Alkali Halides Research Articles

An Atomistic Analysis of the Carpet Growth of KCl Across Step Edges on the Ag(111) Surface.

The carpet growth of alkali halide (AH) layers across step edges of substrates enables the growth of seamless and continuous large domains. Yet, information about how the AH layer adapts continuously to the height difference between the terraces on the two sides of a step is only described by continuum models, which do not give details of the ionic displacements. Here, we present a first study of thin epitaxial KCl(100) layers grown on the Ag(111) surface by scanning tunneling microscopy that provides atomistic details for the first time. Measurements were performed at room temperature. Using a Cl--decorated tip, we resolved the ionic arrangement and hence the KCl lattice distortion in the carpet growth region, in some cases even by imaging both types of ions. Our findings demonstrate the ability of the KCl lattice to distort locally over a short distance of four KCl unit cells as a result of the attractive interaction between the ions and the Ag atoms at and close to the steps. For Ag step edges covered by the KCl carpet, we observe a tendency to straighten along the ⟨110⟩ direction of the KCl layer. In addition, the carpet growth induces the formation of Ag microterraces, i.e., the splitting of higher Ag steps into multiple Ag steps of monatomic height during the KCl deposition at elevated temperatures. These microterraces have a minimum width determined by an energetically preferred fitting to the KCl lattice and allow for the carpet growth, while growth across higher Ag steps is not observed.

Intermolecular interaction potential maps from energy decomposition for interpreting reactivity and intermolecular interactions.

The electrostatic potential (ESP) has been widely used to visualize electrostatic interactions about a molecule. However, electrostatic effects are often insufficient for capturing the entirety of an interaction or a reaction of interest. In this investigation, intermolecular interaction potential maps (IMIPs), constructed from the potentials derived from energy decomposition analysis (EDA) using density functional theory, were developed and applied to provide unique insight into molecular interactions and reactivity. To this end, rather than constructing a potential map from probe point charge interactions, IMIPs were constructed from probe interactions with small molecular fragments, including CH3+, CH3-, benzene, and atomic probes including alkali metals, transition metals, and halides. The interaction potentials are further decomposed producing IMIPs for each interaction component using EDA (electrostatic, orbital, steric, etc.). The IMIPs are applied to the study of various interactions including cation-π and anion-π interactions, electrophilic and nucleophilic aromatic substitution, Lewis acid activation, π-stacking, endohedral fullerenes, and select organometallics which reveal fundamental insight into the positional preferences and physical origins of the interactions that otherwise would be difficult to uncover through other surface analyses.

Pressure as the driving force for mechanochemical reactions on the example of salt metathesis of alkali halides upon ball milling

We report an in situ X-ray diffraction study of the mechanochemical ion metathesis between sodium iodide (NaI) and potassium chloride (KCl) to form sodium chloride (NaCl) and potassium iodide (KI)...

Pop the bubbles and let the current flow: mechanochemistry of micron and nano-sized openings in dielectric layers

Thin or ultra-thin dielectric layers have been widely used in various applications such as capacitors, piezo-electrics, and solar cells. This study explains the mechanism and chemistry of creating nano- and micron-sized openings in atomic-layer-deposited aluminum oxide-based dielectric layers using the alkali metal salt selenization technique. The necessary components for this mechanism are excess methyl groups present in the dielectric layer, supply of selenium and alkali metals, and a minimum annealing temperature. It is shown and explained that to create openings in the dielectric layer, heavier alkali halide metal salts require less energy, or - in other words - a lower annealing temperature. The overall hypothesis is explained via a thermodynamic approach with supportive thermochemical reactions. Thus, an easy way to engineer the dielectric layer to form openings at low temperatures is presented, beneficial for various applications like photovoltaics, optoelectronics, or micro-electro-mechanical systems (MEMS).

Ultrafast Time-Domain Spectroscopy Reveals Coherent Vibronic Couplings upon Electronic Excitation in Crystalline Organic Thin Films.

