A theoretical study on calculation of ionic radii using limiting equivalent conductivities

Graphs of the total radius (the distance between an anionic nuclei and a cationic nuclei in a crystal) of sodium halides and alkali metal fluorides versus total limiting equivalent conductivities were plotted. For the hard ions Na⁺ and F⁻, whose behaviour approaches a hard spherical model, it was determined that radii values could be obtained using differences in limiting equivalent conductivities and ionic crystal data. From the determined radii of sodium and fluoride ions and known crystal data, radii of other alkali metal halides were calculated.

Phase behaviors of ionic liquids attributed to the dual ionic and organic nature

Ionic liquids (ILs), also known as room-temperature molten salts, are solely composed of ions with melting points usually below 100 °C. Because of their low volatility and vast amounts of species, ILs can serve as ‘green solvents' and ‘designer solvents' to meet the requirements of various applications by fine-tuning their molecular structures. A good understanding of the phase behaviors of ILs is certainly fundamentally important in terms of their wide applications. This review intends to summarize the major conclusions so far drawn on phase behaviors of ILs by computational, theoretical, and experimental studies, illustrating the intrinsic relationship between their dual ionic and organic nature and the crystalline phases, nanoscale segregation liquid phase, IL crystal phases, as well as phase behaviors of their mixture with small organic molecules.     

Thermodynamic properties of the mixtures of some ionic liquids with alcohols using a simple equation of state

In the present work, we have used a simple equation of state called the GMA EoS to calculate the density of three ionic liquid mixtures including 1-butyl-3-methylimidazolum hexafluorophosphate, [BMIM] [PF₆] + methanol, 1-butyl-3-methylimidazolum tetrafluoroborate, [BMIM] [BF₄] + methanol, and [BMIM] [BF₄] + ethanol at different temperatures, pressures, and compositions. The isothermal compressibility, excess molar volumes, and excess Gibbs molar energy of these mixtures have been computed using this equation of state. The values of statistical parameters show that the GMA EoS can predict these thermodynamic properties very well within the experimental errors. The results show that isothermal compressibility of ionic liquids is lower than alcohols and the effect of temperature and pressure on the isothermal compressibility of ionic liquids is lower than alcohols. The excess molar volumes and excess molar Gibbs energy for these ionic liquid mixtures with alcohols are all negative at various temperatures and pressures over the whole composition range. The results have been interpreted in terms of intermolecular interactions and structural factors of the ionic liquids and alcohols.     

An electrostatic correction for improved crystal density predictions of energetic ionic compounds

In recent years, there has been considerable interest in predicting the crystal densities of both molecular and ionic energetic compounds using the computed volumes V_m of the isolated gas phase molecules or ions. The surfaces enclosing the volumes are taken to be the 0.001 au (electrons/bohr³) contours of the molecules’ and ions’ electronic densities. For molecular solids, it is known that the ratio M/V_m (M = molecular mass) gives densities that are overall reasonably good, although they can be markedly improved by introduction of an electrostatic interaction correction term. For ionic solids, the subject of this paper, the ratio M/V_m (M = formula unit mass) is not nearly as effective; V_m tends to be significantly larger than the effective volumes of the ions in the crystal, leading to underestimated densities, with an average absolute error of 0.089 g/cm³. The correction term that improves molecular crystal densities is not applicable in the case of ionic solids; however we show, for a database of 25 compounds plus five test cases, that an average absolute error of 0.033 g/cm³ can be achieved by combining M/V_m with terms involving the average positive and negative potentials and areas on the cationic and anionic surfaces. The root-mean-square error is 0.040 g/cm³.     

复杂离子晶体马德隆常数研究

以电中性的平行六面体晶胞为计算单元建立了一种马德隆常数计算公式, 并按围绕参考晶胞 的壳层数N且以参考晶胞体心为原点计算离子晶体的马德隆常数α(N), 第一壳层有26个晶胞 ，第二、第三、…第N壳层分别有98，218，…，24N2+2个晶胞,α(N)是收敛很快的级数并 且很容易计算.计算了几种复杂离子晶体马德隆常数和相应的马德隆能(静电相互作用能). 关键词： 马德隆常数 静电相互作用能 复杂离子晶体     

Ultrasound-assisted of alkali chloride separation using bulk ionic liquid membrane

An ultrasonic-assisted separation of alkali chloride (LiCl, NaCl, and KCl) salts have been carried out using of an hydrophobic ionic liquid membrane (ILM). The ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and tributyl phosphate mixture have been used as ILM. An ultrasonic probe with different frequencies (25, 100, and 250) kHz have been applied as source of ultrasound generator with different times of sonication (2, 5, and 10) min in three phases system containing feed, ILM, and receiver in osmotic U-shaped tube. Also, 250, 500, and 1000 ppm of the feed (alkali chloride) concentration have been used to separate. The frequency of 250 kHz with higher sonication time provides optimum condition for separation of LiCl with lower feed concentration. The thermodynamic properties such as density and speed of sound and the related thermodynamic properties have been calculated to optimize ILM composition (x_IL = 0.45) for ultrasound-separation.     

Some studies on the relative stabilities of the NaCl,CsCl and cubic ZnS structures in ionic crystals

The problem of the relative stability of ionic structures is still unsolved current semi-empirical theories wrongly predict the caesium halides to have the NaCl structure. We point out here that these theories also predict some of the other alkali halides to occur in cubic ZnS structure. To understand these discrepancies, we study the effect of various interactions (such as second neighbour repulsion, van der Waals interaction and differences in ionic compressibilities) on the relative stability of simple structures. The results throw into question the radius ratio approach. It is suggested that one could allow for the presence of three-body interactions by relaxing the requirement that the repulsion interaction should be strictly proportional to the number of neighbours. Such an approach might explain the relative stability of simple ionic structures.     

Semi-quantitative theory of the structures of simple ionic crystals

A simple theory is developed which shows that the regions of stability of the CsCl, NaCl and ZnS structures can be demarcated in a two-dimensional plot of the radius ratio versus the strength of the van der Waals interaction. There is good agreement with experiment. The effect of pressure on these structures is explained qualitatively. The increased occurrence of the ZnS structure and the decreased stability of the CsCl structure in the A²⁺ B²⁻ crystals compared to the A⁺B⁻ crystals is also explained. Finally it is shown that the radius ratio and the polarizabilities of the ions are the important factors that determine the structures of AB₂ crystals.     

Dielectric behaviour of anisotropic ionic crystals

In the case of cubic ionic crystals Havinga has shown that the temperature variation of dielectric constant could be described in terms of volume and temperature effects. By extending his formalism to anisotropic, ionic crystals it has been shown that unlike in cubic ionic crystals where the volume effect consists of a change in the number per unit volume of the polarizable particles and their polarizability with volume, in the case of anisotropic ionic crystals, in addition to these, a variation in the anisotropy of polarizability due to uneven thermal expansion also has to be taken into account. This method of analysis has been examined by taking rutile as an example.     

Excess molar volumes and isentropic compressibilities of binary liquid mixtures containing n-alkanes at 298.15 K

Excess molar volumes (V ^E) and deviation in isentropic compressibilities (Δβ _s) have been investigated from the density ρ and speed of sound u measurements of six binary liquid mixtures containing n-alkanes over the entire range of composition at 298.15 K. Excess molar volume exhibits inversion in sign in one binary mixture, i.e., n-heptane + n-hexane. Remaining five binary mixtures, n-heptane + toluene, cyclohexane + n-heptane, cyclohexane + n-hexane, toluene + n-hexane and n-decane + n-hexane show negative excess molar volumes over the whole composition range. However, the large negative values of excess molar volume becomes domainant in toluene + n-hexane mixture. Deviation in isentropic compressibility is negative over the whole range of composition in the case of all the six binary mixtures. Existence of specific intermolecular interactions in the mixtures has been analyzed in terms of excess molar volume and deviation in isentropic compressibility.     

