Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It has many important industrial applications, chiefly in ceramics as a flux for glazes and enamels and in the production of glasses.
Structure Boron trioxide has three known forms, one amorphous and two crystalline.
Amorphous form The amorphous form (g-) 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 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-) 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- 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- at several gigapascals (9.5 GPa).
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, (H2O(g)) and metaboric acid (HBO2) at around 170 °C, and further heating above 300 °C will produce more steam and diboron trioxide. The reactions are:
H3BO3 → HBO2 + H2O
2 HBO2 → + H2O
Boric acid goes to anhydrous microcrystalline in a heated fluidized bed. Carefully controlled heating rate avoids gumming as water evolves.
Boron oxide will also form when diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:
2B2H6(g) + 3O2(g) → 2(s) + 6H2(g)
B2H6(g) + 3H2O(g) → (s) + 6H2(g)
Reactions Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.
Applications Major component of borosilicate glass Fluxing agent for glass and enamels An additive used in glass fibres (optical fibres) The inert capping layer in the Liquid Encapsulation Czochralski process for the production of gallium arsenide single crystal As an acid catalyst in organic synthesis As a starting material for the production of other boron compounds, such as boron carbide
See also Boron suboxide Boric acid Sassolite Tris(2,2,2-trifluoroethyl) borate
References
L. McCulloch (1937): "A Crystalline Boric Oxide". Journal of the American Chemical Society, volume 59, issue 12, pages 2650–2652.
I.Vishnevetsky and M.Epstein (2015): "Solar carbothermic reduction of alumina, magnesia and boria under vacuum". Solar Energy, volume 111, pages 236-251
External links National Pollutant Inventory: Boron and compounds Australian Government information US NIH hazard information. See NIH. Material Safety Data Sheet CDC - NIOSH Pocket Guide to Chemical Hazards - Boron oxide