The coherent coupling between electronic excitations and vibrational modes of molecules largely affects the optical and charge transport properties of organic semiconductors and molecular solids. To analyze these couplings by means of ultrafast spectroscopy, highly ordered crystalline films with large domains are particularly suitable because the domains can be addressed individually, hence allowing azimuthal polarization-resolved measurements. Impressive examples of this are highly ordered crystalline thin films of perfluoropentacene (PFP) molecules, which adopt different molecular orientations on different alkali halide substrates. Here, we report polarization-resolved time-domain vibrational spectroscopy with 10 fs time resolution and Raman spectroscopy of crystalline PFP thin films grown on NaF(100) and KCl(100) substrates. Coherent oscillations in the time-resolved spectra reveal vibronic coupling to a high-frequency, 25 fs, in-plane deformation mode that is insensitive to the optical polarization, while the coupling to a lower-frequency, 85 fs, out-of-plane ring bending mode depends significantly on the crystalline and molecular orientation. Comparison with calculated Raman spectra of isolated PFP molecules in vacuo supports this interpretation and indicates a dominant role of solid-state effects in the vibronic properties of these materials. Our results represent a first step toward uncovering the role of anisotropic vibronic couplings for singlet fission processes in crystalline molecular thin films.

Study on the role of alkali halides on the mutarotation and dehydration of d-xylose in aqueous solution

Although the xylose mutarotation and transformation have been investigated largely separately, their relationship has been rarely systematically elaborated. The effect of several factors such as xylose concentration, temperature, and salt concentration, affecting the mutarotation of xylose are discussed. Nine alkali halides (LiCl, NaCl, KCl, LiBr, NaBr, KBr, LiI, NaI, and KI) are used to test salt effects. The relationship between xylose rotation rate constant (kM), specific optical rotation at equilibrium ([α]eqm), α/β ratio, H chemical shift difference (ΔΔδ), Gibbs free energy difference (ΔG), hydrogen ion or hydroxide ion concentration ([H+] or [OH−]), and xylose conversion is discussed. Different salts dissolved in water result in different pH of the solutions, which affect the mutarotation of xylose, with the nature of both cation and anion. Shortly, the smaller the cation radius is and the larger the anion radius is, the greater the mutarotation rate is. In the dehydration of xylose to furfural in salty solutions, xylose conversion is positively correlated to mutarotation rate, H+ or OH− concentration, and the energy difference between α-xylopyranose and β-xylopyranose. Although the [α]eqm of xylose is positively correlated with α/β configuration ratio, there is no obvious correlation with xylose dehydration. The conversion to furfural in chlorides is superior to that in bromines and iodides, which is due to the fact that the pH of chloride salts is smaller than that of the corresponding bromide and iodized salts. Higher H+ concentration prefers to accelerate the formation of furfural. In basic salt solutions, the xylulose selectivity is higher than that of furfural at the initial stage of reaction. The furfural selectivity and carbon balance are better in acidic condition rather than in basic condition. In H2O-MTHF (2-Methyltetrahydrofuran) biphasic system, the optimal furfural selectivity of 81.0 % is achieved at 190 °C in 1 h with the assistance of LiI and a little HCl (0.2 mmol, 8 mmol/L in aqueous phase). A high mutarotation rate represents rapid xylose conversion, but a high furfural selectivity prefers in acidic solutions, which would be perfect if organic solvents were available to form biphasic systems.

Statistical Thermodynamic Analysis of the Effect of Chemical Composition on Changes in the Melting Temperatures of Alkali Metal Halides

Statistical Thermodynamic Analysis of the Effect of Chemical Composition on Changes in the Melting Temperatures of Alkali Metal Halides

NMR Spectroscopy and Multiscale Modeling Shed Light on Ion-Solvent Interactions and Ion Pairing in Aqueous NaF Solutions.

