Search

The melting temperature is important for materials design because of its relationship with thermal stability, synthesis, and processing conditions. Current empirical and computational melting point estimation techniques are limited in scope, computational feasibility, or interpretability. We report the development of a machine learning methodology for predicting melting temperatures of binary ionic solid materials. We evaluated different machine-learning models trained on a dataset of the melting points of 476 non-metallic crystalline binary compounds using materials embeddings constructed from elemental properties and density-functional theory calculations as model inputs. A direct supervised-learning approach yields a mean absolute error of around 180 K but suffers from low interpretability. We find that the fidelity of predictions can further be improved by introducing an additional unsupervised-learning step that first classifies the materials before the melting-point regression. Not only does this two-step model exhibit improved accuracy, but the approach also provides a level of interpretability with insights into feature importance and different types of melting that depend on the specific atomic bonding inside a material. Motivated by this finding, we used a symbolic learning approach to find interpretable physical models for the melting temperature, which recovered the best-performing features from both prior models and provided additional interpretability. [ABSTRACT FROM AUTHOR]

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid–solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4]. [ABSTRACT FROM AUTHOR]

Computing the solubility of crystals in a solvent using atomistic simulations is notoriously challenging due to the complexities and convergence issues associated with free-energy methods, as well as the slow equilibration in direct-coexistence simulations. This paper introduces a molecular-dynamics workflow that simplifies and robustly computes the solubility of molecular or ionic crystals. This method is considerably more straightforward than the state-of-the-art, as we have streamlined and optimised each step of the process. Specifically, we calculate the chemical potential of the crystal using the gas-phase molecule as a reference state, and employ the S0 method to determine the concentration dependence of the chemical potential of the solute. We use this workflow to predict the solubilities of sodium chloride in water, urea polymorphs in water, and paracetamol polymorphs in both water and ethanol. Our findings indicate that the predicted solubility is sensitive to the chosen potential energy surface. Furthermore, we note that the harmonic approximation often fails for both molecular crystals and gas molecules at or above room temperature, and that the assumption of an ideal solution becomes less valid for highly soluble substances. [ABSTRACT FROM AUTHOR]

We study the performance of spin-component-scaled second-order Møller–Plesset perturbation theory (SCS-MP2) for the prediction of the lattice constant, bulk modulus, and cohesive energy of 12 simple, three-dimensional covalent and ionic semiconductors and insulators. We find that SCS-MP2 and the simpler scaled opposite-spin MP2 (SOS-MP2) yield predictions that are significantly improved over the already good performance of MP2. Specifically, when compared to experimental values with zero-point vibrational corrections, SCS-MP2 (SOS-MP2) yields mean absolute errors of 0.015 (0.017) Å for the lattice constant, 3.8 (3.7) GPa for the bulk modulus, and 0.06 (0.08) eV for the cohesive energy, which are smaller than those of leading density functionals by about a factor of two or more. We consider a reparameterization of the spin-scaling parameters and find that the optimal parameters for these solids are very similar to those already in common use in molecular quantum chemistry, suggesting good transferability and reliable future applications to surface chemistry on insulators. [ABSTRACT FROM AUTHOR]

Organic ionic plastic crystals (OIPCs), which consist of organic molecular ions, are considered excellent candidates for solid electrolytes due to their high ionic conductivity in solid phases. Molecular ions undergo either rotational or conformational relaxation at certain temperatures in OIPCs. There have been molecular simulations to understand the rotational motion. The polarizability of ions was, however, often ignored in simulations due to the high computational cost. Since the polarizability may affect the translational diffusion, the ionic conductivity, and the phase transition of ionic liquids, it should be of interest to investigate how the polarizability would affect the rotational diffusion of ions in solid phases. In this work, we perform extensive atomistic molecular dynamics simulations for two different kinds of OIPCs, 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF6]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). We employ various simulation models for ions by turning on and off the polarization in their interaction potentials. We find that the polarizability hardly affects the density, the crystalline structure, and the phase transition of both OIPCs. However, a certain rotational motion, especially the rotational diffusion of PF 6 − in [MMIM][PF6] OIPCs, is enhanced by a factor of up to four when the polarizability is turned on. The PF 6 − in [MMIM][PF6] OIPCs undergoes rotational hopping motions more significantly due to polarizability. We find that the rotational diffusion of a certain ion can be enhanced only when the polarization results in a significant change in the dipole moment of the neighboring ions around the ion. [ABSTRACT FROM AUTHOR]

