Evolutionary Algorithm Directed Synthesis of Mixed Anion Compounds LaF2X (X=Br,I) and LaFI2

Mixed-anion compounds have attracted growing attentions, but their synthesis is challenging, making a rational search desirable. Here, we explored LaF₃-LaX₃ (X=Cl, Br, I) system using ab initio structure searches based on evolutionary algorithms, and predicted LaF₂X and LaFX₂ (X=Br, I), which are respectively isostructural with LaHBr₂ and YH₂I, consisting of layered La-F blocks with single and double ordered honeycomb lattices, separated by van der Waals gaps. We successfully synthesized these compounds: LaF₂Br and LaFI₂ crystallize in the predicted structure, while LaF₂I is similar to the predicted one but with different layer stacking. LaF₂I exhibits fluoride ion conductivity comparable to that of non-doped LaF₃, and has the potential to show better ionic conductivity upon appropriate doping, given the theoretically lower diffusion energy barrier and the presence of soft iodine anions. This study shows the structure prediction using evolutionary algorithms will accelerate the discovery of mixed-anion compounds in future, in particular those with an ordered anion arrangement.     

Superionic fluoride ceramics RF<Subscript>3</Subscript> and R<Subscript>0.95</Subscript>Sr<Subscript>0.05</Subscript>F<Subscript>2.95</Subscript> (R = La,Ce, Pr,Nd) prepared by hot pressing

RF₃ and R_0.95Sr_0.05F_2.95 (R = La, Ce, Pr, Nd) ceramic specimens were prepared by hot pressing at 1173 K under pressure of 3 × 10⁸ Pa for 20 min. The ionic conductivity value was determined by means of impedance spectroscopy in vacuum from 293 to 823 K. For LaF₃ at 350 K, the single crystal / ceramics conductivity ratio is about 5. The difference decreases at higher temperature and disappears about 500 K. The ionic conductivity activation energy is 0.30 ± 0.05 eV. For La_0.95Sr_0.05F_2.95, the conductivity of ceramics below 500 K is slightly lower that of single crystals. At T > 500 K, the conductivity values of ceramic and single crystal specimens practically coincide. The ionic conductivity of hot pressed ceramics is about 10^?2 S/cm at 500 K and activation energy is 0.25 ± 0.02 eV.     

Superionic Transport in Fluoride Composites and Glasses

Data on previous conductimetric studies of ionic transport in fluoride composites and vitreous materials are compiled. The F^- conductivity of composites 21SrF₂ · 79LaF₃ (content, mol %) and 34NaF · 66PbF₂ reaches 0.007 S cm^-1 at 500 K and that of glasses based on InF₃ and PbF₂, 6 × 10^-5 S cm^-1 at 400 K, which exceeds that of -PbF₂ and LaF₃ crystals.     

Anisotropic conductivity of PbFCl

Single crystals of undoped PbFCl have been grown. The ionic conductivity of these crystals has been measured parallel and perpendicular to the crystallographic c-axis, and has been compared with the ionic conductivity of undoped β-PbF₂ and PbCl₂ single crystals.According to Greenwood the crystal structures involved favour the presence of anion Frenkel defects. In spite of the fact that conductivity experiments on undoped materials are not conclusive, the present results, together with structural considerations, favour the presence of Schottky defects.     

Ionic conductivity of bismuth oxofluoride BiO<Subscript>0.1</Subscript>F<Subscript>2.8</Subscript> with tysonite-type structure (LaF<Subscript>3</Subscript>)

The complex impedance method in the temperature range of 291–660 K was used to study conductivity of oxofluoride BiO_0.1F_2.8 belonging to the tysonite structural type (LaF₃). Bismuth oxofluoride was synthesized using a solid-phase method at 770–870 K for 1–2 h in an argon atmosphere. Heterovalent substitution of fluoride ions F⁻ by oxygen ions O²⁻ in the anionic BiF₃ matrix sublattice results in high ionic conductivity (∼0.1 S/cm at 660 K) of BiO_0.1F_2.8 ceramic samples.     

