Structure of the α-${\mathrm{Cr}}_2$${\mathrm{O}}_3$ (0001) surface: An ab initio total-energy study

The structure of the basal plane of the α phase of ${\mathrm{Cr}}_2$${\mathrm{O}}_3$ has been investigated using periodic ab initio Hartree-Fock theory. The Cr-terminated surface, which is nonpolar but is charged, is found to be stable. However, a large-scale relaxation of the surface layer away from the ideal bulk-terminated structure has been found. The top layer ${\mathrm{Cr}}^{3+}$ ions move inward toward the second-layer ${\mathrm{O}}^{2\mathrm{-}}$ ions by nearly 50% of their original interlayer spacing. This results in a slight lowering of the Cr ionicity in the surface layer, but no large-scale changes in bond character have been found. We therefore propose that the mechanism behind this dramatic reconstruction is purely electrostatic in origin. © 1996 The American Physical Society.