{{chembox
| Watchedfields = changed
| verifiedrevid = 443427049o
| ImageFile = B2O3powder.JPG
| ImageFile2 = Kristallstruktur Bortrioxid.png
| ImageName = Crystal structure of B<sub>2</sub>O<sub>3</sub> <ref name=a1/>
| OtherNames = boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria<br />
Boric acid anhydride
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 452485
| InChI = 1/B2O3/c3-1-5-2-4
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 30163
| Gmelin = 11108
| SMILES = O=BOB=O
| InChIKey = JKWMSGQKBLHBQQ-UHFFFAOYAI
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/B2O3/c3-1-5-2-4
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = JKWMSGQKBLHBQQ-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 1303-86-2
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 483W67CPF4
| PubChem = 518682
| RTECS = ED7900000
| EINECS = 215-125-8
}}
|Section2={{Chembox Properties
| Formula = B<sub>2</sub>O<sub>3</sub>
| MolarMass = 69.6182 g/mol
| Appearance = white, glassy solid
| Density = 2.460 g/cm<sup>3</sup>, liquid;<br/>
2.55 g/cm<sup>3</sup>, trigonal;<br/>
3.11–3.146 g/cm<sup>3</sup>, monoclinic
| MeltingPtC = 450
| MeltingPt_notes = (trigonal) <br> 510 °C (tetrahedral)
| BoilingPtC = 1860
| BoilingPt_notes = ,<ref>{{cite book|url=https://books.google.com/books?id=ZrxSWmueNMQC&pg=PA496|page=496|title=High temperature corrosion and materials chemistry: proceedings of the Per Kofstad Memorial Symposium. Proceedings of the Electrochemical Society|publisher=The Electrochemical Society|year=2000|isbn=978-1-56677-261-7}}</ref> sublimes at 1500 °C<ref name="patnaik" />
| Solubility = 1.1 g/100mL (10 °C) <br> 3.3 g/100mL (20 °C) <br> 15.7 g/100mL (100 °C)
| SolubleOther = partially soluble in [[methanol]]
| pKa = ~ 4
| MagSus = -39.0·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
|Section4={{Chembox Thermochemistry
| DeltaHf = -1254 kJ/mol
| DeltaGf = -832 kJ/mol
| Entropy = 80.8 J/mol K
| HeatCapacity = 66.9 J/mol K
}}
|Section7={{Chembox Hazards
| GHSPictograms = {{GHS08}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|360FD}}
| PPhrases = {{P-phrases|201|202|281|308+313|405|501}}
| MainHazards = Irritant<ref name=PGCH/>
| NFPA-H = 2
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S = 
| FlashPt = noncombustible
| AutoignitionPt = 
| LD50 = 3163 mg/kg (oral, mouse)<ref>{{IDLH|1303862|Boron oxide}}</ref>
| PEL = TWA 15 mg/m<sup>3</sup><ref name=PGCH/>
| REL = TWA 10 mg/m<sup>3</sup><ref name=PGCH/>
| IDLH = 2000 mg/m<sup>3</sup><ref name=PGCH/>
}}
}}
'''Boron trioxide''' or '''diboron trioxide''' is the [[boron oxide|oxide of boron]] with the formula {{chem2|B2O3}}. It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called '''boric oxide'''<ref name=mccu1937/> or '''boria'''.<ref name=vish2015/>   It has many important industrial applications, chiefly in metallurgy as a [[flux (metallurgy)|flux]] and in the production of [[glass]]es. 

==Structure==
Boron trioxide has two known forms, amorphous and crystalline.

===Amorphous form===
The amorphous form (g-{{chem2|B2O3}}) is by far the most common. It is thought to be composed of [[boroxol ring]]s 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.<ref name=ferl2008/><ref name=ferl2012/> It is now recognized, from experimental and theoretical studies,<ref name=hung2009/><ref name=soper2011/><ref name=jooc2000/><ref name=zwan2005/><ref name=mico1997/> that the fraction of boron atoms belonging to boroxol rings in glassy {{chem2|B2O3}} is somewhere between 0.73 and 0.83, with 0.75 ({{frac|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.<ref name=alde2015/>

===Crystalline form===
The crystalline form (α-{{chem2|B2O3}}) can be obtained with prologed [[annealing (metallurgy)|annealing]] under pressure.<ref name=a1/>) is exclusively composed of BO<sub>3</sub> triangles. This trigonal, [[trapezohedron|quartz-like]] network undergoes a [[coesite]]-like transformation to [[monoclinic]] β-{{chem2|B2O3}} at several gigapascals (9.5 GPa).<ref name=braz2003/>

