Fe/MgO(001) model catalysts: Electron microscopic and spectroscopic studies

An ultrathin iron film deposited on a MgO single crystal in UHV is heated up to 500°C. AES/LEED measurements combined with TEM observations performed on small single crystal platelets show that the film is transformed into small particles below 300°C. Around 380°C, surface diffusion induces a spectacular growth of some particles. Upon further heating, the evolution of the system depends on the shape of the particles. In most cases, the particles become square and orient themselves along the MgO(110) direction (epitaxial configuration); however, they are unstable: they progressively dissolve in the MgO matrix, leading to a substitutional (Mg_xFe_1−x)O solid solution. In other cases, the iron particles remain shapeless and stable, suggesting that dissolution only occurs when the particles grow epitaxially.