Crossover between ionic/covalent and pure ionic bonding in magnesium oxyde clusters

An empirical potential with fluctuating charges is proposed for modelling (MgO)_n clusters in both the molecular (small n) and bulk (n->infty) regimes. Vectorial polarization forces are explicitely taken into account in the self-consistent determination of the charges. Our model predicts cuboid cluster structures, in agreement with previous experimental and theoretical results. The effective charge transferred between magnesium and oxygen smoothly increases from 1 to 2, with an estimated crossover size above 300 MgO molecules

Modeling of defect properties of silver and its ions in alkali fluorides

Electron paramagnetic resonance studies have shown that the silver ion, Ag+, can act as a trap for both electrons and holes in irradiated alkali halide crystals, forming AgOand Ag2+ ions. In this paper we report the results of a simulation study using an embedded cluster approach, called ICECAP, thereby obtaining the ground state configuration and the charge stability of silver in the alkali fluorides, LiF, NaF, and KF. The ICECAP code combines the quantum cluster treated by the Hartree-Fock approximation and the embedding shell model lattice self-consistently. The calculations suggest that a Jahn-Teller distortion of the lattice will occur for the case of the Ag2+ ion, thereby causing the local environment of the ion to descend to tetragonal symmetry. The latter has been confirmed by an electron paramagnetic resonance (EPR) study for Ag doped in KF. © 1994 IOP Publishing Ltd

Density functional study of structural and electronic properties of cube-like MgO clusters

The structural and electronic properties of neutral cube-like MgO clusters comprising of 8, 18, 32, 48 and 64 atoms have been studied using density functional theory. Geometry optimization of all these stoichiometric clusters was done with C2v symmetry. The binding energy and the nearest-neighbor separation in these clusters show a progressive approach towards their corresponding bulk values. The densities of states indicate an increase in the width of valence bands with increasing cluster size. Total charge density plots show a relatively stronger dependence of magnesium charge density on the coordination number as compared to that of oxygen in the 64-atom cluster. © 1995

Neutral atoms in ionic lattices: Excited states of KCl:Ag(0)

The optical-absorption spectrum of a cationic Ag0 atom in a KCl crystal has been studied theoretically by means of a series of cluster models of increasing size. Excitation energies have been determined by means of a multiconfigurational self-consistent field procedure followed by a second-order perturbation correlation treatment. Moreover results obtained within the density-functional framework are also reported. The calculations confirm the assignment of bands I and IV to transitions of the Ag-5s electron into delocalized states with mainly K-4s,4p character. Bands II and III have been assigned to internal transitions on the Ag atom, which correspond to the atomic Ag-4d to Ag-5s transition. We also determine the lowest charge transfer (CT) excitation energy and confirm the assignment of band VI to such a transition. The study of the variation of the CT excitation energy with the Ag-Cl distance R gives additional support to a large displacement of the Cl ions due to the presence of the Ag0 impurity. Moreover, from the present results, it is predicted that on passing to NaCl:Ag0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of involved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is explained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R dependence of the s2-sp transitions due to s2 ions like Tl+

Neutral atoms in ionic lattices: Stability and ground state properties of KCl:Ag(0)

The equilibrium geometry of Ag0 centers formed at cation sites in KCl has been investigated by means of total-energy calculations carried out on clusters of different sizes. Two distinct methods have been employed: First, an ab initio wave-function based method on embedded clusters and second, density-functional theory ~DFT! methods on clusters in vacuo involving up to 117 atoms. In the ab initio calculations the obtained equilibrium Ag0 -Cl2 distance Re is 3.70 Å, implying a large outward relaxation of 18%, along with 7% relaxation for the distance between Ag0 and the first K1 ions in ^100& directions. A very similar result is reached through DFT with a 39-atom cluster. Both approaches lead to a rather shallow minimum of the total-energy surface, the associated force constant of the A1g mode is several times smaller than that found for other impurities in halides. These conclusions are shown to be compatible with available experimental results. The shallow minimum is not clearly seen in DFT calculations with larger clusters. The unpaired electron density on silver and Cl ligands has been calculated as function of the metal-ligand distance and has been compared with values derived from electron-paramagnetic resonance data. The DFT calculations for all cluster sizes indicate that the experimental hyperfine and superhyperfine constants are compatible when Re is close to 3.70 Å. The important relation between the electronic stability of a neutral atom inside an ionic lattice and the local relaxation is established through a simple electrostatic model. As most remarkable features it is shown that ~i! the cationic Ag0 center is not likely to be formed inside AgCl, ~ii! in the Ag0 center encountered in SrCl2, the silver atom is probably located at an anion site, and ~iii! the properties of a center-like KCl:Ag0 would experience significant changes under hydrostatic pressures of the order of 6 GPa