Crystallization of molten α-{{chem2|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&nbsp;°C.<ref name=aziz1985/>  Its proposed crystal structure in enantiomorphic space groups P3<sub>1</sub>(#144); P3<sub>2</sub>(#145)<ref name=gurr1970/><ref name=stro1971/> (e.g., γ-glycine) has been revised to enantiomorphic space groups P3<sub>1</sub>21(#152); P3<sub>2</sub>21(#154)<ref name=effe2001/>(e.g., α-quartz).

==Preparation==
Boron trioxide is produced by treating [[borax]] with [[sulfuric acid]] in a [[fusion furnace]]. At temperatures above 750&nbsp;°C, the molten boron oxide layer separates out from [[sodium sulfate]]. It is then decanted, cooled and obtained in 96–97% purity.<ref name=patnaik/>

Another method is heating [[boric acid]] above ~300&nbsp;°C. Boric acid will initially decompose into steam, (H<sub>2</sub>O<sub>(g)</sub>) and [[metaboric acid]] (HBO<sub>2</sub>) at around 170&nbsp;°C, and further heating above 300&nbsp;°C will produce more steam and diboron trioxide. The reactions are:

:H<sub>3</sub>BO<sub>3</sub> → HBO<sub>2</sub> + H<sub>2</sub>O

:2 HBO<sub>2</sub> → {{chem2|B2O3}} + H<sub>2</sub>O

Boric acid goes to anhydrous microcrystalline {{chem2|B2O3}} in a heated fluidized bed.<ref name=akca1996/>  Carefully controlled heating rate avoids gumming as water evolves.  Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.<ref name=more1961/>

Boron oxide will also form when [[diborane]] (B<sub>2</sub>H<sub>6</sub>) reacts with oxygen in the air or trace amounts of moisture:

:2B<sub>2</sub>H<sub>6</sub>(g) + 3O<sub>2</sub>(g) → 2{{chem2|B2O3}}(s) + 6H<sub>2</sub>(g)

:B<sub>2</sub>H<sub>6</sub>(g) + 3H<sub>2</sub>O(g) → {{chem2|B2O3}}(s) + 6H<sub>2</sub>(g)<ref name=AIRP2011/>

==Applications==
*[[Ceramic flux|Fluxing agent]] for [[glass]] and [[vitreous enamel|enamels]]
*Starting material for synthesizing other [[boron]] compounds such as [[boron carbide]]
*An additive used in glass fibres ([[optical fibre]]s)
*Component used in the production of [[borosilicate glass]]
*The inert capping layer in the Liquid Encapsulation Czochralski process for the production of [[gallium arsenide]] single crystal
*As an acid [[catalyst]] in [[organic synthesis]]

==See also==
*[[boron suboxide]]
*[[boric acid]]
*[[sassolite]]
*[[Tris(2,2,2-trifluoroethyl) borate]]

==References==

<references>

<ref name=PGCH>{{PGCH|0060}}</ref>

<ref name=mccu1937>L. McCulloch (1937): "A Crystalline Boric Oxide". ''Journal of the American Chemical Society'', volume 59, issue 12, pages 2650–2652. {{doi|10.1021/ja01291a05}}</ref>

<ref name=vish2015>I.Vishnevetsky and M.Epstein (2015): "Solar carbothermic reduction of alumina, magnesia and boria under vacuum". ''Solar Energy'', volume 111, pages 236-251 {{doi|10.1016/j.solener.2014.10.039}}</ref>

<ref name=ferl2008>{{cite journal |author1=Ferlat, G. |author2=Charpentier, T. |author3=Seitsonen, A. P. |author4=Takada, A. |author5=Lazzeri, M. |author6=Cormier, L. |author7=Calas, G. |author8=Mauri. F. | title = Boroxol Rings in Liquid and Vitreous B<sub>2</sub>O<sub>3</sub> from First Principles | journal = Phys. Rev. Lett. | year = 2008 | volume = 101 |issue=6 | pages = 065504 | doi=10.1103/PhysRevLett.101.065504 |pmid=18764473 |bibcode=2008PhRvL.101f5504F }}</ref>

