Mixed-valence Compounds: AuO2 and AuS.

Mixed-valence compounds are of great interest due to their interesting properties and wide applications. Recently, gold (Au) chemistry has experienced an unprecedented development. However, Au with mixed-valence states in Au-O compounds has not been reported thus far. Here, two hitherto unknown AuO2 and AuS compounds with mixed-valence character were identified with the aid of first-principles swarm structure searching calculations. AuO2 consists of quasi-square AuO4 moiety and AuO6 octahedron in which Au shows the mixed-valence states of III and V, the first example in Au-O binary compounds. AuS contains the linear AuS2 and quasi-square AuS4 units exhibiting AuI/III mixed-valence states. The analysis of electronic property demonstrates that AuO2 and AuS are narrow band gap semiconductors with strong hybridization between Au 5d and O 2p or S 3p. With the increase of pressure, Au-O and Au-S compounds show completely different thermodynamic stabilities, resulting from distinct shifting of pressure-induced atomic orbital energy levels of O and S atoms. Our work provides an opportunity for understanding mixed-valence character in Au-O and Au-S compounds.