Simple Ionic Solids Research Articles

A program system for self-consistent embedded potentials for ionic crystals

Embedded-cluster models of crystalline solids are important to allow accurate wave function methods to be applicable to solids. The ab initio model potential method for embedding ionic solids has historically been shown to be a viable tool. While useful, the method has been limited by the need to generate such potentials for each crystal structure and the lack of a freely available program for generating ab initio model potentials. Herein, this is remedied by showcasing a new, AFL licensed, program, SCEPIC, which can be used in combination with Molcas or OpenMolcas codes to derive ab initio model potentials for ionic crystals.The applicability of ab initio model potentials derived via SCEPIC is evaluated for three simple ionic solids: MgO, CaO and CaF2. The following questions are addressed: (i) the capability of the method to reproduce the density matrix from periodic density functional theory calculations, (ii) the feasibility of performing geometry optimisations, (iii) the possibility to model band gaps of insulators and (iv) the ligand-field splitting of Ni-doped MgO. Going beyond the classical restriction of parametrising ab initio model potentials only at the Hartree–Fock level-of-theory, this work additionally address the sensitivity of results to the underlying Hamiltonian used to derive the potentials. The results demonstrate that good agreement with periodic density functional theory calculations can be achieved, geometry optimisations are feasible and accurate band gaps and ligand-field splittings can be computed.

Application of XDM to ionic solids: The importance of dispersion for bulk moduli and crystal geometries.

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.

Ambient Heat Capacities and Entropies of Ionic Solids: A Unique View Using the Debye Equation

Entropies of solids are obtained experimentally as integrals of measured heat capacities over the temperature range from zero to ambient. Correspondingly, the Debye phonon distribution equation for solids provides a theoretical connection between these two chemical thermodynamic measures. We examine how the widely applicable Debye equation illuminates the relation between the corresponding experimental measures using more than 250 ionic solids. Estimation of heat capacities for simple ionic solids by the Dulong-Petit heat capacity limit, by the Neumann-Kopp elemental sum, and by the ion sum method is examined in relation to the Debye equation. We note that, and explain why, the ambient temperature heat capacities and entropies of ionic solids are found to be approximately equal, and how deviations from equality may be related to the Debye temperature, ΘD, which characterizes the Debye equation. It is also demonstrated that Debye temperatures may be readily estimated from the experimental ratio of ambient heat capacity to entropy, C(p)/S(p), rather than requiring resort to elaborate theoretical or experimental procedures for their determination. Correspondingly, ambient mineral entropies and heat capacities are linearly correlated and may thus be readily estimated from one another.

Flexoelectric polarization of barium strontium titanate in the paraelectric state

The strain-gradient-induced polarization (flexoelectric effect) was investigated in Ba0.67Sr0.33TiO3 (BST) ceramic at temperatures above the 21 °C Curie point. At 23 °C the flexoelectric coefficient μ12 was more than one order of magnitude greater than the highest value measured in lead magnesium niobate ceramic. Over the temperature range of linear Curie–Weiss behavior, the coefficient μ12 was roughly proportional to the dielectric permittivity; however, the constant of proportionality was higher than predicted for simple ionic solids. The unexpected behavior in the BST ceramic suggests the need for a broader database of flexoelectric coefficients.

A Window on the Solid State: Part I: Structures of Metals; Part II: Unit Cells of Metals; Part III: Strucutres of Ionic Solids; Part IV: Unit Cells of Ionic Solids (Abstract of Volume 5D, Number 2)

A Window on the Solid State helps students understand and instructors present the structural features of solids. Parts I and II were published previously by JCE Software (1) and Macintosh versions of Parts I and II are also available (2). Parts I and II have been updated to include improvements in art and minor changes in logic. Parts III and IV expand the collection to include the structures of simple ionic solids using the visual effects available in an interactive computer medium. The package provides a tour of the structures commonly used to introduce features of the solid state. Full Text Article

Self-consistent tight-binding theory of elasticity in ionic solids.

