Boron trioxide

{{chembox | Watchedfields = changed | verifiedrevid = 443427049o | ImageFile = B2O3powder.JPG | ImageFile2 = Kristallstruktur Bortrioxid.png | ImageName = Crystal structure of B₂O₃<ref name=a1/> | OtherNames = boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria<br /> Boric anhydride | IUPACName = Diboron trioxide | 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 = B₂O₃ | MolarMass = 69.6182 g/mol | Appearance = white, glassy solid | Density = 2.460 g/cm³, liquid;<br/> 2.55 g/cm³, trigonal;<br/> 3.11–3.146 g/cm³, 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 = −39.0·10⁻⁶ cm³/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/m³<ref name=PGCH/> | REL = TWA 10 mg/m³<ref name=PGCH/> | IDLH = 2000 mg/m³<ref name=PGCH/> }} }} '''Boron trioxide''' or '''diboron trioxide''' is the 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 ceramics as a flux for glazes and enamels and in the production of glasses.

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

===Amorphous form=== The amorphous form (g-{{chem2|B2O3}}) is by far the most common. It 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.<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 = 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/><ref>{{cite journal |last1=Alderman |first1=Oliver |title=Boroxol ring dissolution in molten and glassy B2O3 by neutron and x-ray diffraction difference methods |journal=The Journal of Chemical Physics |date=2025-02-03 |volume=162 |issue=5 |page=054502 |doi=10.1063/5.0248235 |url=https://pubs.aip.org/aip/jcp/article/162/5/054502/3333270/Boroxol-ring-dissolution-in-molten-and-glassy-B2O3|url-access=subscription }}</ref>

===Crystalline α form=== The crystalline form (α-{{chem2|B2O3}}) is exclusively composed of BO₃ triangles. Its crystal structure was initially believed to be the enantiomorphic space groups P3₁(#144) and P3₂(#145), like γ-glycine;<ref name=gurr1970/><ref name=stro1971/> but was later revised to the enantiomorphic space groups P3₁21(#152) and P3₂21(#154) in the trigonal crystal system, like α-quartz<ref name=effe2001/>

Crystallization of α-{{chem2|B2O3}} from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prolonged annealing of the amorphous solid ~200&nbsp;°C under at least 10 kbar of pressure.<ref name=aziz1985/><ref name=a1/>

===Crystalline β form=== The trigonal network undergoes a coesite-like transformation to monoclinic β-{{chem2|B2O3}} at several gigapascals (9.5 GPa).<ref name=braz2003/>

==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, (H₂O_(g)) and metaboric acid (HBO₂) at around 170&nbsp;°C, and further heating above 300&nbsp;°C will produce more steam and diboron trioxide. The reactions are:

:H₃BO₃ → HBO₂ + H₂O

:2 HBO₂ → {{chem2|B2O3}} + H₂O

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.

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

:2B₂H₆(g) + 3O₂(g) → 2{{chem2|B2O3}}(s) + 6H₂(g)

:B₂H₆(g) + 3H₂O(g) → {{chem2|B2O3}}(s) + 6H₂(g)<ref name=AIRP2011/>

==Reactions== Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.<ref name=more1961/>

==Applications== *Major component of borosilicate glass *Fluxing agent for glass and enamels{{Citation needed|date=December 2022}} *An additive used in glass fibres (optical fibres) *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 *As a starting material for the production of other boron compounds, such as boron carbide

==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 B₂O₃ from First Principles | journal = Phys. Rev. Lett. | year = 2008 | volume = 101 |issue=6 | article-number = 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 B₂O₃|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 B₂O₃ 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|article-number=365402|doi=10.1088/0953-8984/23/36/365402|pmid=21865633|bibcode=2011JPCM...23.5402S|s2cid=5291179 |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|bibcode=2005SSNMR..27....5Z }}</ref>

<ref name=mico1997>{{cite journal|last1=Micoulaut|first1=M.|title=The structure of vitreous B₂O₃ 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 |article-number=455104 |doi=10.1088/0953-8984/27/45/455104 |pmid=26499978 |bibcode=2015JPCM...27S5104A |s2cid=21783488 |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 |bibcode=1970AcCrB..26..906G }}</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 B₂O₃ 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 |url-access=subscription }}</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 | url-access = subscription }}</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 | bibcode = 1970AcCrB..26..906G }}</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 B₂O₃ | journal = Acta Crystallographica B | year = 1971 | volume = 27 | issue = 8 | pages = 1662–1663 | doi = 10.1107/S0567740871004515 | bibcode = 1971AcCrB..27.1662S }}</ref>

<ref name=effe2001>{{cite journal | last1 = Effenberger | first1 = H. | last2 = Lengauer | first2 = C. L. | last3 = Parthé | first3 = E. | title = Trigonal B₂O₃ 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 |bibcode=1996CEPPI..35..311K }}</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 }}</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] *[https://www.nlm.nih.gov/toxnet/index.html 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