Role of oxide ionicity in electronic screening at oxide/metal interfaces

Metal and oxygen core-level Auger and photoemission spectra have been measured for ultrathin epitaxial films of α-Cr₂O₃ and α-Fe₂O₃ on Pt(111). The binding-energy shifts of monolayer films relative to thicker films suggest a much smaller extent of screening by the metal substrate than that reported earlier for MgO/ Ag(001) [S. Altieri, L. H. Tjeng, F. C. Voogt, T. Hibma, and G. A. Sawatsky, Phys. Rev. B 59, R2517 (1999)]. This finding suggests that the higher degree of bond covalency in α-Cr₂O₃ and α-Fe₂O₃ relative to MgO results in more effective intrinsic screening of charge-transfer processes, rendering the metal much less effective for extrinsic screening. As a result, the metal is expected to have a negligible effect on the optical, electronic, and magnetic properties of the oxide.