constant curves, suggesting that no phase transition exists within the measured temperature range (120–420 K). The relative displacement of cations and anions, the turned polarization of molecular electric moment and macrocyclic molecule rotator are the main factors to determine the trend of dielectric constant.

Structural and optical properties of (Ag,Cu)(In,Ga)Se2 polycrystalline thin film alloys

The structural and optical properties of pentenary alloy (Ag,Cu)(In,Ga)Se2 polycrystalline thin films were characterized over the entire compositional range at a fixed (Cu + Ag)/(In + Ga) ratio. Films deposited at 550 °C on bare and molybdenum coated soda-lime glass by elemental co-evaporation in a single-stage process with constant incident fluxes exhibit single phase chalcopyrite structure, corresponding to 122 spacegroup (I-42d) over the entire compositional space. Unit cell refinement of the diffraction patterns show that increasing Ag substitution for Cu, the refined ao lattice constant, (Ag,Cu)-Se bond length, and anion displacement increase in accordance with the theoretical model proposed by Jaffe, Wei, and Zunger. However, the refined co lattice constant and (In,Ga)-Se bond length deviated from theoretical expectations for films with mid-range Ag and Ga compositions and are attributed to influences from crystallographic bond chain ordering or cation electronegativity. The optical band gap, derived from transmission and reflection measurements, widened with increasing Ag and Ga content, due to influences from anion displacement and cation electronegativity, as expected from theoretical considerations for pseudo-binary chalcopyrite compounds.