First-Principles Simulations of Interstitial Atoms in Ionic Solids
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a,
T. BRUDEVOLL
a,
W.SCHULZa,
and N.E. CHRISTENSEN a a Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia The atomic and electronic structure of the radiation-induced interstitial atoms in MgO and KCl crystals representing two broad classes of ionic solids are calculated and compared. The first-principles full potential LMTO method is applied to a 16-atom supercell. For both crystals the energetically most favourable configuration is a dumbbell centered at a regular anion site. Its (110) and (111) orientations are very close in energy which permits the dumbbell to rotate easily on a lattice site. The mechanism and the relevant activation energy for thermally activated diffusion hops from the dumbbell equilibrium position to the cube face and cube center are discussed in the light of the available experimental data for MgO. In order to interpret recent experimental data on Raman spectroscopy, the local vibrational frequences are calculated for the dumbbell in KC1 (the so-called H center). A strong coupling is found between its stretching molecular mode and the breathing mode of the nearest cations whose frequency is predicted.
I. INTRODUCTION Ceramics based on the MgO-A12 0 3 system are known as prospective materials for fusion reactors, for which purpose they have to maintain structural and electrical integrity under irradiation with fast neutrons, -y-rays, and high energy particles [1]. Such irradiations produce a number of Frenkel defects, i.e. interstitial atoms and vacancies. Secondary diffusion-controlled reactions between these primary radiation defects can result in the appearance of defect clusters, dislocation loops, and voids affecting considerably the mechanical properties of the ceramics. This has been a main motivation for the intensive study in recent years of the mechanisms and kinetics of radiation damage in oxide materials, in particular MgO and A120 3 [2,3]. Several kinds of radiation-induced point defects have been investigated, the simplest ones are called F+ and F centers (0 vacancy with one and two trapped electrons, respectively) [4]. However, surprisingly little is known so far about their counterparts, interstitial oxygen atoms, O, whereas similar radiation-induced defects in alkali halides with the same crystal structure and high ionicity, e.g. KCl and KBr, are relatively well studied experimentally and theoretically [5,6]. These interstitials are unstable with respect to the formation of chemical bonds with regular halogen ions, X-, which leads to the X2 quasi-molecules (called H centers) centered on halogen lattice sites. Their diffusion occurs via X2 bond breaking, and the resulting X' atom moves along the (110) axis through the cube face center, the saddle point of the diffusion path
[6]. In contrast, practically nothing is known about corresponding properties of 02 atoms in MgO [4,7], including the mechanism and activation energy of their thermal diffus
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