Ba with Unusual Oxidation States in Ba Chalcogenides under Pressure.

The preparation of compounds with novel atomic oxidation states and emergent properties is of fundamental interest in chemistry. As s-block elements, alkali-earth metals invariably show a +2 formal oxidation state at normal conditions, and among them, barium (Ba) presents the strongest chemical reactivity. Herein, we propose that novel valence states of Ba can be achieved in pressure-induced chalcogenides, where it also shows a feature of 5d-elements. First-principles swarm-intelligence structural search calculations identify three novel stoichiometric compounds: BaCh ₄ (Ch = O, S) containing Ba ²⁺ , Ba ₃ Ch ₂ (Ch = S, Se, Te) with Ba ⁺ and Ba ²⁺ , and Ba ₂ Ch (Ch = Se, Te) with Ba ⁺ cations. The pressure-induced drop of the Ba 5d level relative to Ba 6s is responsible for this unusual oxidation state. These compounds display captivating structural characters, such as Ba-centered polyhedra and chain-shaped Ch units. More interestingly still, the interaction between two Ba ⁺ ions ensures their structural stability.