Abstract
Lanthanide nitride materials have recently been shown to act as catalysts for mild condition ammonia synthesis through reaction mechanisms differing from existing industrial catalysts. However, an in-depth study of the electronic structure of GdN, the prototypical lanthanide nitride, has not yet been completed. We report on an extensive in situ x-ray photoelectron spectroscopy and near-edge x-ray absorption study of the surface of epitaxial gadolinium (Gd) pre and post exposure to molecular nitrogen (N2), compared with a near-stoichiometric gadolinium nitride (GdN) thin film. In this study, the nitrogen 1𝑠 core level is used as a proxy to probe the facile breaking of molecular nitrogen (N2) by the pure gadolinium (Gd) surface at ambient temperature. We also investigate the evolution of Gd 3𝑑, 4𝑑, and 4𝑝 core levels with nitrogen exposure compared with Gd 3+ atomic multiplet calculations. We find that the exchange splitting of the Gd 4𝑠 and 5𝑠 levels suggests that 4𝑓 electrons are not significantly involved in bonding with nitrogen, which is well aligned with ongoing computational assumptions in the literature. This confirms that Gd-N bonding is predominately ionic, with the valence and conduction bands being of N 2𝑝 and Ln 5𝑑 and 6𝑠 character, respectively. The valence band suggests that the formation of GdN by exposure of a Gd surface to N2 does not result in complete nitridation, at least for the dose used and depth studied. Indeed, the surface of epitaxial Gd, post exposure to N2, suggests the coexistence of both pure Gd and GdN, as confirmed by density of states calculations.