Barium oxide, also known as baria, is a white hygroscopic non-flammable compound with the formula BaO.

[citation needed] Barium oxide is used as a coating for hot cathodes, for example, those in cathode-ray tubes.

[3] It is most known for its use in the Brin process, named after its inventors, a reaction that was used as a large scale method to produce oxygen before air separation became the dominant method in the beginning of the 20th century, as BaO can be a source of pure oxygen through heat fluctuation.

BaO(s) + ½O2(g) ⇌ BaO2(s) It oxidises to BaO2 by formation of a peroxide ion ([O−O]2−, or O2−2) — with the same charge of O2−, and therefore keeping the electrochemical balance with the most stable Ba2+.

The complete peroxidation of BaO to BaO2 occurs at moderate temperatures by oxygen uptake within the BaO rock-salt lattice: Calculations using Density Functional Theory (DFT) suggest that the oxygen incorporation reaction is exothermic, and that the most energetically favoured occupation site is indeed the peroxide ion at the oxide lattice — other than interstitial positions, for instance.