Mesoporous inorganic salts with crystal defects: unusual catalysts and catalyst supports

We proposed a strategy to synthesize mesoporous inorganic salt particles using the special properties of ionic liquid (IL) mixtures, and hollow mesoporous LaF₃, NdF₃, and YF₃ particles were synthesized and characterized using different techniques. The size of the mesopores in the salt particles was about 4 nm, and the materials were full of crystal defects. The LaF₃, NdF₃ and YF₃ particles were used as the catalysts for the cyanosilylation reaction of benzaldehyde using trimethylsilyl cyanide, and Ru/LaF₃ and Ru/NdF₃, in which Ru nanocatalysts were supported on the LaF₃ and NdF₃ particles with mesopores, were used to catalyze hydrogenations of benzene to cyclohexane and levulinic acid (LA) to γ-valerolactone (GVL). It was discovered that the activities of these catalysts were unprecedentedly high for these reactions. Detailed study showed that both the crystal defects and the mesopores in the salt particles played crucial roles for the extremely high catalytic activity.     

High temperature crystal structures and superionic properties of SrCl2, SrBr2, BaCl2 and BaBr2

The structural properties of the binary alkaline-earth halides SrCl₂, SrBr₂, BaCl₂ and BaBr₂ have been investigated from ambient temperature up to close to their melting points, using the neutron powder diffraction technique. Fluorite-structured SrCl₂ undergoes a gradual transition to a superionic phase at 900–1100 K, characterised by an increasing concentration of anion Frenkel defects. At a temperature of 920(3) K, the tetragonal phase of SrBr₂ undergoes a first-order transition to a cubic fluorite phase. This high temperature phase shows the presence of extensive disorder within the anion sublattice, which differs from that found in superionic SrCl₂. BaCl₂ and BaBr₂ both adopt the cotunnite crystal structure under ambient conditions. BaCl₂ undergoes a first-order structural transition at 917(5) K to a disordered fluorite-structured phase. The relationship between the (disordered) crystal structures and the ionic conductivity behaviour is discussed and the influence of the size of the mobile anion on the superionic behaviour is explored.     

Synthesis of highly fluorescent LaF3:Ln/LaF3 core/shell nanocrystals by a surfactant-free aqueous solution route

A surfactant-free aqueous solution route has been established for the synthesis of LaF₃:Ln³⁺/LaF₃ core/shell nanocrystals (Ln=Ce, Tb, Nd) heated at 75 °C at ambient pressure. All the as-prepared nanocrystals with spherical shape have an average size around 20 nm, and consist of well crystallized hexagonal phases. The X-ray photoelectron spectra was used to confirm that the LaF₃ shells have coated the LaF₃:Ce³⁺, Tb³⁺ cores. Compared with that of the original cores under the same conditions, the emission intensity of the LaF₃:Ce³⁺, Tb³⁺/LaF₃ and LaF₃:Nd³⁺/LaF₃ core/shell nanocrystals increased significantly of 120% and 60%, respectively. The quantum yield of the LaF₃:Ce³⁺, Tb³⁺/LaF₃ core/shell nanocrystals reached about 27% in aqueous solution. These results indicate that a significant reduction of the quenching from the surface of the core nanocrystals can be obtained by the synthesis of the core/shell structures, and this method can provide more desirable lanthanide-doped nanocrystals for potential biological applications.     

Influence de l'oxygène sur la conductivité électrique du fluorure de baryum monocristallin

The influence of oxygen on the electrical properties of single crystals of BaF₂ was investigated in the temperature range 450–1150°C. Two types of studies have been undertaken: total ac conductivity and cation transport measurements. The results confirm an anion Frenkel disorder model in MeF₂. Substitution of the fluorine atoms by oxygen in the lattice supposes the creation of the vacancies V^·_F. The evaluation of intrinsic, extrinsic and association domains as a function of oxygen partial pressure has been studied. Values for the energy of formation of the intrinsic defects, the motion energies of V^·_F-vacancies and association or precipitation energy have been deduced from the slopes of the conductivity curves.     

First-Principles Design of Na-ion Superionic Conductors: Interstitial-Based Na Diffusion in NaCuZrS3

In recent years all-solid-state sodium-ion batteries (SS-SIBs) have drawn significant attention due to their potential to be safer and lower cost than lithium-ion batteries. However, the lack of sodium solid-state electrolytes with high ionic conductivity has become one of the major challenges. Here, with first-principles computation we took NaCuZrS₃, consisting of earth-abundant and environmentally benign elements only, as an example to study Na-ion transport in the post-perovskite-like structure and used computation-guided design to improve its potential as a solid-state electrolyte. With ab initio molecular dynamics simulation and nudged elastic band calculation, we studied possible diffusion mechanisms in this material and found that Na ion interstitials have a favorable migration barrier of 0.22 eV, which is among the smallest in the literature reported values. Considering the large formation energy of Frenkel defects, we proposed doping strategy to introduce extra Na interstitials in the material. Our study suggests that the post-perovskite-like sulfides are worth of exploration for applications in SS-SIBs.     

