The structure of vitreous boron oxide

A new study of the structure of vitreous boron oxide has been made under greatly improved conditions. Using Rh Kα radiation with the method of fluorescence excitation, reliable intensity values were measured to 4πsinθ/λ = 20 Å⁻¹, and a rigorous interpretation was obtained in terms of pair functions. Each boron is triangularly surrounded by 3 oxygen atoms with a B–O distance of 1.37 Å. Each oxygen atom is bonded to 2 boron atoms. The major part of the glass is made up of boroxol groups B₃O₆ (a hexagonal ring of 3 boron atoms and 3 oxygen atoms with 3 corner oxygen atoms outside the ring). These groups are linked together by sharing the corner oxygen atoms, with randomness in orientation about the B–O bond directions at the shared oxygen atoms. Within the ring the B–O–B angle is 120° and at the shared oxygen it is approximately 130°. There is satisfactory agreement between the calculated and measured pair function distribution curves. In vitreous B₂O₃ the scheme of bonding does not fill space, and any boroxol group has neighboring groups which are not directly connected to it by B–O and O–B bonds. The measured pair function areas indicate neutral atoms, so that the bonding is predominantly covalent.