<ref name=ferl2012>{{cite journal|last1=Ferlat|first1=G.|last2=Seitsonen|first2=A. P.|last3=Lazzeri|first3=M.|last4=Mauri|first4=F.|title=Hidden polymorphs drive vitrification in B<sub>2</sub>O<sub>3</sub>|journal=Nature Materials Letters|volume=11|issue=11|pages=925–929|date=2012|doi=10.1038/NMAT3416|pmid=22941329|arxiv=1209.3482|bibcode=2012NatMa..11..925F|s2cid=11567458}}</ref>

<ref name=hung2009>{{cite journal|last1=Hung|first1=I.|display-authors=etal|title=Determination of the bond-angle distribution in vitreous B<sub>2</sub>O<sub>3</sub> by rotation (DOR) NMR spectroscopy|journal=Journal of Solid State Chemistry|date=2009|volume=182|issue=9|pages=2402–2408|doi=10.1016/j.jssc.2009.06.025|bibcode=2009JSSCh.182.2402H}}</ref>

<ref name=soper2011>{{cite journal|last1=Soper|first1=A. K.|title=Boroxol rings from diffraction data on vitreous boron trioxide|journal=J. Phys.: Condens. Matter|date=2011|volume=23|issue=36|page=365402|doi=10.1088/0953-8984/23/36/365402|pmid=21865633|bibcode=2011JPCM...23.5402S|url=https://zenodo.org/record/889447}}</ref>

<ref name=jooc2000>{{cite journal|last1=Joo|first1=C.|display-authors=etal|title=The ring structure of boron trioxide glass|journal=Journal of Non-Crystalline Solids|date=2000|volume=261|issue=1–3|pages=282–286|doi=10.1016/s0022-3093(99)00609-2|bibcode=2000JNCS..261..282J}}</ref>

<ref name=zwan2005>{{cite journal|last1=Zwanziger|first1=J. W.|title=The NMR response of boroxol rings: a density functional theory study|journal=Solid State Nuclear Magnetic Resonance|date=2005|volume=27|issue=1–2|pages=5–9|doi=10.1016/j.ssnmr.2004.08.004|pmid=15589722}}</ref>

<ref name=mico1997>{{cite journal|last1=Micoulaut|first1=M.|title=The structure of vitreous B<sub>2</sub>O<sub>3</sub> obtained from a thermostatistical model of agglomeration|journal=Journal of Molecular Liquids|date=1997|volume=71|issue=2–3|pages=107–114|doi=10.1016/s0167-7322(97)00003-2}}</ref>

<ref name=alde2015>{{cite journal |last1=Alderman, O. L. G. Ferlat, G. Baroni, A. Salanne, M. Micoulaut, M. Benmore, C. J. Lin, A. Tamalonis, A. Weber, J. K. R. |title=Liquid B2O3 up to 1700K: X-ray diffraction and boroxol ring dissolution |journal=Journal of Physics: Condensed Matter |date=2015 |volume=27 |issue=45 |page=455104 |doi=10.1088/0953-8984/27/45/455104 |pmid=26499978 |bibcode=2015JPCM...27S5104A |url=https://hal.archives-ouvertes.fr/hal-01502196/file/Alderman%20et%20al.%20-%202015%20-%20Liquid%20B%202%20O%203%20up%20to%201700%20K%20x-ray%20diffraction%20and.pdf }}</ref>

<ref name=a1>{{cite journal |author1=Gurr, G. E. |author2=Montgomery, P. W. |author3=Knutson, C. D. |author4=Gorres, B. T. | title = The Crystal Structure of Trigonal Diboron Trioxide | journal = Acta Crystallographica B | year = 1970 | volume = 26 | issue = 7 | pages = 906–915 | doi = 10.1107/S0567740870003369 }}</ref>

<ref name=braz2003>{{cite journal |author1=Brazhkin, V. V. |author2=Katayama, Y. |author3=Inamura, Y. |author4=Kondrin, M. V. |author5=Lyapin, A. G. |author6=Popova, S. V. |author7=Voloshin, R. N. | title = Structural transformations in liquid, crystalline and glassy B<sub>2</sub>O<sub>3</sub> under high pressure | journal = JETP Letters | year = 2003 | volume = 78 | issue = 6 | pages = 393–397 | doi = 10.1134/1.1630134 | url = http://www.jetpletters.ac.ru/ps/47/article_679.shtml |bibcode=2003JETPL..78..393B |s2cid=189764568 }}</ref>

