Boron trioxide is one of the oxides of boron. It is a white, glassy solid with the formula B2O3. It is almost always found as the vitreous form; however, it can be crystallized after extensive annealing. Glassy boron oxide is thought to be composed ofboroxol 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 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. The crystalline form is exclusively composed of BO3 triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals.
Preparation
Boron trioxide is produced by treating borax with sulfuric acid in a fusion furnace. At temperatures above 750 °C, the molten boron oxide layer separates out from sodium sulfate. It is then decanted, cooled and obtained in 96–97% purity. Another method is heating boric acid above ~300 °C. Boric acid will initially decompose into steam, and metaboric acid at around 170 °C, and further heating above 300 °C will produce more steam and diboron trioxide. The reactions are: Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed. Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated. Crystallization of molten α-B2O3 at ambient pressure is strongly kinetically disfavored. Threshold conditions for crystallization of the amorphous solid are 10 kbar and ~200 °C. Its proposed crystal structure in enantiomorphic space groups P31; P32 has been revised to enantiomorphic space groups P3121; P3221. Boron oxide will also form when diborane reacts with oxygen in the air or trace amounts of moisture:
