Boron trioxide ordiboron trioxide is theoxide of boron with the formulaB2O3. It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also calledboric oxide[6] orboria.[7] It has many important industrial applications, chiefly in ceramics as aflux for glazes and enamels and in the production ofglasses.
The amorphous form (g-B2O3) is by far the most common. It is thought to be composed ofboroxol 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.[8][9] It is now recognized, from experimental and theoretical studies,[10][11][12][13][14] that the fraction of boron atoms belonging to boroxol rings in glassyB2O3 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.[15][16]
The crystalline form (α-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;[17][18] but was later revised to the enantiomorphic space groups P3121(#152) and P3221(#154) in thetrigonal crystal system, like α-quartz[19]
Crystallization of α-B2O3 from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prolongedannealing of the amorphous solid ~200 °C under at least 10 kbar of pressure.[20][1]
Boron trioxide is produced by treatingborax withsulfuric acid in afusion furnace. At temperatures above 750 °C, the molten boron oxide layer separates out fromsodium sulfate. It is then decanted, cooled and obtained in 96–97% purity.[3]
Another method is heatingboric acid above ~300 °C. Boric acid will initially decompose into steam, (H2O(g)) andmetaboric acid (HBO2) at around 170 °C, and further heating above 300 °C will produce more steam and diboron trioxide. The reactions are:
H3BO3 → HBO2 + H2O
2 HBO2 →B2O3 + H2O
Boric acid goes to anhydrous microcrystallineB2O3 in a heated fluidized bed.[22] Carefully controlled heating rate avoids gumming as water evolves.
Boron oxide will also form whendiborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:
Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.[24]
^abGurr, 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.Bibcode:1970AcCrB..26..906G.doi:10.1107/S0567740870003369.
^L. McCulloch (1937): "A Crystalline Boric Oxide".Journal of the American Chemical Society, volume 59, issue 12, pages 2650–2652.doi:10.1021/ja01291a05
^I.Vishnevetsky and M.Epstein (2015): "Solar carbothermic reduction of alumina, magnesia and boria under vacuum".Solar Energy, volume 111, pages 236-251doi:10.1016/j.solener.2014.10.039
^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 (6) 065504.Bibcode:2008PhRvL.101f5504F.doi:10.1103/PhysRevLett.101.065504.PMID18764473.
^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 (9):2402–2408.Bibcode:2009JSSCh.182.2402H.doi:10.1016/j.jssc.2009.06.025.
^Micoulaut, M. (1997). "The structure of vitreous B2O3 obtained from a thermostatistical model of agglomeration".Journal of Molecular Liquids.71 (2–3):107–114.doi:10.1016/s0167-7322(97)00003-2.
^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.Bibcode:1970AcCrB..26..906G.doi:10.1107/S0567740870003369.
^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.S2CID97795834.
^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.Bibcode:1996CEPPI..35..311K.doi:10.1016/0255-2701(95)04142-7.
![Crystal structure of B2O3[1]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aB2O3powder.JPG)
