Dipole-induced-dipole polarizabilities of symmetric clusters

Symmetry arguments are applied to the determination of closed-form solutions to the classical dipole-induced-dipole model of interaction polarizabilities for polyhedral clusters of cubic and icosahedral symmetry. Analytical conditions for the range of applicability of the model and onset of the 'polarization catastrophe' are given. These conditions are expressed as transformed versions of the hyperbolic curve of divergence that is already present in the classical case of an interacting pair. Clusters based on the cube and the icosahedron support unique patterns of induced dipoles, but other physically accessible distributions are found for the octahedron (equatorial dipoles antiparallel to a large central moment), tetrahedron and dodecahedron (central and mean cage dipole moments antiparallel)

Nuclear quadrupole coupling of O-17 and S-33 in ionic solids: Invalidation of the Sternheimer model by short-range corrections

Electronic structure calculations of electric field gradients (EFG) at the nuclei of the anions F-, O2-, and S2- for distorted octahedral environments representing the condensed phases of LiF, MgO, and MgS show dominant short-range corrections to the Sternheimer model. In this model, the EFG at the nucleus is proportional to that calculated from charges external to the ion with the constant of proportionality reflecting screening by the anion electron density. The short-range corrections suppress and even reverse the Sternheimer predictions and act in such a way as to damp the site specificity of the nuclear quadrupole coupling constants (NQCC) observed in magnetic resonance experiments. In contrast, the Sternheimer model is shown to describe accurately the calculated EFG at the nucleus of the Na+ cation. The physical origin of the short-range corrections is connected to the well-characterized short-range contributions to the induced dipoles and quadrupoles on ions at low-symmetry sites in the condensed phase

A transferable representation of the induced multipoles in ionic crystals

Electronic structure calculations of the induced dipole and quadrupole moments on a fluoride ion at low symmetry sites in a model crystalline environments are described. The results are used to characterize the short range contributions to the induced multipoles on the F- ion that arise from overlap between the wavefunctions of the ions. These are represented by general functions of the positions of the surrounding ions, suitable for use in a computationally tractable simulation model. The present calculations are designed to test the transferability to other classes of materials of this representation which, to date, largely has been deduced from calculations on distorted alkali halide crystals. First, the induced multipoles in mixed crystals of the alkali halides are considered, in order to check that they are predicted reliably by combining the representations deduced for the corresponding pure materials. Second, polarization effects in the alkaline earth fluorides MgF2 and CaF2 in a locally distorted fluorite crystal structure are examined. It is found that the material-specific parameters in the representation are related through simple functions of the ionic radii. This relationship holds between MgF2 and CaF2 in the fluorite structure, but also extends to the alkali fluorides previously studied. In order to illustrate the significance of polarization effects, the polarization model for CaF2 is combined with an ab initio pair potential for the repulsion and dispersion interactions and used in a molecular dynamics simulation. Inclusion of the polarization term improves greatly the calculated phonon frequencies in the crystal

Overlap-model and ab initio cluster calculations of ion properties in distorted environments

Electronic structure calculations on eight alkali fluorides and chlorides are performed in which the first shell of cations about a central anion is distorted and induced anion dipole and quadrupole moments are obtained. The charge-density overlap model used is validated against previous full-cluster calculations of distortion-induced moments and overall gives the same physical account of these properties for small anions and cations. Its use allows the calculations to be extended to RbF and RbCl. The overlap model mimics the full-cluster description of short-range damping of the multipole moments, and the induced moments are used to fit a simple model of the short-range interactions. Fitted parameter sets are shown to transfer between systems in a physically transparent manner. © 2001 Elsevier Science B.V

Overlap model and ab initio cluster calculations of polarisabilities of ions in solids

A recently developed overlap model for exchange-induction is used to simulate in-crystal anion polarisabilities for alkali halides and chalcogenides (LiF, NaF, KF, LiCl, NaCl, KCl, LiBr, NaBr, KBr, MgO, CaO, SrO, MgS, CaS and SrS) in overall qualitative agreement with results of ab initio cluster calculations and experiment. Extension to AgF supports the proposal that crystal-field splitting causes significant enhancement of cation polarisability for d10 systems, in contrast to the demonstrated insensitivity of s2 and p6 spherical cations

Short-range contributions to the polarization of cations

Charge-overlap cluster calculations [Wheatley, R. J. Chem. Phys. Lett. 1998, 294, 487], in which the electronic structure of a central ion is modified self-consistently by the frozen densities of its neighbors, are used to model induced dipoles in the condensed-phase alkali metal halides. Two distinct effects of overlap are identified: exchange-repulsion and Coulomb corrections. For anions, both substantially reduce the dipole from its asymptotic value; for cations, the two short-range effects are opposed, producing small net short-range corrections of different sign for different alkali metal cations. © 2001 American Chemical Society