Intra-atomic and interatomic Coulomb contributions to the total energy are added to the tight-binding theory of bonding in simple ionic solids. They are found to remove the principal discrepancies with experiment found earlier for the bulk modulus and Grueneisen constant for the alkali halides and alkaline-earth chalcogenides. The analysis is extended also to predict the full elastic tensor for these systems and the pressure dependence of that tensor, in good accord with experiment where it exists.

Surface excitons on ionic crystals

We have recorded electronic EEL spectra of (001) faces of a number of simple ionic solids, with higher resolution and much lower beam currents than in previous work. We are able to confirm reports that some, but not all, solids show surface excitons at lower energy than the bulk values. We propose a new model for surface excitons, involving the excitation of an electron to a Rydberg-like state outside the crystal surface.

Surface excitons on ionic crystals

We have recorded electronic EEL spectra of (001) faces of a number of simple ionic solids, with higher resolution and much lower beam currents than in previous work. We are able to confirm reports that some, but not all, solids show surface excitons at lower energy than the bulk values. We propose a new model for surface excitons, involving the excitation of an electron to a Rydberg-like state outside the crystal surface.

Effective interatomic interactions and lattice mechanical properties of the NiO crystal for both paramagnetic and antiferromagnetic phases

The antiferromagnetic oxide crystals behave rather unusually in respect to many physical properties, and the various attempts to understand them in terms of models which satisfactorily describe the simple ionic solids show conspicuous discrepancies between theory and experiment. One of the major difficulties common to all the existing calculations is that it is not possible to give a consistent description of the elastic properties and the dispersion of phonons of such a crystal in the antiferromagnetic phase within the framework of a single model. Moreover, so far, the magnetic and the mechanical properties of these crystals have been studied, one independent of the other. Apart from this difficulty, a simultaneous description of the dielectric properties and the phonon dispersion with the existing models also presents several problems. In a previous work we have suggested a model which is based on a microscopic analysis of the energy expression of an assembly of ions. Assuming a specific form of the superexchange interaction that is suggested by various experimental and theoretical investigations and incorporating the same in the above model, we have been able to reduce significantly the discrepancies mentioned above for the case of NiO crystal. In addition to this, it has also been possible to present a unified description of the magnetic and the mechanical properties of this crystal with a single model. The specific properties that we have attempted to correlate with a single set of model parameters are: the cohesive and the stability properties, the elastic and the dielectric properties, the dispersion relation of phonons and magnons, and the sublattice magnetization properties. We also indicate the rough order of magnitude of the different interactions implied in the model and the relations that they bear to the unusual features of this solid. Some of the limitations of the present model and the directions of further refinement are also discussed.

Unified study of the lattice mechanical properties of the AgCl crystal

The silver halide crystals have many peculiarities in their physical properties which are uncommon among the simple ionic solids. The different theoretical models used to calculate the properties of this group of solids have concentrated on explaining one or the other of them. But these calculations have one major difficulty common to all. It has not been possible to give a consistent description of the dielectric properties and the dispersion of phonons in the framework of a single model. We have shown from a microscopic analysis of the energy expression of a system of ions occupying arbitrary positions that there exist two types of short-range polarization mechanisms, one arising out of the overlap of the unperturbed wave functions and another due to the perturbation of the wave function. These two effects taken together resolve the incompatibility mentioned above. Based on this analysis we have proposed a model in terms of which we have correlated the cohesion, the stability, the phase-transition properties, the elastic and the dielectric properties, and the phonon dispersion of the AgCl crystal. It is seen that the present model broadly reproduces all the characteristic features of this crystal. Moreover, we have tried to assess in a rough way the relative importance of the different interactions envisaged and their relation to the typicalities of this solid. Finally, some of the limitations of the present calculation are also pointed out.

