SURFACE-STRUCTURE AND BONDING IN THE STRONGLY CORRELATED METAL-OXIDESNIO AND UO2

We show atomic resolution elevated temperature scanning tunneling micr oscope images of the NiO (001) and UO2 (111) surfaces. When imaging th e empty states structure of NiO, a fourfold symmetric hole state is ob served around defects and step edges which influences the second neare st neighbor Ni ions. A similar effect is not observed on the UO2 (111) surface. We explain this difference in terms of the degree of covalen t coupling that is involved in the bonding of these oxides. Modeling o f the electronic structure through the local spin density approximatio n incorporating the Hubbard U is used to create charge density maps wh ich show that there is a significant element of nonsphericity of charg e distribution around the Ni ions in NiO, but that the distribution is spherical in UO2. (C) 1998 American Vacuum Society.