Boron trioxide
Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
3D model (JSmol)
ECHA InfoCard 100.013.751 Edit this at Wikidata
EC Number
  • 215-125-8
RTECS number
  • ED7900000
  • InChI=1S/B2O3/c3-1-5-2-4 checkY
  • InChI=1/B2O3/c3-1-5-2-4
  • O=BOB=O
Molar mass 69.6182 g/mol
Appearance white, glassy solid
Density 2.460 g/cm3, liquid;

2.55 g/cm3, trigonal;
3.11–3.146 g/cm3, monoclinic

Melting point 450 °C (842 °F; 723 K) (trigonal)
510 °C (tetrahedral)
Boiling point 1,860 °C (3,380 °F; 2,130 K) ,[2] sublimes at 1500 °C[3]
1.1 g/100mL (10 °C)
3.3 g/100mL (20 °C)
15.7 g/100mL (100 °C)
Solubility partially soluble in methanol
Acidity (pKa) ~ 4
-39.0·10−6 cm3/mol
66.9 J/mol K
80.8 J/mol K
-1254 kJ/mol
-832 kJ/mol
Occupational safety and health (OHS/OSH):
Main hazards
GHS labelling:
P201, P202, P281, P308+P313, P405, P501
NFPA 704 (fire diamond)
Flash point noncombustible
Lethal dose or concentration (LD, LC):
3163 mg/kg (oral, mouse)[5]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 15 mg/m3[4]
REL (Recommended)
TWA 10 mg/m3[4]
IDLH (Immediate danger)
2000 mg/m3[4]
Supplementary data page
Boron trioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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.[6] It is now recognized, from experimental and theoretical studies,[7][8][9][10][11] that the fraction of boron atoms belonging to boroxol rings in glassy B2O3 is somewhere between 0.73 and 0.83, with 0.75 (34) 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.[12]

The crystalline form (α-B2O3) (see structure in the infobox[1]) 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).[13]


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.[3]

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 → B2O3 + H2O

Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed.[14] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[15]

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.[16] Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145)[17][18] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)[19](e.g., α-quartz).

Boron oxide will also form when diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:

2B2H6(g) + 3O2(g) → 2B2O3(s) + 6H2(g)
B2H6(g) + 3H2O(g) → B2O3(s) + 6H2(g)[20]


See also


  1. ^ a b 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.
  2. ^ High temperature corrosion and materials chemistry: proceedings of the Per Kofstad Memorial Symposium. Proceedings of the Electrochemical Society. The Electrochemical Society. 2000. p. 496. ISBN 978-1-56677-261-7.
  3. ^ a b Patnaik, P. (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 978-0-07-049439-8. Retrieved 2009-06-06.
  4. ^ a b c d NIOSH Pocket Guide to Chemical Hazards. "#0060". National Institute for Occupational Safety and Health (NIOSH).
  5. ^ "Boron oxide". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  6. ^ Ferlat, G.; Charpentier, T.; Seitsonen, A. P.; Takada, A.; Lazzeri, M.; Cormier, L.; Calas, G.; Mauri. F. (2008). "Boroxol Rings in Liquid and Vitreous B2O3 from First Principles". Phys. Rev. Lett. 101 (6): 065504. Bibcode:2008PhRvL.101f5504F. doi:10.1103/PhysRevLett.101.065504. PMID 18764473.; Ferlat, G.; Seitsonen, A. P.; Lazzeri, M.; Mauri, F. (2012). "Hidden polymorphs drive vitrification in B2O3". Nature Materials Letters. 11 (11): 925–929. arXiv:1209.3482. Bibcode:2012NatMa..11..925F. doi:10.1038/NMAT3416. PMID 22941329. S2CID 11567458.
  7. ^ Hung, I.; et al. (2009). "Determination of the bond-angle distribution in vitreous B2O3 by rotation (DOR) NMR spectroscopy". Journal of Solid State Chemistry. 182 (9): 2402–2408. Bibcode:2009JSSCh.182.2402H. doi:10.1016/j.jssc.2009.06.025.
  8. ^ Soper, A. K. (2011). "Boroxol rings from diffraction data on vitreous boron trioxide". J. Phys.: Condens. Matter. 23 (36): 365402. Bibcode:2011JPCM...23.5402S. doi:10.1088/0953-8984/23/36/365402. PMID 21865633.
  9. ^ Joo, C.; et al. (2000). "The ring structure of boron trioxide glass". Journal of Non-Crystalline Solids. 261 (1–3): 282–286. Bibcode:2000JNCS..261..282J. doi:10.1016/s0022-3093(99)00609-2.
  10. ^ Zwanziger, J. W. (2005). "The NMR response of boroxol rings: a density functional theory study". Solid State Nuclear Magnetic Resonance. 27 (1–2): 5–9. doi:10.1016/j.ssnmr.2004.08.004. PMID 15589722.
  11. ^ 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.
  12. ^ 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" (PDF). Journal of Physics: Condensed Matter. 27 (45): 455104. Bibcode:2015JPCM...27S5104A. doi:10.1088/0953-8984/27/45/455104. PMID 26499978.((cite journal)): CS1 maint: multiple names: authors list (link)
  13. ^ Brazhkin, V. V.; Katayama, Y.; Inamura, Y.; Kondrin, M. V.; Lyapin, A. G.; Popova, S. V.; Voloshin, R. N. (2003). "Structural transformations in liquid, crystalline and glassy B2O3 under high pressure". JETP Letters. 78 (6): 393–397. Bibcode:2003JETPL..78..393B. doi:10.1134/1.1630134. S2CID 189764568.
  14. ^ 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.
  15. ^ Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M". General Electric. ((cite journal)): Cite journal requires |journal= (help)
  16. ^ Aziz, M. J.; Nygren, E.; Hays, J. F.; Turnbull, D. (1985). "Crystal Growth Kinetics of Boron Oxide Under Pressure". Journal of Applied Physics. 57 (6): 2233. Bibcode:1985JAP....57.2233A. doi:10.1063/1.334368.
  17. ^ 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.
  18. ^ 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.
  19. ^ 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. S2CID 97795834.
  20. ^ AirProducts (2011). "Diborane Storage & Delivery" (PDF). Archived from the original (PDF) on 2015-02-04. Retrieved 2013-08-21. ((cite journal)): Cite journal requires |journal= (help)