Ionic liquid (IL)-based solid polymer electrolytes (SPE) with stable thermal properties and low electrical resistivity have been evaluated. Two candidates for the polymer component of the SPE, poly(ethylene glycol) diacrylate (PEGDA) and Nafion, were considered. Differential scanning calorimetry analysis and electrical resistivity tests revealed that PEGDA, in comparison to Nafion, enables the formation of uniform SPEs with lower electrical resistivity and better thermal stability within a range of 25 °C–170 °C. Therefore, PEDGA was selected for further evaluation of the IL component effect on the resulting SPE. Six IL candidates, including 1-butyl-3-methylimidazolium methanesulfonate ± methanesulfonic acid (BMIM.MS ± MSA), diethylmethylammonium triflate ±bis(trifluoromethanesulfonyl)imine (Dema.OTF±HTFSI), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ± bis(trifluoromethanesulfonyl)imine (BMIM.TFSI ± HTFSI), were selected to test the effect of hydrophobicity/hydrophilicity of the IL on the resulting SPE. Fourier transformation infrared spectrometer analysis revealed that the BMIM.MSA-based electrolytes have the highest tendency to absorb from the environment and keep the moisture, while Dema.OTF has the fastest curing time. The SPE candidates were further evaluated for absorption characteristics of different gasses and vapors, such as N2, O2, ethanol vapor, and diluted CO/N2, that were tested with the in situ quartz crystal microbalance (QCM) technique. Among all six candidates, BMIM.MS showed the largest N2 and O2 absorption capacity from the environment. Dema.OTF + HTFSI, meanwhile, demonstrated a higher level of interactions with the ethanol vapor. In the case of CO/N2, QCM analysis revealed that BMIM.MS+MSA has the largest, ∼13 µg/cm2, absorption capacity that is reached within 400 s of being exposed to the gas mixture. [ABSTRACT FROM AUTHOR]

Previous theoretical studies of calamitic (i.e., rod-like) ionic liquid crystals (ILCs) based on an effective one-species model led to indications of a novel smectic-A phase with a layer spacing being much larger than the length of the mesogenic (i.e., liquid–crystal forming) ions. In order to rule out the possibility that this wide smectic-A phase is merely an artifact caused by the one-species approximation, we investigate an extension that accounts explicitly for cations and anions in ILCs. Our present findings, obtained by grand canonical Monte Carlo simulations, show that the phase transitions between the isotropic and the smectic-A phases of the cation–anion system are in qualitative agreement with the effective one-species model used in the preceding studies. In particular, for ILCs with mesogens (i.e., liquid–crystal forming species) carrying charged sites at their tips, the wide smectic-A phase forms, at low temperatures and within an intermediate density range, in between the isotropic and hexagonal crystal phases. We find that in the ordinary smectic-A phase, the spatial distribution of the counterions of the mesogens is approximately uniform, whereas in the wide smectic-A phase, the small counterions accumulate in between the smectic layers. Due to this phenomenology, the wide smectic-A phase could be interesting for applications, which hinge on the presence of conductivity channels for mobile ions. [ABSTRACT FROM AUTHOR]

Dispersion corrections are essential in the description of intermolecular interactions; however, dispersion-corrected functionals must also be transferrable to hard solids. The exchange-hole dipole moment (XDM) model has demonstrated excellent performance for non-covalent interactions. In this article, we examine its ability to describe the relative stability, geometry, and compressibility of simple ionic solids. For the specific cases of the cesium halides, XDM-corrected functionals correctly predict the energy ranking of the B1 and B2 forms, and a dispersion contribution is required to obtain this result. Furthermore, for the lattice constants of the 20 alkali halides, the performance of XDM-corrected functionals is excellent, provided that the base functional's exchange enhancement factor properly captures non-bonded repulsion. The mean absolute errors in lattice constants obtained with B86bPBE-XDM and B86bPBE-25X-XDM are 0.060 Å and 0.039 Å, respectively, suggesting that delocalization error also plays a minor role in these systems. Finally, we considered the calculation of bulk moduli for alkali halides and alkaline-earth oxides. Previous claims in the literature that simple generalized gradient approximations, such as PBE, can reliably predict experimental bulk moduli have benefited from large error cancellations between neglecting both dispersion and vibrational effects. If vibrational effects are taken into account, dispersion-corrected functionals are quite accurate (4 GPa–5 GPa average error), again, if non-bonded repulsion is correctly represented. Careful comparisons of the calculated bulk moduli with experimental data are needed to avoid systematic biases and misleading conclusions. [ABSTRACT FROM AUTHOR]

