Barium in High Oxidation States in Pressure-Stabilized Barium Fluorides

The oxidation state of an element influences its chemical behavior of reactivity and bonding. Finding unusual oxidation state of elements is a theme of eternal pursuit. As labeled by an alkali-earth metal, barium (Ba) invariably exhibits an oxidation state of +2 by a loss of two 6s valence electrons while its inner 5p closed shell is known to remain intact. Here, we show through the reaction with fluorine (F) at high pressure that Ba exhibits a hitherto unexpected high oxidation state greater than +2 in three pressure-stabilized F-rich compounds BaF₃, BaF₄, and BaF₅, where Ba takes on the role of a 5p element by opening up its inert 5p shell. Interestingly enough, these pressure-stabilized Ba fluorides share common structural features of Ba-centered polyhedrons but exhibit a diverse variety of electronic properties showing semiconducting, metallic, and even magnetic behaviors. Our work modifies the traditional knowledge on the chemistry of alkali-earth Ba element established at ambient pressure and highlights the major role of pressure played in tuning the oxidation state of elements