The balance between ion solvation and ion pairing in aqueous solutions modulates chemical and physical processes from catalysis to protein folding. Yet, despite more than a century of investigation, experimental determination of the distribution of ion-solvation and ion-pairing states remains elusive, even for archetypal systems like aqueous alkali halides. Here, we combine nuclear magnetic resonance (NMR) spectroscopy and multiscale modeling to disentangle ion-solvent interactions from ion pairing in aqueous sodium fluoride solutions. We have developed a high-accuracy method to collect experimental NMR resonance frequencies for both ions as functions of temperature and concentration. Comparison of these data with resonance frequencies for nonassociating salts allows us to differentiate the influence of solvation and ion pairing on NMR spectra. These high-quality experimental NMR data are used to validate our modeling framework comprising polarizable force field molecular dynamics (MD) simulations and quantum chemical calculations of NMR resonance frequencies. Our experimental and theoretical resonance frequency shifts agree over a wide range of temperatures and concentrations. Structural analysis reveals how both trends are dominated by interactions with water molecules. For the more sensitive 19F nucleus, the NMR resonance frequency decreases as hydrogen bonds between fluoride and water molecules are reduced in number with increased temperature and molality. Through a detailed analysis of the theoretical NMR resonance frequencies for both ions, we show that NMR spectroscopy can distinguish both contact ion pairs and single-solvent-separated ion pairs from free ions. This quantitative framework can be applied directly to other systems.

Ligand‐Controlled Copper‐Catalyzed Halo‐Halodifluoromethylation of Alkenes and Alkynes Using Fluorinated Carboxylic Anhydrides

AbstractPolyhalogenated molecules are often found as bioactive compounds in nature and are used as synthetic building blocks. Fluoroalkyl compounds hold promise for the development of novel pharmaceuticals and agrochemicals, as the introduction of fluoroalkyl groups is known to improve lipophilicity, membrane permeability, and metabolic stability. Three‐component 1,2‐halo‐halodifluoromethylation reactions of alkenes are useful for their synthesis. However, general methods enabling the introduction of halodifluoromethyl (CF2X) and halogen (X’) groups in the desired combination of X and X’ are lacking. To address this gap, for the first time, we report a three‐component halo‐halodifluoromethylation of alkenes and alkynes using combinations of commercially available fluorinated carboxylic anhydrides ((CF2XCO)2O, X=Cl and Br) and alkali metal halides (X’=Cl and Br). In situ prepared fluorinated diacyl peroxides were identified as important intermediates, and the use of appropriate bipyridyl‐based ligands and a copper catalyst was essential for achieving high product selectivity. The synthetic utility of the polyhalogenated products was demonstrated by exploiting differences in the reactivities of their C−X and C−X’ bonds to achieve selective derivatization. Finally, the reaction mechanism and ligand effect were investigated using experimental and theoretical methods to provide important insights for the further development of catalytic reactions.

The electric double layer structure and interface energy of uranium electrode in molten LiCl-KCl-UCl3 mixture: A molecular dynamics simulation study

The electric double layer structure and interface energy of uranium electrode in molten LiCl-KCl-UCl3 mixture, as well as the surface energies of metallic uranium and molten salts, were investigated using molecular dynamics simulations. The surface energies were calculated, respectively, from the pressure tensor and the energy difference for molten salt and uranium, and interface energy was evaluated from the Gibbs free energy difference between systems with and without interface. The surface energies of molten alkali halides are between 76.4 and 201.8 mJ·m−2 and decrease with the increase of radius of the halide anion or the alkali cation for molten salt with the same alkali cation or the same halide anion. The surface energy of uranium depends on the allotrope morphology and crystal facet, and the surface energies of α-U, β-U, and γ-U are 2.36, 1.45, and 1.69 J·m−2 in average for the seven low index crystal facets. The interface energy of uranium and molten LiCl-KCl-UCl3 mixture is about 2.71 J·m−2 and increases with the increase of charge density on the uranium slabs. The molten LiCl-KCl-UCl3 mixture in anodic zone has higher coordination number than that in cathodic zone but similar coordination distance.