Three‐Body Interaction in the Fluorite Structure Ionic Crystals

AbstractThe importance of the three‐body interaction in the study of simple ionic solids with centrosymmetric structure is both, theoretically and empirically well established. But so far there is no attempt to investigate its significance in the properties of ionic crystals belonging to fluorite and antifluorite structures. In contrast to said alkali halides the absence of the centre of symmetry for this structure implies complications which make a direct estimate of this effect quite difficult. From an analysis of the elastic data the inadequacy of a two‐body central interaction to describe the elastic constants of such crystals is demonstrated. An order of estimate is made of the three‐body interaction in the elastic properties of the calcium fluorite crystal from the HF wavefunctions of the ions. For the other crystals where the Clementi wavefunctions for the ions are not available the estimate is done directly from the elastic constants. The order of magnitude of this interaction obtained for the different crystals is comparable and lies in the range observed for other ionic solids. Further this investigation apart from indicating the importance of the three‐body interaction also accounts for the anomalies in the previous calculations. Finally some of the limitations of the present calculation are pointed out.

Self-trapping of electronic excitations and radiolysis of ionic solids

Abstract This paper describes the structure of self-trapped holes and self-trapped excitons in some simple ionic solids and discusses the role of the self-trapped exciton in radiolysis.

Lattice energies: calculations from pressure—volume data, and uncertainties in the results

A method of calculating lattice energies is developed, which reproduces directly the observed pressure—volume curve of some simple ionic solids. The results in general are close to the experimental values, and show deviations usually between 5 and 10 kJ mole −1. The calculated values are not systematically too high or too low. Some estimates are made of the effect of errors in the P–V curve on the calculated lattice energies.

Thermotransport of 85Sr and 54Mn in potassium chloride single crystals

The thermotransport of 85Sr and 54Mn tracers in single crystals of KCl was measured in the temperature range 950–1025 K, by means of Soret effect measurements. The mean value of the impurity heat of transport was −3.18±0.09 eV for Sr in pure or Sr-doped crystals, and was −5.45±0.83 eV for Mn in pure or Mn-doped crystals. There was no significant difference between values from pure and doped crystals for Sr, but for Mn this may not be the case. A discussion of the experimental and theoretical limitations of such measurements as a source of information about the heats of transport for individual jumps was given, and a comparison made with earlier thermotransport measurements in simple ionic solids.

THERMOTRANSPORT OF 85Sr AND 54Mn IN POTASSIUM CHLORIDE SINGLE CRYSTALS

The thermotransport of 85Sr and 54Mn tracers in single crystals of KCl was measured in the temperature range 950–1025 K, by means of Soret effect measurements. The mean value of the impurity heat of transport was −3.18±0.09 eV for Sr in pure or Sr‐doped crystals, and was −5.45±0.83 eV for Mn in pure or Mn‐doped crystals. There was no significant difference between values from pure and doped crystals for Sr, but for Mn this may not be the case. A discussion of the experimental and theoretical limitations of such measurements as a source of information about the heats of transport for individual jumps was given, and a comparison made with earlier thermotransport measurements in simple ionic solids.

Application of infrared spectroscopy to order-disorder problems in simple ionic solids

Infrared powder transmission spectra have been obtained on selected pairs of structures related by distortional and order-disorder phase transformations. It is shown that lowering of space group symmetry causes resolvable and interpretable detail to appear in the spectrum only when distortions are extreme. Ordering tends to cause dramatic changes in the spectra. In both cases a factor group analysis is necessary to interpret the results.

On the channelling of protons in ionic solids

Abstract This paper is concerned with the theoretical aspects of the channelling of protons in the simple ionic solids of the rock-salt and caesium chloride structures. It is shown that the electrostatic fields occurring between the constituent positive and negative ions can affect the trajectories of protons travelling along certain crystal directions. In particular, the ions surrounding the 〈100〉 direction of the rock-salt structure form a series of electrostatic quadrupole lenses which constrain the proton's path to the central region of the channel. This quadrupole effect is compared to the channelling produced by the interaction with the exponentially screened Coulomb potential of the nuclei, and is shown to be significant only at relatively low proton energies below 10 kev.