Bulk terminated NaCl(111) on aluminum: A polar surface of an ionic crystal?

Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl-2 on AI(111) and Al(100) or by conversio n of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorp tion. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio cal culations of the electronic structure of these islands show that each of th e Na atoms carries half a positive elementary charge, leaving the islands n eutral and explaining the existence of an otherwise unstable surface.