A white or yellowish solid, BaO, obtained by heating barium in oxygen or by the thermal decomposition of barium carbonate or nitrate; cubic; r.d. 5.72; m.p. 1923°C; b.p. 2000°C. When barium oxide is heated in oxygen the peroxide, BaO2, is formed in a reversible reaction that was once used as a method for obtaining oxygen (the Brin process). Barium oxide is now used in the manufacture of lubricating-oil additives.