Boron trioxide (or diboron 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 (amorphous) form; however, it can be crystallized after extensive annealing (that is, under prolonged heat).
Glassy boron oxide (g-B2O3) 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.
The crystalline form (α-B2O3) (see structure in the infobox) is exclusively composed of BO3 triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals (9.5 GPa).
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 boron trioxide. The reactions are:
H3BO3 → HBO2 + H2O
2 HBO2 → B2O3 + H2O
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 (compare liquid and crystal densities). Threshold conditions for crystallization of the amorphous solid are 10 kbar and ~200 °C. Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145) (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)(e.g., α-quartz).
Boron oxide will also form when diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:
^ abGurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The Crystal Structure of Trigonal Diboron Trioxide". Acta Crystallographica B. 26 (7): 906–915. doi:10.1107/S0567740870003369.
^Micoulaut, M. (1997). "The structure of vitreous B2O3 obtained from a thermostatistical model of agglomeration". Journal of Molecular Liquids. 71 (2–3): 107–114. doi:10.1016/s0167-7322(97)00003-2.
^Alderman, O. L. G. Ferlat, G. Baroni, A. Salanne, M. Micoulaut, M. Benmore, C. J. Lin, A. Tamalonis, A. Weber, J. K. R. (2015). "Liquid B2O3 up to 1700K: X-ray diffraction and boroxol ring dissolution". Journal of Physics: Condensed Matter. 27 (45): 455104. doi:10.1088/0953-8984/27/45/455104. PMID26499978.CS1 maint: multiple names: authors list (link)
^Kocakuşak, S.; Akçay, K.; Ayok, T.; Koöroğlu, H. J.; Koral, M.; Savaşçi, Ö. T.; Tolun, R. (1996). "Production of anhydrous, crystalline boron oxide in fluidized bed reactor". Chemical Engineering and Processing. 35 (4): 311–317. doi:10.1016/0255-2701(95)04142-7.
^Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M". General Electric. Cite journal requires |journal= (help)
^Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The crystal structure of trigonal diboron trioxide". Acta Crystallographica B. 26 (7): 906–915. doi:10.1107/S0567740870003369.
^Strong, S. L.; Wells, A. F.; Kaplow, R. (1971). "On the crystal structure of B2O3". Acta Crystallographica B. 27 (8): 1662–1663. doi:10.1107/S0567740871004515.
^Effenberger, H.; Lengauer, C. L.; Parthé, E. (2001). "Trigonal B2O3 with Higher Space-Group Symmetry: Results of a Reevaluation". Monatshefte für Chemie. 132 (12): 1515–1517. doi:10.1007/s007060170008.