Effect of LaF<Subscript>3</Subscript> admixture on thermal stability of borosilicate glasses

Oxyfluoride glass-ceramics based on the aluminosilicate glass-matrix with the nano-phase of fluoride is an interesting material for optoelectronics. A new glass from the SiO₂–B₂O₃–Na₂O–LaF₃ system in which nanocrystallization of LaF₃ could be obtained as well is presented. Thermal stability of glass and the crystalline phases formed upon heat treatment were determined by DTA/DSC and XRD methods, respectively. The effect of the glass composition on thermal stability was investigated by the SEM method. It has been found that the addition of LaF₃ increases the tendency to decomposition of the borosilicate glass. In glasses with the ratio B₂O₃/(Na₂O+3La₂F₆)<1 it is possible to obtain the immersed crystallization of LaF₃ in transparent glassy matrix. The process is preceded by LaOF formation. Glasses with the composition B₂O₃/(Na₂O+3La₂F₆)≥1 revealed the tendency to La(BSiO₅) crystallization.     

On the mechanisms of ionic conductivity in BaLiF3: a molecular dynamics study

The mechanisms of ionic conductivity in BaLiF(3) are investigated using molecular simulations. Direct molecular dynamics simulations of (quasi) single crystalline super cell models hint at the preferred mobility mechanism which is based on fluoride interstitial (and to a smaller extent F(-) vacancy) migration. Analogous to previous modeling studies, the energy related to Frenkel defect formation in the ideal BaLiF(3) crystal was found as 4-5 eV which is in serious controversy to the experimentally observed activation barrier to ionic conductivity of only 1 eV. However, this controversy could be resolved by incorporating Ba(2+)? Li(+) exchange defects into the elsewise single crystalline model systems. Indeed, in the neighborhood of such cation exchange defects the F(-) Frenkel defect formation energy was identified to reduce to 1.3 eV whilst the cation exchange defect itself is related to a formation energy of 1.0 eV. Thus, our simulations hint at the importance of multiple defect scenarios for the ionic conductivity in BaLiF(3).     

Physical properties of mixed-valence Tl 3 I TlIIICl6 investigated by conductivity and positron annihilation lifetime measurements: Effect of crushing the crystals

For a better understanding of isotope exchange in solid Tl₄Cl₆ the effects of crushing the crystals were investigated by conductivity and by positron annihilation lifetime measurements. As in untreated Tl₄Cl₆, the conductivity variation with temperature shows a break at about 450 K. The activation energies, 0.53 eV and 0.70 eV, respectively, below and above 450 K, are very close to those in the untreated material but the absolute values of conductivity are lower after crushing which is attributed to the trapping of the mobile defects of dislocation. The positron lifetime variation with increasing temperature shows some contribution of extrinsic defects, annealing at about 413 K. On further heating or cooling cycles, the lifetime changes are controlled by the production of intrinsic cation vacancies, whose formation enthalpy, 0.39 eV, is close to that derived for untreated Tl₄Cl₆. The shape of the initial part of the curves would indicate that crushing does not directly create appreciable concentrations of cation vacancies but would rather produce annealable defects, possibly dislocations, favouring the formation and/or trapping of such vacancies.     

Selective Fluoride Electrode Mechanism Studies

Abstract

We wanted to determine the mechanism of charge transport through the LaF₃ crystal in Orion's 94-09 and 94-09A fluoride electrodes. Voltage was applied to the electrode while it was in 15% Sodium Acetate Solution (pH 6.3) with or without (F¹⁸). We concluded that the mechanism of conduction through LaF₃ was one of ionic diffusion. F¹⁸ accumulated on the electrode surface very rapidly. Diffusion into the body of the LaF₃ (deeper than 300 Å) was also very rapid.     

Calorimetric analysis of NaF and NaLaF4

Heat flux calorimetry was used to measure the heat capacity of NaF between 630 K and 1100 K. The results agreed well with previous measurements known from the literature. Similar experiments were performed on NaLaF₄. This compound was synthesized from NaF and LaF₃ and analyzed by X-ray diffraction. The heat capacity of NaLaF₄ was determined for two temperature ranges: from 4 K to 400 K, for which adiabatic calorimetry, and from 623 K to 1213 K for which differential scanning heat flux calorimetry was used. The two heat capacity series fit smoothly; the resulting C_p,m function was used in a reassessment of the binary system (NaF + LaF₃).     

Defect processes in MgAl2O4 spinel

In perfect normal MgAl₂O₄ spinel the Mg²⁺ ions occupy tetrahedral 8a sites and Al³⁺ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.     