A hierarchy of simplified Hartree-Fock (HF), density functional theory (DFT) methods, and their combinations has been recently proposed for the fast electronic structure computation of large systems. The covered methods are a minimal basis set Hartree–Fock (HF-3c), a small basis set global hybrid functional (PBEh-3c), and its screened exchange variant (HSE-3c), all augmented with semiclassical correction potentials. Here, we extend their applicability to inorganic covalent and ionic solids as well as layered materials. The new methods have been dubbed HFsol-3c, PBEsol0-3c, and HSEsol-3c, respectively, to indicate their parent functional as well as the correction potentials. They have been implemented in the CRYSTAL code to enable routine application for molecular as well as solid materials. We validate the new methods on diverse sets of solid state benchmarks that cover more than 90 solids ranging from covalent, ionic, semi-ionic, layered, and molecular crystals. While we focus on structural and energetic properties, we also test bandgaps, vibrational frequencies, elastic constants, and dielectric and piezoelectric tensors. HSEsol-3c appears to be most promising with mean absolute error for cohesive energies and unit cell volumes of molecular crystals of 1.5 kcal/mol and 2.8%, respectively. Lattice parameters of inorganic solids deviate by 3% from the references, and vibrational frequencies of α-quartz have standard deviations of 10 cm−1. Overall, this shows an accuracy competitive to converged basis set dispersion corrected DFT with a substantial increase in computational efficiency. [ABSTRACT FROM AUTHOR]

Tasker type III polar terminations of ionic crystals carry a net surface charge as well as a dipole moment and are fundamentally unstable. In contact with electrolytes, such polar surfaces can be stabilized by adsorption of counterions from the solution to form electric double layers. In a previous work [T. Sayer et al., J. Chem. Phys 147, 104702 (2017)], we reported on a classical force field based molecular dynamics study of a prototype model system, namely, a NaCl(111) slab interfaced with an aqueous NaCl solution on both sides. A serious hurdle in the simulation is that the finite width of the slab admits an electric field in the solid perturbing the theoretical charge balance at the interface of semi-infinite systems [half the surface charge density for NaCl(111)]. It was demonstrated that the application of a finite macroscopic field E canceling the internal electric field can recover the correct charge compensation at the interface. In the present work, we expand this method by applying a conjugate electric displacement field D. The benefits of using D instead of E as the control variable are two fold: it does not only speed up the convergence of the polarization in the simulation but also leads to a succinct expression for the biasing displacement field involving only structural parameters which are known in advance. This makes it feasible to study the charge compensating phenomenon of this prototype system with density functional theory based molecular dynamics, as shown in this work. [ABSTRACT FROM AUTHOR]

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P122i4][PF6], was investigated for CO2 and N2 absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO2 compared with N2. The CO2 absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N2 showed low absorption at both temperatures. The simulations suggest that the significant increase in CO2 absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO2 separations from CO2/N2 mixture. [ABSTRACT FROM AUTHOR]

In this work, we study how including charge transfer into force fields affects the predicted elastic and vibrational Γ-point properties of ionic crystals, in particular those of rock salt. In both analytical and numerical calculations, we find that charge transfer generally leads to a negative contribution to the Cauchy pressure, PC ≡ C12 - C66, where C12 and C66 are elements of the elastic tensor. This contribution increases in magnitude with pressure for different charge-transfer approaches in agreement with results obtained with density functional theory (DFT). However, details of the charge-transfer models determine the pressure dependence of the longitudinal optical-transverse optical splitting and that for partial charges. These last two quantities increase with density as long as the chemical hardness depends at most weakly on the environment while experiments and DFT find a decrease. In order to reflect the correct trends, the charge-transfer expansion has to be made around ions and the chemical (bond) hardness has to increase roughly exponentially with inverse density or bond lengths. Finally, the adjustable force-field parameters only turn out meaningful, when the expansion is made around ions. [ABSTRACT FROM AUTHOR]