Apparent contrast of molecularly thin films of water at ionic crystal surfaces

We focus the apparent contrast of water islands formed at ionic crystal surfaces in air at room temperature (RT) using the non-contact monitoring mode of a scanning force microscope (SFM). Their apparent heights of water islands formed on LiF(001), CaF₂(111) and BaF₂(111) surfaces have been measured to be about 2, 1 and 1 nm, respectively. The water growth of a NaF(001) surface has two phases (phase I and phase II). The apparent heights of phase I and phase II have been measured to be about 2 and 1.5 nm, respectively. The apparent height of water films is shown to be strongly modified by the dielectric constant of sample and water, and the true thickness of water islands is discussed.     

Sensitivities of ionic explosives

ABSTRACT

We have investigated the relevance for ionic explosive sensitivity of three factors that have been demonstrated to be related to the sensitivities of molecular explosives. These are (1) the maximum available heat of detonation, (2) the amount of free space per molecule (or per formula unit) in the crystal lattice and (3) specific features of the electrostatic potential on the molecular or ionic surface. We find that for ionic explosives, just as for molecular ones, there is an overall tendency for impact sensitivity to increase as the maximum detonation heat release is greater. This means that the usual emphasis upon designing explosives with large heats of detonation needs to be tempered somewhat. We also show that a moderate detonation heat release does not preclude a high level of detonation performance for ionic explosives, as was already demonstrated for molecular ones. Relating the free space per formula unit to sensitivity may require a modified procedure for ionic explosives; this will continue to be investigated. Finally, an encouraging start has been made in linking impact sensitivities to the electrostatic potentials on ionic surfaces, although limited so far to ammonium salts.     

Entropy and ionic conductivity

It is known that the ionic conductivity can be obtained by using the diffusion constant and the Einstein relation. We derive it here by extracting it from the steady electric current which we calculate in three ways, using statistics analysis, an entropy method, and an entropy production approach.     

The relation between the radii and the densities of magnetic skyrmions

Compared with the traditional magnetic bubble, a skyrmion has a smaller size, and better stability and therefore is considered as a very promising candidate for future memory devices. When skyrmions are manipulated, erased and created, the density of skyrmions can be varied, however the relationship between the radii and the densities of skyrmions needs more exploration. In this paper, we study this problem both theoretically and by using the lattice simulation. The average radius of skyrmions as a function of material parameters, the strength of the external magnetic field and the density of skyrmions is obtained and verified. With this explicit function, the skyrmion radius can be easily predicted, which is helpful for the future study of skyrmion memory devices.     

Compression mechanism of GaF3 and FeF3: a high-pressure X-ray diffraction study

The VF₃-type compounds MF₃ with M = Fe and Ga have been studied by high-pressure energy-dispersive X-ray diffraction. The compression mechanism was found to be highly anisotropic for both compounds, with the c-axis showing little pressure dependence. The volume reduction is mainly achieved through coupled rotations of the MF₆ octahedra around the c-axis, which reduces the length of the a-axis. The compression mechanism of both compounds is reasonably well described in terms of deformation of an 8/3/c2 sphere-packing model up to the pressures where the fluorine atoms become hexagonally close-packed. It is proposed that both compounds enter a phase with the fluorine atom arranged in a “super-dense” sphere packing at higher pressures. The zero pressure bulk modulus of FeF₃ and GaF₃ was determined as 12(2) and 37(3) GPa, respectively, and a scaling relation between the zero pressure bulk modulus and unit cell volumes was found for TiF₃, CrF₃, FeF₃ and GaF₃.     

