Redox behavior of iron at the surface of an O(100) single crystal studied by ambient-pressure photoelectron spectroscopy

Graphical Abstract We have studied the oxidation and reduction of iron in an Fe-doped MgO single crystal by , and using ambient-pressure XPS and NEXAFS. Surface charging of the crystal was rendered manageable by the elevated temperatures and the gas atmospheres. The oxidation state of iron was found to shift reversibly between the and states, with a strong asymmetry in the rates; while oxidation by or was nearly complete at , reduction by began at , and was still incomplete at . Grazing-incidence XRD characterization of the crystal indicated the presence of octahedral, nanoscale inclusions assigned to the magnesioferrite spinel (). It is proposed that the redox behavior observed involves interconversion between the rock-salt (O) and spinel phases, with the more open lattice containing enabling more rapid ion diffusion and thus more facile oxidation compared to reduction.