Boron trioxide

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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 (pK_a)	~ 4
Hazards
MSDS	External MSDS
EU classification	Repr. Cat. 2
NFPA 704	0 2 1
LD₅₀	3150 mg/kg (oral, rat)
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (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 (what?) 9.5 GPa.^[6]

1 Preparation
2 Hardness
3 Applications
4 See also
5 References
6 External links

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

References

^ 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.
^ ^a ^b Patnaik, P. (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.
^ Eckert, H. (1992). "Structural characterization of noncrystalline solids and glasses using solid state NMR". Progress in Nuclear Magnetic Resonance Spectroscopy 24 (3): 159–293. doi:10.1016/0079-6565(92)80001-V.
^ Hwang, S.-J.; Fernandez, C.; Amoureux, J. P.; Cho, J.; Martin, S. W.; Pruski, M. (1997). "Quantitative study of the short range order in B₂O₃ and B₂S₃ 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.
^ 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.
^ 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. http://www.jetpletters.ac.ru/ps/47/article_679.shtml.
^ 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.
^ 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. http://dash.harvard.edu/handle/1/3645198.
^ 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.
^ Mukhanov, V. A.; Kurakevich, O. O.; Solozhenko, V. L. (2008). "On the Hardness of Boron (III) Oxide". Journal of Superhard Materials 30: 71. doi:10.3103/S1063457608010097.