Ligand-Controlled Copper-Catalyzed Halo-Halodifluoromethylation of Alkenes and Alkynes Using Fluorinated Carboxylic Anhydrides.

Polyhalogenated molecules are often found as bioactive compounds in nature and are used as synthetic building blocks. Fluoroalkyl compounds hold promise for the development of novel pharmaceuticals and agrochemicals, as the introduction of fluoroalkyl groups is known to improve lipophilicity, membrane permeability, and metabolic stability. Three-component 1,2-halo-halodifluoromethylation reactions of alkenes are useful for their synthesis. However, general methods enabling the introduction of halodifluoromethyl (CF2X) and halogen (X') groups in the desired combination of X and X' are lacking. To address this gap, for the first time, we report a three-component halo-halodifluoromethylation of alkenes and alkynes using combinations of commercially available fluorinated carboxylic anhydrides ((CF2XCO)2O, X=Cl and Br) and alkali metal halides (X'=Cl and Br). In situ prepared fluorinated diacyl peroxides were identified as important intermediates, and the use of appropriate bipyridyl-based ligands and a copper catalyst was essential for achieving high product selectivity. The synthetic utility of the polyhalogenated products was demonstrated by exploiting differences in the reactivities of their C-X and C-X' bonds to achieve selective derivatization. Finally, the reaction mechanism and ligand effect were investigated using experimental and theoretical methods to provide important insights for the further development of catalytic reactions.

Molecular-Resolution Imaging of Ionic Liquid/Alkali Halide Interfaces with Varied Surface Charge Densities via Atomic Force Microscopy.

When in contact with charged solid surfaces, ionic liquids (ILs) are known to form solvation structures consisting of alternating cation and anion layers. This phenomenon is considered to originate from the adsorption layer of counterions overcompensating the surface charge, so-called overscreening. However, the response of these layers to surfaces with near-zero or extremely high surface charge density (σ) remains inadequately understood. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: nearly zero-charged RbI(100) and highly charged RbI(111), by employing frequency modulation atomic force microscopy with atomic resolution. Two commonly used ILs are examined in this study: 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3mpyr][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). On RbI(100) surfaces with near zero σ, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. These results support the argument that overscreening occurs under low σ, even approaching zero, and reconcile conflicting experimental conclusions about low σ systems. On RbI(111) surfaces with high σ, we identify solvation structures consisting of two consecutive counterion layers. This structure aligns with the theoretically predicted crowding; a phenomenon rarely observed in commonly used ILs due to typically unreachable σ in electrochemical IL/electrode systems. Our findings indicate that alkali halide(111) surfaces are potentially valuable for exploring the crowding phenomenon in ILs, addressing the current scarcity of experimental observations.

Remote vapor-phase dual alkali halide salts assisted quasi-van der Waals epitaxy of m-phase ZrO2 thin films with high dielectric constant and stable flexible properties

High-κ dielectric constant and wideband gap of ZrO2 material render it as an excellent candidate for transistor gate dielectric layers. However, current reported synthesis techniques suffer the problems of high precursor volatilization rate, ultrasmall grains with low dielectric constant, and high leakage current, which largely impede its application in electronic devices. Here, the quasi-van der Waals epitaxy growth of compact m-phase ZrO2 thin films has been developed, in which the stable supply of Zr source is realized by the tuned sublimation of ZrC powder with remote vapor-phase dual halide salts assistant. The formation of m-phase ZrO2 is due to the lower Gibbs free energy, in which the crystal nucleates at the etched hole edges of mica substrate, thus forming hexagonal shape polycrystal grains and merging as the continuous thin films. The microstructures and Raman spectrum characterization reveal the two dominated growth orientations and good crystal qualities, which indicate the uniform dielectric constant. The excellent growth reproducibility could be easily adapted to thin metal substrates, such as tungsten, molybdenum, and stainless steel, where the adhesion strength is strong because of the higher density of interfacial chemical bonding. Meanwhile, the metal–insulator–metal flexible capacitors show the high dielectric constant of 23–26 and low leakage current density of 10−4 A/cm2 at large voltage and only exhibit the decreased capacitance density of 7% after several hundred bending cycles. Our work paves a way to achieve the high-quality dielectric thin films on various substrates by the unique chemical vapor deposition design strategy.

