Boron trioxide

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Boron trioxide
Crystal structure of B2O3 [1]
Kristallstruktur Bortrioxid.png
Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
1303-86-2 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:30163 YesY
ChemSpider 452485 YesY
ECHA InfoCard 100.013.751
EC Number 215-125-8
PubChem 518682
RTECS number ED7900000
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] sublimates at 1500 °C[3]
1.1 g/100mL (10 °C)
3.3 g/100mL (20 °C)
15.7 100 g/100mL (100 °C)
Solubility partially soluble in methanol
Acidity (pKa) ~ 4
66.9 J/mol K
80.8 J/mol K
-1254 kJ/mol
-832 kJ/mol
Main hazards Irritant Xi[4]
Safety data sheet See: data page
Repr. Cat. 2
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point noncombustible
Lethal dose or concentration (LD, LC):
3163 mg/kg (oral, mouse)[5]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 15 mg/m3[4]
REL (Recommended)
TWA 10 mg/m3[4]
IDLH (Immediate danger)
2000 mg/m3[4]
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
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 a large number of 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 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).[12]


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 water steam 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.[13] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[14]

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


See also[edit]


  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 1-56677-261-3. 
  3. ^ a b Patnaik, P. (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 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 and Health. 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: 065504. doi:10.1103/PhysRevLett.101.065504. ; Ferlat, G.; Seitsonen, A. P.; Lazzeri, M.; Mauri, F. (2012). "Hidden polymorphs drive vitrification in B2O3". Nature Materials Letters. doi:10.1038/NMAT3416. 
  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: 2402–2408. 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: 365402. doi:10.1088/0953-8984/23/36/365402. 
  9. ^ Joo, C.; et al. (2000). "The ring structure of boron trioxide glass". Journal of Non-Crystalline Solids. 261: 282–286. 
  10. ^ Zwanziger, J. W. (2005). "The NMR response of boroxol rings: a density functional theory study". Solid State Nuclear Magnetic Resonance. 27: 5–9. doi:10.1016/j.ssnmr.2004.08.004. 
  11. ^ Micoulaut, M. (1997). "The structure of vitreous B2O3 obtained from a thermostatistical model of agglomeration". Journal of Molecular Liquids. 71: 107–114. 
  12. ^ 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. doi:10.1134/1.1630134. 
  13. ^ 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. 
  14. ^ Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M". General Electric. 
  15. ^ 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. doi:10.1063/1.334368. 
  16. ^ 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. 
  17. ^ 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. 
  18. ^ 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. 
  19. ^ AirProducts (2011). "Diborane Storage & Delivery" (PDF). 

External links[edit]