The linear and second-order nonlinear optical susceptibilities of three ionic organic crystals, 4-N,N-dimethylamino-4' -N'-methyl-stilbazolium tosylate (DAST), 4-N,N-dimethylamino-4'-N'- methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS), and 4-N,N-dimethylamino-4'-N'- phenyl-stilbazolium hexafluorophosphate (DAPSH), have been calculated by adopting a two-step multi-scale procedure, which consists in calculating: (i) the ion properties using ab initio or density functional theory methods and then (ii) in accounting for the crystal environment effects using classical electrostatic models. Provided that the ionic properties are evaluated at the second-order M0ller-Plesset level and that the dressing field effects using point charges are accounted for, the agreement with experiment is excellent and enables to explain the origin of the larger χ(2) response of DAPSH with respect to DAST and DSTMS. The study has also demonstrated that including the dressing field leads to a decrease of the χ(2) response of ionic crystals whereas its effect is opposite for molecular crystals. Moreover, the results have also demonstrated that this multi-scale approach can be used to interpret the impact of the nature and position of the counterion on the linear and nonlinear optical susceptibilities of ionic crystals. Finally, it has been shown that the use of a conventional exchange-correlation functional like B3LYP leads to severe overestimations of χ(1) but large underestimations of χ(2) whereas the use of homogeneous dipole field is not recommended because it usually leads to overestimations of the linear and nonlinear optical susceptibilities. [ABSTRACT FROM AUTHOR]

We tested the isotropic periodic sum (IPS) method for computing Madelung energies of ionic crystals. The performance of the method, both in its nonpolar (IPSn) and polar (IPSp) forms, was compared with that of the zero-charge and Wolf potentials [D. Wolf, P. Keblinski, S. R. Phillpot, and J. Eggebrecht, J. Chem. Phys. 110, 8254 (1999)]. The results show that the IPSn and IPSp methods converge the Madelung energy to its reference value with an average deviation of ~10-4 and ~10-7 energy units, respectively, for a cutoff range of 18-24a (a/2 being the nearest-neighbor ion separation). However, minor oscillations were detected for the IPS methods when deviations of the computed Madelung energies were plotted on a logarithmic scale as a function of the cutoff distance. To remove such oscillations, we introduced a modified IPSn potential in which both the localregion and long-range electrostatic terms are damped, in analogy to the Wolf potential. With the damped-IPSn potential, a smoother convergence was achieved. In addition, we observed a better agreement between the damped-IPSn and IPSp methods, which suggests that damping the IPSn potential is in effect similar to adding a screening potential in IPSp. [ABSTRACT FROM AUTHOR]

Self-assembly of a binary monolayer of charged particles is modeled using molecular dynamics and statistical mechanics. The equilibrium phase diagram for the system has three distinct phases: an ionic crystal; a geometrically ordered crystal with disordered charges; and a fluid. We show that self-assembly occurs near the phase transition between the ionic crystal and the fluid, and that the rate of ordering is sensitive to the applied pressure. By assuming an Arrhenius form for the rate of ordering, an optimality condition for the temperature and pressure is derived that maximizes the rate. Using the Clausius-Clapeyron equation, the optimal point on the phase boundary is expressed in terms of the thermodynamic changes in state variables across the boundary. The predicted optimal temperature and pressure conditions are in good agreement with numerical simulations and result in self-organization rates five times that of a simulation without applied pressure. [ABSTRACT FROM AUTHOR]