Effect of solution‐processed cesium carbonate on Cu(In,Ga)Se2 thin‐film solar cells

AbstractHeavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process.

Ionic 2D Materials

2D materials are an exciting topic with a wide variety of applications in many research fields, from optoelectronics, photonics or qubit devices to catalysis. So far, these layered nanoscopic materials have been obtained by different procedures. In top-down approaches, from simple mechanical "Scotch tape" exfoliation to liquid exfoliation to break the weak non-covalent interactions between layers. In this project, the challenge was to obtain 2D flakes of a 3D ionic starting material with cubic rock salt structure (e.g. alkali halides NaCl, NaF, KCl, KF or CsF). They will have physical properties with substantial differences from those of known 2D materials. For example, there are few large bandgap 2D materials, almost exclusively BN layers. The BN bulk system has a band gap around 4 eV and its monolayer 6 eV, while the employed alkali halides in this project, the bulk vales are 10.0 eV (CsF), 10.9 eV (KF), 11.7 eV (NaF), KCl eV (8.5) and eV NaCl (8.6). This will open many applications that are now only restricted to BN layers. The crucial point is how it is possible to exfoliate an isotropic 3D material with ionic interactions in three spatial directions.(1,2) The intercalation of bromine molecules in CsF was corroborated, many years ago, by X-ray single-crystal diffraction.(1,3, see Figure) The intercalated structure is analogous to that of other conventional 2D materials; for example, graphene also with bromine molecules. This intercalation into the 3D structure of the alkali halide has two effects, one is to generate an anisotropic 2D structure with alternated monoatomic CsF layers and layers of Br2 molecules. The second important effect is that now for the exfoliation of the CsF-Br2 compound, only the non-ionic F···Br-Br interactions must be broken. Preliminary DFT calculations indicate that such contact energies are of the same order of magnitude as other non-covalent interactions of common 2D materials.The intercalation of such 2D materials has been performed by the intercalation of Br2 and IBr molecules combined with different exfoliation techniques (sonication-centrifugation, ball milling, microwaves..). The characterization of the 2D flakes has been performed using Atomic Force Microscopy. The final goal of this project is to make a profit from the large gap of such 2D systems for optoelectronic properties, such as single-photon emission.(1) Desmarteau, D. D.; Grelbig, T.; Hwang, S. H.; Seppelt, K., Angewandte Chemie-International Edition in English 1990, 29 (12), 1448-1449(2) Ruiz, E.; Alvarez, S., Journal of the American Chemical Society 1995, 117 (10), 2877-2883.(3) Drews, T.; Marx, R.; Seppelt, K., Chemistry-a European Journal 1996, 2 (10), 1303-1307. Figure 1

In Situ High-Pressure Correlated Transportation of Heavy Rare-Earth Perovskite Nickelates as Batch Synthesized within Eutectic Molten Salts at MPa-pO2.

The multiple magneto-/electrical quantum transitions discovered with d-band correlated metastable perovskite oxides, such as rare-earth nickelate (ReNiO3), enable applications in artificial intelligence and multifunctional sensors. Nevertheless, to date such investigation merely focuses on ReNiO3 with light or middle rare-earth composition, while the analogous explorations toward heavy rare-earth (ReHNiO3, ReH after Gd) are impeded by their ineffective material synthesis relying on GPa pressure. Herein, for the first time we synthesized the powder of ReHNiO3 in grams/batch with ∼1000 times lower pressure and ∼300 °C lower temperature in comparison to the previous ∼101 milligram/batch results, assisted by their eutectic precipitation and heterogeneous growth within alkali-metal halide molten salt at MPa oxygen pressures. Further in situ characterizations under high pressures within a diamond anvil cell reveal a distinguishing pressure predominated bad metal transport within the nonequilibrium state of ReHNiO3 showing high-pressure sensitivity up to 10 GPa, and the temperature dependences in electrical transportations are effectively frozen.

