Lone-pair State Stabilized Variable-Valency Vacancy-Ordered Double Perovskites

We report the prediction of a new family of vacancy-ordered double perovskites (VODP) Ba₂SnX₆ (X = I, Br, Cl) and compare their structural, electronic and optical properties with the well-studied, already-synthesized Cs₂SnX₆ family. The isolated lowest conduction band of Cs₂SnX₆ is derived from the hybridized Sn-5s + X-p lone-pair states. Upon Ba substitution, this originally unoccupied hybridized lone-pair (HLP) band becomes occupied, and the resulting material is remarkably stable, suggesting an HLP-stabilization mechanism for designing new variable-valency (or mixed-valency) VODPs. Preliminary calculations also suggest that Ba₂SnX₆ have very strong near-edge optical absorption.