X-ray powder diffraction, a fundamental technique of structure research in physics, chemistry, and biology, is extended into the femtosecond time domain of atomic motions. This allows for mapping (macro)molecular structure generated by basic chemical and biological processes and for deriving transient electronic charge density maps. In the experiments, the transient intensity and angular positions of up to 20 Debye Scherrer reflections from a polycrystalline powder are measured and atomic positions and charge density maps are determined with a combined spatial and temporal resolutions of 30 pm and 100 fs. We present evidence for the so far unknown concerted transfer of electrons and protons in a prototype material, the hydrogen-bonded ionic ammonium sulfate [(NH4)2SO4]. Photoexcitation of ammonium sulfate induces a sub-100 fs electron transfer from the sulfate groups into a highly confined electron channel along the c-axis of the unit cell. The latter geometry is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps derived from the diffraction data display a periodic modulation of the channel’s charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. Our results set the stage for femtosecond structure studies in a wide class of (bio)molecular materials. [ABSTRACT FROM AUTHOR]

An efficient real space method is derived for the evaluation of the Madelung’s potential of ionic crystals. The proposed method is an extension of Evjen’s method. It takes advantage of a general analysis of the potential convergence in real space. Indeed, we show that the series convergence is exponential as a function of the number of canceled multipolar moments in the unit cell. The method proposed in this work reaches such an exponential convergence rate. Its efficiency is comparable to Ewald’s method. However, unlike the latter, it uses only simple algebraic functions. [ABSTRACT FROM AUTHOR]

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pédrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U4+-doped Cs2GeF6. Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF6)2- and (UF6Cs8)6+ clusters embedded in Cs2GeF6, in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF6 volume and their U–F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f17s1 configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals. [ABSTRACT FROM AUTHOR]

Despite extensive experimental, theoretical, and simulation efforts, a unified description of ionic phase transitions and criticality has not yet emerged. In this work, we investigate the phase behavior of the restricted primitive model of elec0trolyte solutions on the simple cubic lattice using grand canonical Monte Carlo simulations and finite-size scaling techniques. The phase diagram of the system is distinctly different from its continuum-space analog. We find order-disorder transitions for reduced temperatures T*≤=0.51, where the ordered structures resemble those of the NaCl crystal. The order-disorder transition is continuous for 0.15≤=T*≤=0.51 and becomes first order at lower temperatures. The line of first-order transitions is a line of three-phase coexistence between a disordered and two ordered phases. The line of continuous, second-order transitions meets this line of triple points at a tricritical point at T*≃0.1475. We locate the line of continuous transitions, and the line of triple points using finite-size scaling techniques. The tricritical temperature is estimated by extrapolation of the size-dependent tricritical temperatures obtained from a sixth-order Landau expansion of the free energy. Our calculated phase diagram is in qualitative agreement with mean-field theories. [ABSTRACT FROM AUTHOR]

The curvature energies of triply periodic minimal surfaces (TPMS) and zero equipotential surfaces (ZEPS) of ionic crystals are both quantized with the Euler-Poincaré characteristic as the "quantum number," and the curvature energy of the TPMS larger than that of the corresponding ZEPS. Quantization is imposed by the charge-defined metric. [ABSTRACT FROM AUTHOR]

An embedding procedure suitable for Hartree–Fock studies of ionic systems is presented and implemented on the semiempirical level. In this approach model clusters are embedded in finite fields of pseudoatoms with average self-consistent orbital occupation numbers or fixed orbital occupation numbers for best charge equilibration of cluster atoms. These pseudoatoms are of the same kind and described with the same set of parameters as the real cluster atoms. The implementation in the semiempirical self-consistent field molecular orbital method SINDO1 is described. Calculations are performed for the systems TiO2 and MgO and the influence of the embedding on geometry and electronic structure is discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an embedded density functional approach to study adsorption on crystalline surfaces. Following ideas suggested by Cortona, Wesolowski, and Warshel, we divide the total system into a quantum cluster and the surrounding lattice whose density is assumed to be the same as in the ideal crystal. In this case the Kohn–Sham Hamiltonian for electrons in the cluster contains additional terms corresponding to the Coulomb, exchange, correlation, and ‘‘nonadditive kinetic energy’’ potentials from the environment. Test calculations for the He and Ar dimers, X–H2O molecular complexes (X=Li+, Na+, K+, F- or Cl-) and water adsorption on the (001) surface of the NaCl crystal suggest that this model provides a promising alternative for cluster models employed earlier for calculations of defects and adsorption on ionic crystals. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ab initio model potentials initially developed as effective core potentials, have been proposed as embedding potentials in the field of embedded cluster calculations on impurities in ionic crystals [J. Chem. Phys. 89, 5739 (1988)] and, since then, efficiently used in the theoretical study of bulk and surface problems. These potentials bring into an ab initio cluster calculation, classical and quantum mechanical interactions with a frozen crystalline environment (Madelung, short-range Coulomb, exchange, and orthogonality) at a reasonable cost. In this paper, we extend the ab initio model potential embedding method in order to include the effects of dipole polarization and site relaxation of lattice ions external to the cluster, which are represented by an empirical shell model. We apply the method to the ab initio calculation of local distortions around a Mn2+ impurity in CaF2 lattice in the ground state (6A1g) and two excited states (4T1g,4A1g) of the cubic MnF6-8 embedded cluster. In this material, x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure measurements exist which provide quantitative experimental values of first and second coordination shell distortions around Mn2+ in its ground state; the comparison with these experiments is very satisfactory. The calculated distortions in the excited states are indirectly validated by calculations of absorption/emission transitions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a simple method to incorporate electrostatic forces into the extended Hückel scheme, thereby extending its range of application to ionic systems. A series of applications to alkali halide molecules and clusters shows the importance of including the Madelung energy in such calculations. The amount of charge transfer, the position of the valence levels, and the band gap are found to depend on the inclusion of the electrostatic potential. The calculated bulk and surface electronic structure compares well with experimental findings. [ABSTRACT FROM AUTHOR]