Enhanced conductivity in ionic conductor-insulator composites: numerical models in two and three dimensions

We describe a two-dimensional (2D) and a three-dimensional (3D) percolation model for ionic conductor-insulator composites such as copper(I) bromide-titanium dioxide (CuBr-TiO₂) or lithium iodide-alumina (LiI-Al₂O₃). These composites present an enhanced conductivity closely related to the insulator concentration. This effect is explained by the formation of highly conducting space charge regions near the phase boundaries which are represented by good conductor bonds. Our numerical model takes into account grain size and correlation effects. The dimension has a leading role for the conduction properties. In the 2D case, the good conductor bonds do not percolate, whatever the insulator concentration, and the maximum conductivity of the composite samples is of the same order as that of the ionic conductor grains. The behavior of the system is very different in the 3D case where, for a large domain of composition, the good conductors percolate through the regions between the conductor grains. For the CuBr-TiO₂ composites the conductivity versus composition curve is bell-shaped. Conversely, in the LiI-Al₂O₃ system, a linear relation between the conductivity and the insulator volume fraction is obtained in the experiments. Our model gives a plausible interpretation of the conductivity in both systems. Received 10 April 2001     

Radiation-induced electronic and ionic charge storage and release in sapphire

Radiation-induced thermally stimulated relaxation (TSR) processes in the reduced f -Al 2 O 3 (sapphire) crystal were investigated at 290-650 v K by means of the TS current (TSC), ionic depolarisation current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarisation of sapphire wide (～75 v K) and asymmetric ionic dipolar TSDC peak at T max , 590 v K (disorientation of the anion vacancy-related dipoles) was detected. This peak correlates with the wide TSEE peak at T max , 615 v K, the radiation-induced electrical degradation (RIED) yield rise above 550 v K ( T max , 745 v K) and the chromium emission line broadening in ruby. Above 450-500 v K the anion vacancy hopping (migration) starts. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphire (especially in vacuo near the sample surface, grain boundaries, dislocations) in various TSR (TSC, TSDC, TS heat release and bleaching) and RIED phenomena. Surface structure and impurity content, surrounding atmosphere (vacuum or air) and electric fields determine these phenomena.     

Ionic and Mixed Conductors for Electrochemical Devices

The contribution gives a brief overview on the use of ionic and mixed conductors for electrochemical devices such as fuel cells, batteries, sensors, permeation membranes or electrochemical windows. Special emphasis is laid on the understanding of thermodynamics and kinetics of charge carrier concentration and mobilities, in particular on the dependence of the properties on the control parameters. A second part of the contribution is devoted to the use of this knowledge to optimise electrochemical properties and devices. In this context the introduction of interfaces and the use of the spacing of interfaces as practical “degrees of freedom” are discussed with regard to future applications.     

On ionic conduction in the α-phase of PbSnF4

The structure re-refinement of tetragonal PbSnF₄ was executed using data reported earlier. The rms amplitude in M2(Sn) site is larger in (001) plane than along the c-axis and the thermal vibrations of F3 fluorine ions are very large, i.e. B_eq = 12.415(20) A². M2F₈ polyhedra form a distorted cube. The fluorine ions on the F3 site are present between the M2 ion layers. The fluorine ions on the F3 site are located between the M2 ion layers and their weak bonds to Sn ions play an important role in the conduction. The ionic conductivity in the (001) plane is higher than that in the 001 direction.     

Predicting the viscosity and electrical conductivity of ionic liquids on the basis of theoretically calculated ionic volumes

Selected physical properties of the ionic liquids might be quantitatively predicted based on the volumes of the ions these systems are composed of. It is demonstrated that the ionic volumes calculated using relatively simple theoretical quantum chemistry methods can be utilised to estimate the viscosities and electrical conductivities of various commonly used ionic liquids. The fitting formulas of the exponential form are offered and their predictive usefulness is verified. The quality of such predictions is discussed on the basis of several ionic liquids involving [Tf₂N]^? and [BF₄]^? anions and 16 various cations. The dependence of the viscosity and electrical conductivity of the ionic liquids on the temperature is also investigated and the temperature-dependent equations are derived and compared to the experimentally measured values.