<ref name=aziz1985>{{cite journal |author1=Aziz, M. J. |author2=Nygren, E. |author3=Hays, J. F. |author4=Turnbull, D. | title = Crystal Growth Kinetics of Boron Oxide Under Pressure | journal = Journal of Applied Physics | year = 1985 | volume = 57 | issue = 6 | page = 2233 | doi = 10.1063/1.334368 | url = http://dash.harvard.edu/handle/1/3645198 |bibcode=1985JAP....57.2233A }}</ref>

<ref name=gurr1970>{{cite journal | title = The crystal structure of trigonal diboron trioxide | last1 = Gurr | first1 = G. E. | last2 = Montgomery | first2 = P. W. | last3 = Knutson | first3 = C. D. | last4 = Gorres | first4 = B. T. | journal = Acta Crystallographica B | year = 1970 | volume = 26 | issue = 7 | pages = 906–915 | doi = 10.1107/S0567740870003369 }}</ref>

<ref name=stro1971>{{cite journal | last1 = Strong | first1 = S. L. | last2 = Wells | first2 = A. F. | last3 = Kaplow | first3 = R. | title = On the crystal structure of B<sub>2</sub>O<sub>3</sub> | journal = Acta Crystallographica B | year = 1971 | volume = 27 | issue = 8 | pages = 1662–1663 | doi = 10.1107/S0567740871004515 }}</ref>

<ref name=effe2001>{{cite journal | last1 = Effenberger | first1 = H. | last2 = Lengauer | first2 = C. L. | last3 = Parthé | first3 = E. | title = Trigonal B<sub>2</sub>O<sub>3</sub> with Higher Space-Group Symmetry: Results of a Reevaluation | journal = Monatshefte für Chemie | year = 2001 | volume = 132 | issue = 12 | pages = 1515–1517 | doi = 10.1007/s007060170008 | s2cid = 97795834 }}</ref>

<ref name=patnaik>{{cite book | last = Patnaik | first = P. | year = 2003 | title = Handbook of Inorganic Chemical Compounds | publisher = McGraw-Hill | page = 119 | isbn =978-0-07-049439-8 | url = https://books.google.com/books?id=Xqj-TTzkvTEC&pg=PA119 | access-date = 2009-06-06 }}</ref>

<ref name=akca1996>{{cite journal  | title = Production of anhydrous, crystalline boron oxide in fluidized bed reactor |author1=Kocakuşak, S. |author2=Akçay, K. |author3=Ayok, T. |author4=Koöroğlu, H. J. |author5=Koral, M. |author6=Savaşçi, Ö. T. |author7=Tolun, R. | journal = Chemical Engineering and Processing | year = 1996 | volume = 35 | issue = 4 | pages = 311–317 | doi = 10.1016/0255-2701(95)04142-7 }}</ref>

<ref name=more1961>{{cite journal | author = Morelock, C. R. | title = Research Laboratory Report #61-RL-2672M | year = 1961 | publisher = General Electric }}</ref>

<ref name=AIRP2011>{{cite journal | author = AirProducts | title = Diborane Storage & Delivery | year = 2011 | url = http://www.airproducts.com/~/media/downloads/other/D/en-diborane-electronics-technical-bulletin.pdf | access-date = 2013-08-21 | archive-url = https://web.archive.org/web/20150204210427/http://www.airproducts.com/~/media/downloads/other/D/en-diborane-electronics-technical-bulletin.pdf | archive-date = 2015-02-04 | url-status = dead }}</ref>

</references>

==External links==
*[https://web.archive.org/web/20060209040519/http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory: Boron and compounds]
*[https://web.archive.org/web/20050717211845/http://www.nohsc.gov.au/OHSInformation/Databases/ExposureStandards/az/Boron_oxide.htm Australian Government information]
*[http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rn+@rel+1303-86-2 US NIH hazard information]. See [[NIH]].
*[http://hazard.com/msds/mf/baker/baker/files/b3708.htm Material Safety Data Sheet]
*[https://www.cdc.gov/niosh/npg/npgd0060.html CDC - NIOSH Pocket Guide to Chemical Hazards - Boron oxide]

{{Boron compounds}}
{{Oxides}}
{{Oxygen compounds}}
{{Authority control}}

{{DEFAULTSORT:Boron Trioxide}}
[[Category:Boron compounds]]
[[Category:Acidic oxides]]
[[Category:Glass compositions]]
[[Category:Sesquioxides]]