Various dipole interaction models were compared with one another and with literature values for dissociation energies, multipole moments (u, Q, and Ω), and polarizabilities α for the alkali halide molecules. A displaced-shell (D-shell) model, which takes account of the overlap repulsion as in the shell model but also uses the effective electron shell displacements in calculating the electrostatic interactions, gives slightly better dissociation energies and as good or better dipole and quadrupole moments than the other models. All of the models considered here give mean polarizabilities in fairly good agreement with published SCF values; however, none gives consistently good polarizability anisotropies. It is found that both the overlap repulsion (as treated in the shell model) and the effective displacements of the electron shells (as treated in the D-shell model) have large effects on the calculated polarizability anisotropies relative to the DID model. [ABSTRACT FROM AUTHOR]

An iterative self-consistent determination of the long range interaction energy between an admolecule and a ionic crystal is performed within the scheme of local and response potentials and the definition of the generalized electric susceptibilities of the two partners. The multipolar (electrostatic+induction) contributions and the quantum (dispersion+empirical short range) terms are determined as a sum of interactions between the molecule and the atomic planes parallel to the surface, constituting the crystal. The discrete structure of each plane is described with an increasing accuracy by increasing the order of the Fourier expansion in the reciprocal planar lattice. A semianalytical expression of each contribution is given for a symmetric top molecule adsorbed on a NaCl surface as a function of the location of the center of mass and of the orientation of the molecule. [ABSTRACT FROM AUTHOR]

Examines the shear modulus G of colloidal crystals formed from thoroughly deionized suspensions of charged latex spheres. Features and structures of colloidal solids; Various realizations of torsional resonance spectroscopy; Changes of morphology in randomly oriented polycrystalline; Observation of halolike scattering pattern.

Investigates the crystal structure of cobalt and nickel acetate tetrahydrate at room temperature and liquid-helium temperature. Use of neutron powder diffraction of the fully deutered salts; Calculation of the rotational potential experienced by the methyl group; Comparison of different methods of calculating partial charges in the ionic compounds.

We have measured by means of photoluminescence the energy of crystal-field peaks for RbCdF3: Mn2+ and KZnF3: Mn2+ where the value of the Mn2+–F- distance, R, derived by EXAFS is R=2.13±0.01 Å and R=2.08±0.01 Å, respectively. From these data and those for RbMnF3 and KMnF3 we have studied the dependence on R of the B, C, and 10 Dq parameters for the MnF4-6 complex. This analysis reveals that within the experimental errors, B and C are constant in the range 2.07

We evaluate the potential energy of a He atom near an ionic surface by summing a two-body potential as derived from molecular physics. Corrugation parameters and binding energy are derived for a set of four alkali halide surfaces and compared to recent atom scattering data. Finally we comment on the possibility of performing a surface crystallography of ionic crystals by means of atom scattering. [ABSTRACT FROM AUTHOR]