Galvanostatic investigation of electrochemical processes at the LaF<Subscript>3</Subscript>/M (M: Ag,Bi) interface

The electrochemical processes at the interface between solid fluorine-conducting electrolyte LaF₃(Eu²⁺ 0.8 mol %) and silver or bismuth electrodes in the two-electrode cell with nonpolarizable reference electrode are studied using the galvanostatic method. The anodic galvanostatic transients of LaF₃: Eu²⁺/Ag and LaF₃: Eu²⁺/Bi interfaces are linearized on the log(η ? η_max), vs. t coordinates, i.e. the rate of LaF₃|MF _n |M electrode formation is limited by slow surface diffusion of metal adions. The initial portions of cathodic galvanostatic transients in the range of solid-electrolyte lanthanum reduction are approximated by the linear dependence of η on log(1 ? √t/τ). The plots of logI vs. 1/η are linear both for the lanthanum reduction and for silver and bismuth oxidation involving mobile fluoride ion of solid electrolyte, which is typical for two-dimensional growth of new phase.     

Influence of alkaline earth oxides on thermal stability of oxyfluoride glass

A new type of glass from the Na₂O–MeO–Al₂O₃–SiO₂–LaF₃ system where MeO = MgO, CaO, BaO and SrO has been studied. The aim of the investigation was to determine, by means of thermal techniques (DTA and DSC), the influence of alkaline earth ions additions on its thermal stability and the ability of LaF₃ phase to crystallization. The effect of LaF₃ crystallization was analyzed in connection with glass composition expressed by the Al₂O₃/(MeO + Na₂O + 3La₂F₆) ratio varying from 0.4 to 0.8 for the alkaline earth admixtures. The compositions of the glasses have been designed so as to make it possible to define the effect of the charge of the ion modifiers (Na⁺, Me²⁺, La³⁺) on the alumina position in the framework of the glass. Two series of glasses were obtained with a different F⁻ content. The formation of LaF₃ depends directly on the strength of the network and can be control by the Al₂O₃/modifiers ratio as well as the content of fluorine ions. Generally, it can be stated that transparent glass-ceramic with nanocrystallization of LaF₃ can be obtained for Al₂O₃/(Na₂O + MeO + 3La₂F₆) ≤0.6 in the examined glasses. The more the ionicity of the alkaline earth ions the greater the tendency for the crystallization of Me₂LaF₇ and MeF₂. In the glass structure the substitution of oxygen ions by F⁻ ions facilitated the crystallization of LaF₃. Simultaneously, it influenced the thermal stability of the aluminosilicate network and induced the crystallization of appropriate silicates during the heat treatment.     

Colour tuneability and white light generation in Yb<Superscript>3+</Superscript>–Ho<Superscript>3+</Superscript>–Tm<Superscript>3+</Superscript> co-doped SiO<Subscript>2</Subscript>–LaF<Subscript>3</Subscript> nano-glass-ceramics prepared by sol–gel method

Nanostructured transparent glass-ceramics with composition of 95SiO₂–5LaF₃ co-doped with 0.3Yb³⁺, 0.1Ho³⁺ and 0.1Tm³⁺ (mol%) were synthesized by thermal treatment of precursor sol–gel derived glasses. X-ray diffraction and transmission electron microscopy analysis point out the precipitation of hexagonal LaF₃ nanocrystals with diameter ranging from 11 to 20 nm in these nano-glass-ceramics. White light generation by means of efficient blue, green and red up-conversion luminescence under infrared excitation at 980 nm was observed and involved mechanisms were analyzed. Colour tuneability is achieved by varying the up-conversion emission ratios as a function of pump power.     

Defect chemistry and physical properties of Ln0.5Sr0.5FeO3 (Ln: La,Pr)

A defect chemical model for the behavior of acceptor and donor-doped LaFeO₃ as a function of oxygen pressure is proposed. The nonstoichiometric deviation is calculated as a function of oxygen partial pressure, pO₂, at different temperatures. The defect structure of complex oxides is described within the point defects approach.The results show that the conductivity is dependent on pO₂ and temperature. Thermoelectric power values indicate clearly the nature of the dominant specie in charge transport. Stability regimes and compensation mechanisms at various oxygen partial pressures and temperatures are proposed. This model also examines the charge compensation mechanisms that dominate under the different regimes.From equilibrium constants, partial molar enthalpy and partial molar entropy of oxygen in Ln_0.5Sr_0.5Fe O_3?δ (Ln: La, Pr) are estimated.