F-type centres and hydrogen anions in MgO: Hartree-Fock ground states

Abstract

Second-neighbour defect clusters, described in the Hartree-Fock approximation with Kunz-Klein localising potentials out to fourth neighbours, embedded in an infinite shell model lattice are analysed with consistent distortion and polarisation for F+, F (H-)+, and (H2-)0 defects in MgO. Basis sets for the defects are optimised, and are improved by recontraction for the first and second neighbours. Questions of total energy, nearest-neighbour displacements and localisation are addressed for each defect. For the F+ centre, its ground state in relation to the valence band is discussed, as is the localisation of its unrelaxed excited state. For the F+ and (H2-)0 centres, spin densities are evaluated at each step of the calculation, and compared with experiment. Calculations that completely neglect the ion-size effect of all ions except at the defect centre give some qualitatively plausible results. The full-cluster results are used to derive short-range shell model parameters for H-and H2- in MgO. The method and results are critically reviewed.

Ab initioand semiempirical calculations ofH−centers in MgO crystals

“…Similar O ion displacements but larger Mg displacements were obtained in the ab initio ICECAP calculations. 22 The effective charges of Mg and O ions are close to those expected for purely ionic bonding (Ϯ2 e). ͑Note that the Mulliken population analysis was used in ab initio calculations and the Löwdin analysis was used in the semiempirical calculations.͒ Ab initio calculations predict half an electron of the H Ϫ defect to be delocalized, mainly over the NN Mg ions.…”

“…This is compatible with the activation energy value (4.85 eV͒ found for H Ϫ diffusion by the vacancy mechanism using the shell model. 22 The likely alternative is to assume that a substitutional H Ϫ defect leaves the O site to an interstitial position and then moves through the crystal as an interstitial ͓Fig. 3͑b͔͒.…”

“…12 H Ϫ and H 2Ϫ centers in MgO have been a subject of several theoretical studies. [22][23][24] Pandey and Vail 22 have carried out embedded-cluster calculations of these defects, treating 19-atom clusters within the unrestricted Hartree-Fock approximation as implemented into the ICECAP computer code. However, neither information on the defect level position in the band energy gap nor absorption energies have been presented.…”

“…Shell-model calculations assuming a vacancy diffusion mechanism predict a very high energy barrier of 4.85 eV. 22 In the present paper, we have performed a theoretical study of the electronic structure, absorption energy, and diffusion of H Ϫ defects in MgO using two different methods and large quantum clusters.…”

Indirect tunneling in metal-insulator-metal junctions

“…A reasonable value of the radius R of the nonrelaxed state is also obtained. It is to be expected that this value is larger than the radius of 1.66A calculated for the relaxed state [23].…”