The reflection of infrared light by ionic crystals with cubic symmetry such as lithium fluoride, LiF, is analyzed in terms of phonon-polaritons. In contrast to the conventional view on phonon-polaritons that uses the Coulomb gauge and assumes a purely local dielectric response of the material, we here develop an alternative description making use of the Lorentz gauge. This involves retarded interactions between charges, implying a non-local response of the material to electromagnetic radiation. The resulting new phonon-polariton dispersion relation features polaritons with negative group velocity in the frequency range in between the transverse (ωT) and longitudinal frequency (ωL). By contrast, the conventional description predicts, in zero order, the absence of any propagating polaritons in the frequency interval between ωT and ωL. The new dispersion relation provides an efficient, zero-order description of the fine structure within the reststrahlen band of LiF. The local minimum near the middle of the reflectance band is due to excitation of a phonon-polariton whose energy and momentum matches that of the incoming photon. The Lorentz gauge description can also describe off-normal reflection and accounts for the experimentally observed widening of the reflection band with increasing angle of incidence. [ABSTRACT FROM AUTHOR]

A correction to the usual Madelung form for the electrostatic energy of cubic ionic crystals due to the finite size of real atomic ions is derived. [ABSTRACT FROM AUTHOR]

This work is concerned with the lattice energy of periodic assemblies of mass and charge distributions of the form, exp (-αp2), where α is an adjustable positive variable and p_ is the vector from the lattice site or average position. The energy of interaction between two distributions is the density-weighted integral of the interactions between the volume elements of each distribution. Reciprocal space lattice summation formulas derived for particles represented by gaussian smeared-out density distributions are applied to the gaussian potential and a bounded version of the soft-sphere potential for a range of exponents. Two types of spatial broadening are considered, continuous or physical broadening (PB) and broadening resulting from the time average of point particle positions, so-called 'time' broadening (TB). For neutral mass distributions a reciprocal space lattice summation formula is derived which is applied to the bounded soft-sphere potential. For the charged systems, the methodology described in Heyes [J. Chem. Phys. 74, 1924 (1981)] is used, which for the PB case gives the Ewald-like formulas derived by Gingrich and Wilson [Chem. Phys. Lett. 500, 178 (2010)] using a different method. Another expression for the lattice energy of the spread out charge distributions is derived which is cast entirely in terms of a summation over the reciprocal lattice vectors, without the arbitrary charge spreading function used in the Ewald method. The effects of charge spreading on a generalized definition of the Madelung constant (M) for a selection of crystal lattices are shown to be insignificant for route mean square displacements up to values typical of melting of an ionic crystal. When the length scale of the charge distribution becomes comparable to or greater than the mean inter particle spacing, however, the effects of charge broadening on the lattice energy are shown to be significant. In the PB case, M → 0 for the uniform charge density or α → 0 limit, and M ultimately becomes negative in the TB case for a large enough root mean square displacement (or small enough α). [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations for crystalline benzene (C6H6), pyridinium iodide [C5NH6]+I-, and pyridinium nitrate [C5NH6]+NO3- have been performed as a function of temperature and pressure. Despite the similar shape of the benzene molecule and the pyridinium cation, the experimental and simulated data have showed clear differences in their dynamics. Therefore, the rotational dynamics have been explored in detail by comparing thoroughly the existing experimental results together with new quasielastic neutron scattering (QENS) data obtained for (PyH)NO3 and molecular dynamics simulations. The correlation times, activation energy, geometry of motion of benzene molecule and pyridinium cation, isothermal compressibility, and activation volume obtained from the simulations are compared with the experimental results obtained by nuclear magnetic resonance and QENS methods. MD simulations have also revealed that reorientation of the pyridinium cation in pyridinium nitrate between two inequivalent positions is strongly affected by the hydrogen bond N-H···O between the cation and the anion and the influence of temperature on strength of the hydrogen bond is much more important than that of the pressure. [ABSTRACT FROM AUTHOR]

