Boron trioxide

Boron trioxide
Other names boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria Boric acid anhydride
Identifiers
CAS number	1303-86-2 Y
PubChem	518682
ChemSpider	452485 Y
ChEBI	CHEBI:30163 Y
RTECS number	ED7900000
Jmol-3D images	Image 1
SMILES O=BOB=O
InChI InChI=1S/B2O3/c3-1-5-2-4 Y Key: JKWMSGQKBLHBQQ-UHFFFAOYSA-N Y InChI=1/B2O3/c3-1-5-2-4 Key: JKWMSGQKBLHBQQ-UHFFFAOYAI
Properties
Molecular formula	B2O3
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 (trigonal) 510 °C (tetrahedral)
Boiling point	1860 °C,[1] sublimates at 1500 °C[2]
Solubility in water	22 g/L
Solubility	partially soluble in methanol
Acidity (pKa)	~ 4
Hazards
MSDS	External MSDS
EU classification	Repr. Cat. 2
NFPA 704	0 1 0
LD50	3150 mg/kg (oral, rat)
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y trioxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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 (amorphic) form; however, it can be crystallized after extensive annealing. It is one of the most difficult compounds known to crystallize.

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. This view is controversial, however, because no model has ever been made of glassy boron oxide of the correct density containing a large number of six-membered rings. The rings are thought to make a few BO₃ triangles, but mostly link (polymerize) into ribbons and sheets.^[3][4] The crystalline form (α-B₂O₃) see structure in the infobox^[5]) is exclusively composed of BO₃ triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B₂O₃ at several gigapascals and is 9.5 GPa.^[6]

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.^[2]

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.^[7] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.^[8]

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.^[9] Its proposed crystal structure in enantiomorphic space groups P3₁(#144); P3₂(#145)^[10][11] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3₁21(#152); P3₂21(#154)^[12](e.g., α-quartz).

Hardness

The bulk modulus of β-B₂O₃ is rather high (K = 180 GPa). The Vickers hardness of g-B₂O₃ is 1.5 GPa and of β-B₂O₃ is 16 GPa.^[13]

Applications

• Fluxing agent for glass and enamels
• Starting material for synthesizing other boron compounds such as boron carbide
• An additive used in glass fibres (optical fibres)
• It is used in the production of borosilicate glass
• The inert capping layer in the LEC process for the production of gallium arsenide single crystal
• As an acid catalyst in organic synthesis

See also

• boron suboxide
• boric acid
• sassolite

References

1. ^ 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 1566772613. http://books.google.com/?id=ZrxSWmueNMQC&pg=PA496. 
2. ^ ^{a b} Patnaik, Pradyot (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0070494398. http://books.google.com/?id=Xqj-TTzkvTEC&pg=PA119. Retrieved 2009-06-06. 
3. ^ Eckert, H. (1992). "Structural characterization of noncrystalline solids and glasses using solid state NMR". Prog. NMR Spectrosc. 24 (3): 159. doi:10.1016/0079-6565(92)80001-V. 
4. ^ S.-J. Hwang, C. Femandez, J.P. Amoureux, J. Cho, S.W. Martin & M. Pruski. (1997). "Quantitative study of the short range order in B,O, and B,S, by MAS and two-dimensional triple-quantum MAS ¹¹B NMR". Solid State Nuclear Magnetic Resonance 8 (2): 109–121. doi:10.1016/S0926-2040(96)01280-5. PMID 9203284. 
5. ^ G.E. Gurr, P.W. Montgomery, C.D. Knutson, B.T.Gorres (1970). "The Crystal Structure of Trigonal Diboron Trioxide". Acta Cryst. B 26 (7): 906–915. doi:10.1107/S0567740870003369. 
6. ^ V. V. Brazhkin et al. (2003). "Structural transformations in liquid, crystalline and glassy B₂O₃ under high pressure". JETPh Lett. 78 (6): 845. doi:10.1134/1.1630134. http://www.jetpletters.ac.ru/ps/47/article_679.shtml. 
7. ^ Kocakusak, S; Akcay, K; Ayok, T; Kooroglu, H; Koral, M; Savasci, O; 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. 
8. ^ C.R. Morelock, General Electric Research Laboratory Report #61-RL-2672M(1961)
9. ^ "Crystal Growth Kinetics of Boron Oxide Under Pressure". J. Appl. Phys. 57 (6): 2233. 1985. doi:10.1063/1.334368. http://dash.harvard.edu/handle/1/3645198. 
10. ^ Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The crystal structure of trigonal diboron trioxide". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 26 (7): 906–915. doi:10.1107/S0567740870003369. 
11. ^ Strong, S. L.; Wells, A. F.; Kaplow, R. (1971). "On the crystal structure of B2O3". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 27 (8): 1662–1663. doi:10.1107/S0567740871004515. 
12. ^ Effenberger, Herta; Lengauer, Christian L.; Parthé, Erwin (2001). "Trigonal B2O3 with Higher Space-Group Symmetry: Results of a Reevaluation". Monatshefte für Chemie/Chemical Monthly 132 (12): 1515–1517. doi:10.1007/s007060170008. 
13. ^ V. A. Mukhanov, O. O. Kurakevich, and V. L. Solozhenko (2008). "On the Hardness of Boron (III) Oxide". Journal of Superhard Materials 30: 71. doi:10.3103/S1063457608010097. 

External links

• National Pollutant Inventory: Boron and compounds
• Australian Government information
• US NIH hazard information. See NIH.
• Material Safety Data Sheet