Defective and irregular surfaces of oxide crystals

At Keele I have been involved in a project which focused on studying oxygen-exchange reactions on surfaces of oxide crystals. As a preliminary step in accomplishing the purpose of this study, we have considered the energetics and electronic structure of defective and irregular (001) surfaces of MgO [B42] using ab initio calculations based on the local density functional pseudopotential approach. We have obtained structures, electronic densities of states (DOS) and valence electron distributions for a number of fully relaxed systems such as flat (001) surface, step, corner and anticorner. We have also studied the formation of an F-center on all these surfaces. We have found that the F-centre formation energy decreases steadily (being still large) as the coordination of the oxygen site is reduced. At the same time, the F-centre level shows a tendency to approach the top of the valence band. In addition, we have found specific features in the DOS for the corner-anticorner system attributed to oxygens at the corner sites.

In an attempt to understand the micromechanism of the oxygen-exchange reactions on the MgO surface, we looked at the adsorption of a single oxygen atom and an oxygen molecule on (001) MgO and CaO surfaces (including irregularities). GGA and spin-polarisation have been used in this study. In [B45,B48,B49] we show that atomic oxygen adsorption leads to formation of a peroxide O₂^2- molecular ion at the surface with adsorption energies of about 2 eV (chemisorption). An oxygen molecule sticks very weakly to these surfaces (physisorption). The calculated DOS for a peroxide defect at MgO surface might have been observed in UPS and MIES experiments at Clausthal [B47] (cf. [B60]).

The structure, DOS and electronic distribution of stoichiometric, fully and half reduced SnO₂ (110) surfaces have been studied in [B41] using the same approach. The ionic relaxations were found to be moderate. Reduction of the surface gives rise to a broad distribution of states in the gap which is in qualitative agreement with experiment. The role of gradient corrections to the LDA approach has been studied in [B44].

The energetics of N₂O dissociation at CaO flat (001) surface has been studied by DFT in [B50].

Using a combination of semiempirical and ab initio methods we studied in [B53] absorption and luminescence at low-coordinated surface sites of MgO crystal (surface singlet and triplet excitons). We found systematic reduction in transition energies with the lowering of the oxygen coordination in accord with available experimental data.

This work on application of DFT ab initio methods to studying the MgO and CaO (001) surfaces  as well as oxygen adsorption on the same surfaces has been reviewed in [A4-A6].

7/2/1999