An embedded ion method is proposed for accurately calculating the [sup 13]C chemical shift tensors in ionic compounds. The method models an ionic crystal by embedding an ion of interest inside an array of point charges. The potential, produced by an infinite ionic lattice, at the location of the ion of interest can be simulated accurately utilizing a point charge array obtained by the Ewald summation method. The Ewald summation method, as implemented in the computer program EWALD, in conjunction with the quantum-mechanics program GAUSSIAN 98 is used to generate a self-consistent point charge array that simulates the Ewald potential in a defined region at the center of the array. Subsequently, the chemical shift tensor calculation is performed using GAUSSIAN 98 on the ion of interest positioned in the region inside the point charge array in which the Ewald potential is established. The embedded ion method was tested on potassium methyl-trithiocarbonate (KS[sub 2]CSCH[sub 3]) whose crystal lattice is composed of potassium cations and molecular S[sub 2]CSCH[sub 3][sup -] anions. The principal values of the [sup 13]C chemical shift tensors in KS[sub 2]CSCH[sub 3] were measured in a stationary cross polarization nuclear magnetic resonance experiment. It is shown that the correlation between experimental and calculated principal values improves significantly when the C-H bond distances are optimized from their x-ray values. It is further demonstrated that a substantial improvement in the correlation is obtained when the chemical shielding tensor calculation is performed on an S[sub 2]CSCH[sub 3][sup -] anion embedded inside a point charge array obtained by the Ewald summation method. The embedded ion method was completed applying the B3P86/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVDZ quantum-mechanical computations and the various results are compared and analyzed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new compressible ion model for describing the energetic components of the cohesive energy of a fully ionic crystal is developed and tested using previous ab initio results for three cubic phases of MgO. This model is physically highly transparent and improves on previous compressible ion models in two ways. First, the short-range cation-anion interaction and the rearrangement energy needed to convert a free O[sup -] ion plus a free electron into an O[sup -2] ion having a form optimal for its in-crystal environment are decomposed into the major contributions originating from the six outermost anion electrons plus smaller terms generated by the two 2s electrons. This model transfers to the B2 and B3 phases of MgO after parametrization on the ab initio data for the B1 phase even more accurately than previous compressible ion models. Second, the separate modeling of the repulsive (permutation) and attractive (penetration) components of the short-range anion-anion interactions enables the new model to describe their subtle dependencies on the in-crystal anion environment that lie beyond the scope of previous models. The new model is also used to illuminate the behavior of two fluorite oxides. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ab initio computation of uncorrelated short range two-body anion-anion potentials V[sub s][sup 0](r[sub AA]) can yield two apparent anomalies. First, despite the common understanding that the repulsion between two closed shell species arises from the overlap of their wave functions, compression of the anion electron densities sometimes increases V[sub s][sup 0](r[sub AA]), even though the overlap is reduced. Second, attractive V[sub s][sup 0](r[sub AA]) are occasionally predicted at large ionic separations r[sub AA]. These apparent anomalies arise because V[sub s][sup 0](r[sub AA]) is the sum of a permutation term V[sub perm][sup 0](r[sub AA]) arising from interionic electron exchange plus a penetration term V[sub pen][sup 0](r[sub AA]), independent of such exchange, equal to the nonpoint Coulombic electrostatic interaction. This is attractive at realistic r[sub AA] and reduced in magnitude by ionic compression. V[sub perm][sup 0](r[sub AA]) is always repulsive and is decreased by ionic compression except occasionally at large r[sub AA] involving an attractive V[sub s][sup 0](r[sub AA]). The latter increases are explained by analyzing V[sub perm][sup 0](r[sub AA]) into two further terms: one involving V[sub pen][sup 0](r[sub AA]). Uniform electron gas density functional predictions of V[sub perm][sup 0](r[sub AA]) are oversensitive to the ion density, thereby missing compression-induced enhancements of V[sub s][sup 0](r[sub AA]). Ab initio predictions of V[sub pen][sup 0](r[sub AA]) and V[sub perm][sup 0](r[sub AA]) are presented both for "optimal" V[sub s][sup 0](r[sub AA]) computed using anion wave functions optimal for each crystal geometry and for "frozen" V[sub s][sup 0](r[sub AA]), where the entire potential is computed using the anion wave function optimal for a geometry very close to that of the crystal at equilibrium. This data plus the total "frozen" V[sub s](r[sub AA]) consisting of V[sub s][sup 0](r[sub AA]) plus an approximate electron cor... [ABSTRACT FROM AUTHOR]