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: BaCh4 (Ch = O, S) containing Ba2+, Ba3Ch2 (Ch = S, Se, Te) with Ba+ and Ba2+, and Ba2Ch (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.