COMPUTER MODELING OF SURFACES AND DEFECTS ON CERIUM DIOXIDE

The structures and relative stabilities of several CeO2 surfaces, and of anion vacancy centers formed on them, are studied using theoretical molecular mechanics methods. In agreement with previous literature wo rk, the compact (111) surface appears as the most stable one; it is fo llowed by (110), (211) (which spontaneously reconstructs into a steppe d (111) configuration) and (100). The latter case deserves considerati on; the intrinsic instability predicted for it by a simple model, whic h led to it being discarded in previous works, disappears once its sur face oxygen content is properly adjusted. Formation of anion vacancy d efects seems more difficult (actually, not easier than generation of b ulk Ce2O3) on the stable (111) surface than on the others. This sugges ts that different surfaces might be discernible in appropriate redox e xperiments. Anion vacancy pairs, probably important for O-2 uptake and release processes, seem easier to form on the (111) surface. Possible implications of these results for the reactivities of ceria materials having different surfaces exposed are discussed. The simulations repr oduce with similar to 10% error the experimental bulk modulus of CeO2; they predict also as stable a cubic phase of Ce2O3, with a lattice co nstant of 11.16 Angstrom in very good agreement with recent reports.