Unusual valence state in the antiperovskites Sr3SnO and Sr3PbO revealed by x-ray photoelectron spectroscopy

Abstract

The class of antiperovskite compounds A3BO (A = Ca, Sr, Ba; B = Sn, Pb) has attracted interest as a candidate three-dimensional Dirac system with topological surface states protected by crystal symmetry. A key factor underlying the rich electronic structure of A3BO is the unusual valence state of B, i.e., a formal oxidation state of -4. Practically, it is not obvious whether anionic B can be stabilized in thin films, due to its unusual chemistry, as well as the polar surface of A3BO, which may render the growth-front surface unstable. We report x-ray photoelectron spectroscopy measurements of single-crystalline films of Sr3SnO and Sr3PbO grown by molecular beam epitaxy. We observe shifts in the core-level binding energies that originate from anionic Sn and Pb, consistent with density functional theory calculations. Near the surface, we observe additional signatures of neutral or cationic Sn and Pb, which may point to an electronic or atomic reconstruction with possible impact on putative topological surface states.