Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations ( $-57\mathrm{}\%$ for the Fe and $-79\mathrm{}\%$ for the O termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline $\alpha$- ${\mathrm{Fe}}_2{\mathrm{O}}_3$ (0001) films, which were prepared in high oxygen pressures.

• Received 11 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.1038