SINGLE-CRYSTAL STRUCTURAL INVESTIGATION OF BAO2

This paper reports the first structural study of BaO2 by single-crysta l X-ray diffraction. Crystals grown from the melt of Ba (NO3)(2) in an Ag tube often show twin boundaries. Twinning can also be induced by f urther pressing of the crystal surface with a needle. BaO2 is tetragon al I4/mmm, with a = 3.8114(6) (or F4/mmm, with a = 5.3901(6)) and c = 6.8215(11)Angstrom. The structure of BaO2 is sodium chloride-like with the peroxide groups aligned alo ng the c-axis. Each Ba is coordinated to a total of 10 oxygens with eight of the Ba...O distances being 2.7 97(1)Angstrom and the other two 2.664(2)Angstrom. The O-O length in th e peroxide unit is found to be 1.493(2)Angstrom, which follows the tre nd of bond lengths versus bond order in a series of homoatomic oxygen molecules and ions. The crystal appears to be stoichiometric in oxygen .