SERS-active core-satellite nanostructures in a membrane filter-integrated microfluidic device for sensitive and continuous detection of trace molecules

We developed a surface-enhanced Raman scattering (SERS)-active plasmonic core-satellite nanostructure and incorporated it into a membrane filter-integrated microfluidic device for continuous monitoring of molecules in solution. The core-satellite nanostructures were fabricated by immobilizing a high number density of gold nanoparticles (AuNPs) on silica beads.to create many nanogaps among the AuNPs. The sizes of the nanogaps were fine-tuned by adding a silver (Ag) shell to optimize the SERS activity. In addition, citrate molecule, the capping agent of the nanoparticles, was displaced by alkali halides. The displacement not only reduced the SERS signals of citrate but also enhanced the adsorption of target molecules. The alkali halide-treated core-satellite nanostructures were accumulated onto a membrane filter integrated into a microfluidic device, serving as a uniform and sensitive SERS substrate. By increasing the volume of the sample solution flowing through the membrane filter, we increased the number of molecules adsorbed to the nanostructures, amplifying the intensities of their characteristic Raman peaks. Our microfluidic SERS device demonstrated continuous SERS detection of malachite green at a concentration as low as 500 fM. In summary, while various core-satellite nanostructures and microfluidic SERS devices have been reported, the integration of the membrane filter-containing microfluidic device with the core-satellite nanostructures facilitated sensitive and continuous molecule detection in our study.

Calculation of liquidus in eutectic alkali halide mixtures using thermodynamic perturbation theory

A statistical-thermodynamic model that makes it possible to describe with good accuracy phase equilibria between melt and crystal in binary alkali halide mixtures with common cations and anions is proposed. This model is based on the thermodynamic perturbation theory, which takes into account the charge-dipole contribution to the interionic interaction of binary molten salts using a multicomponent mixture of charged hard spheres as a reference system. Specific expressions for chemical potentials, taking into account the charge-induced dipole interaction in the molten phase, are presented within the developed approach. Considering binary eutectic mixtures NaF–NaCl, CsF–CsCl, NaF–CsF, and NaCl–CsCl as an example, the liquidus curves obtained in two series of calculations are shown: with the equation of state and without it. The discrepancies between the calculated and experimental data on the position of eutectic equilibrium for most of the considered mixtures do not exceed about 10 percent on the temperature scale and 5 percent on the composition when using only the tabulated values of ionic radii and polarizabilities without any adjustment.

Electrodeposition of Beta-Tantalum in Alkali Metal Halides and Oxohalide Melts

Data on the electrolytic preparation of β-Ta in halide and oxohalide melts were reported. It was shown that during electrolysis in halide melts containing K2TaF7, the amount β-Ta in the cathodic products can be markedly increased by using a copper cathode with a definite texture. It was found that in chloride-oxofluoride melts the β-Ta content increases monotonically on passing from NaCl to KCl and RbCl melt. The dependencies of the β-Ta content in cathodic products on the ratio of oxygen to tantalum concentrations were studied in chloride-oxofluoride and fluoride-oxofluoride melts. The maxima of these dependences were identified and explained. It was shown that β-Ta could be obtained by electrolysis in molten salts only at temperatures below 850 °C.

Time-of-Flight Spectroscopy of Hyperthermal Molecular Beams of Alkali and Alkaline Earth Metal Halides

Time-of-Flight Spectroscopy of Hyperthermal Molecular Beams of Alkali and Alkaline Earth Metal Halides