Dopant-Induced Diffusion Processes at Metal-Oxide Interfaces Studied for Iron- and Chromium-Doped MgO/Mo(001) Model Systems

X-ray photoelectron spectroscopy reveals spontaneous atom diffusion from a Mo(001) support into a MgO thin film doped with transition-metal ions. The amount of interfacial mixing depends on the nature of the dopants and is considerably larger for Fe than for Cr impurities. DFT calculations find the reason for Mo diffusion in the ability of the dopants to change oxidation state. Cr exclusively occurs as 3+ ion in the rocksalt lattice, whereby the charge mismatch to native Mg2+ ions is compensated for by Mg vacancies. Iron, on the other hand, switches its thermodynamically preferred configuration from 3+ to 2+ with increasing temperature. As a result, Mo atoms from the support move into the Mg vacancies upon sample annealing and become oxidized via charge transfer into the Fe3+ species. Our study unravels a new charge-compensation scheme in doped oxides that proceeds via chemical intermixing at a metal-oxide interface. The mechanism may rationalize the often observed inactivity of doped oxides in charge-transfer reactions.