Unusual Valence State in the Antiperovskites Sr$_3$SnO and Sr$_3$PbO Revealed by X-ray Photoelectron Spectroscopy

TL;DR

This study uses X-ray photoelectron spectroscopy to investigate the valence states of Sn and Pb in Sr3SnO and Sr3PbO antiperovskite films, revealing an unusual negative valence state and surface reconstructions relevant to their topological properties.

Contribution

First direct experimental evidence of the unusual -4 valence state of Sn and Pb in Sr3SnO and Sr3PbO thin films using XPS, supported by DFT calculations.

Findings

Confirmation of negative valence states of Sn and Pb in the bulk

Detection of surface reconstructions affecting valence states

Implications for topological surface states in antiperovskites

Abstract

The class of antiperovskite compounds $A_3B$O ($A$ = Ca, Sr, Ba; $B$ = Sn, Pb) has attracted interest as a candidate 3D Dirac system with topological surface states protected by crystal symmetry. A key factor underlying the rich electronic structure of $A_3B$O 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 $A_3B$O, which may render the growth-front surface unstable. We report X-ray photoelectron spectroscopy (XPS) measurements of single-crystalline films of Sr$_3$SnO and Sr$_3$PbO grown by molecular beam epitaxy (MBE). We observe shifts in the core-level binding energies that originate from anionic Sn and Pb, consistent with density functional theory (DFT) 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.