Boron trioxide

Structure

Boron trioxide has three known forms, one amorphous and two crystalline.

Amorphous form

The amorphous form (g-B₂O₃) is by far the most common. It is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen.

Because of the difficulty of building disordered models at the correct density with many boroxol rings, this view was initially controversial, but such models have recently been constructed and exhibit properties in excellent agreement with experiment. It is now recognized, from experimental and theoretical studies, that the fraction of boron atoms belonging to boroxol rings in glassy B₂O₃ is somewhere between 0.73 and 0.83, with 0.75 = 3/4 corresponding to a 1:1 ratio between ring and non-ring units. The number of boroxol rings decays in the liquid state with increasing temperature.

Crystalline α form

The crystalline form (α-B₂O₃) is exclusively composed of BO₃ triangles. Its crystal structure was initially believed to be the enantiomorphic space groups P3₁(#144) and P3₂(#145), like γ-glycine; but was later revised to the enantiomorphic space groups P3₁21(#152) and P3₂21(#154) in the trigonal crystal system, like α-quartz

Crystallization of α-B₂O₃ from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prolonged annealing of the amorphous solid ~200 °C under at least 10 kbar of pressure.

Crystalline β form

The trigonal network undergoes a coesite-like transformation to monoclinic β-B₂O₃ at several gigapascals (9.5 GPa).