- Boron oxide structure
- BO unit
- Crystal structure
- Vitreous structure
- Properties
- Physical appearance
- Molecular mass
- Taste
- Density
- Melting point
- Boiling point
- Stability
- Nomenclature
- Applications
- Synthesis of boron trihalides
- Insecticide
- Solvent for metal oxides: formation of glasses, ceramics and boron alloys
- Binder
- References
The boron oxide or boric anhydride is an inorganic compound whose chemical formula is B 2 O 3. As boron and oxygen are elements of the p block of the periodic table, and even more so, heads of their respective groups, the electronegativity difference between them is not very high; therefore, B 2 O 3 is expected to be covalent in nature.
B 2 O 3 is prepared by dissolving borax in concentrated sulfuric acid in a melting furnace and at a temperature of 750 ° C; thermally dehydrating boric acid, B (OH) 3, at a temperature of about 300 ° C; or it can also be formed as a product of the reaction of diborane (B 2 H 6) with oxygen.
Boron oxide powder. Source: Materialscientist at English Wikipedia
Boron oxide can have a semi-transparent glassy, or crystalline appearance; the latter by grinding can be obtained in powder form (top image).
Although it may not seem so at first glance, B 2 O 3 is considered one of the most complex inorganic oxides; not only from a structural point of view, but also due to the variable properties acquired by glasses and ceramics to which this is added to their matrix.
Boron oxide structure
BO unit
B 2 O 3 is a covalent solid, so in theory there are no B 3+ or O 2 ions in its structure, but BO bonds. Boron, according to the valence bond theory (TEV), can only form three covalent bonds; in this case, three BO links. As a consequence of this, the expected geometry must be trigonal, BO 3.
The BO 3 molecule is deficient in electrons, especially oxygen atoms; However, several of them can interact with each other to supply said deficiency. Thus, the BO 3 triangles are joined by sharing an oxygen bridge, and are distributed in space as networks of triangular rows with their planes oriented in different ways.
Crystal structure
Boron oxide crystalline structure. Source: Orci
An example of such rows with BO 3 triangular units is shown in the image above. If you look closely, not all the faces of the plans point towards the reader, but the other way. The orientations of these faces may be responsible for how B 2 O 3 is defined at a certain temperature and pressure.
When these networks have a long-range structural pattern, it is a crystalline solid, which can be built from its unit cell. This is where it is said that B 2 O 3 has two crystalline polymorphs: α and β.
Α-B 2 O 3 is produced at ambient pressure (1 atm), and is said to be kinetically unstable; in fact, this is one of the reasons that boron oxide is probably a difficult-to-crystallize compound.
The other polymorph, β-B 2 O 3, is obtained at high pressures in the GPa range; therefore, its density must be greater than that of α-B 2 O 3.
Vitreous structure
Boroxol ring. Source: CCoil
BO 3 networks naturally tend to adopt amorphous structures; These are, they lack a pattern that describes the molecules or ions in the solid. When B 2 O 3 is synthesized, its predominant form is amorphous and not crystalline; in correct words: it is a solid more glassy than crystalline.
B 2 O 3 is then said to be vitreous or amorphous when its BO 3 networks are disordered. Not only this, but they also change the way they come together. Instead of being arranged in a trigonal geometry, they end up linking together to create what researchers call a boroxol ring (top image).
Note the obvious difference between triangular and hexagonal units. The triangular ones characterize the crystalline B 2 O 3, and the hexagonal ones the vitreous B 2 O 3. Another way to refer to this amorphous phase is boron glass, or by a formula: gB 2 O 3 (the 'g' comes from the word glassy, in English).
Thus, gB 2 O 3 networks are composed of boroxol rings and not BO 3 units. However, gB 2 O 3 can crystallize to α-B 2 O 3, which would imply an interconversion of rings to triangles, and would also define the degree of crystallization achieved.
Properties
Physical appearance
It is a colorless, glassy solid. In its crystalline form it is white.
Molecular mass
69.6182 g / mol.
Taste
Slightly bitter
Density
-Crystalline: 2.46 g / mL.
-Vitreous: 1.80g / mL.
Melting point
It does not have a fully defined melting point, because it depends on how crystalline or glassy it is. The purely crystalline form melts at 450 ° C; however, the glassy form melts in a temperature range from 300 to 700ºC.
Boiling point
Again, the reported values do not match on this value. Apparently liquid boron oxide (melted from its crystals or from its glass) boils at 1860ºC.
Stability
It must be kept dry, as it absorbs moisture to transform into boric acid, B (OH) 3.
Nomenclature
Boron oxide can be named in other ways, such as:
-Diboron trioxide (systematic nomenclature).
-Boron (III) oxide (stock nomenclature).
-Boric oxide (traditional nomenclature).
Applications
Some of the uses for boron oxide are:
Synthesis of boron trihalides
Boron trihalogenides, BX 3 (X = F, Cl and Br) can be synthesized from B 2 O 3. These compounds are Lewis acids, and with them it is possible to introduce boron atoms to certain molecules to obtain other derivatives with new properties.
Insecticide
A solid mixture with boric acid, B 2 O 3 -B (OH) 3, represents a formula that is used as a household insecticide.
Solvent for metal oxides: formation of glasses, ceramics and boron alloys
Liquid boron oxide is capable of dissolving metal oxides. From this resulting mixture, once cooled, solids composed of boron and metals are obtained.
Depending on the amount of B 2 O 3 used, as well as the technique, and the type of metal oxide, a rich variety of glasses (borosilicates), ceramics (boron nitrides and carbides), and alloys (if used) can be obtained. metals only).
In general, glass or ceramics acquire greater resistance and strength, and also greater durability. In the case of glasses, they end up being used for optical and telescope lenses, and for electronic devices.
Binder
In the construction of steel smelting furnaces, magnesium-based refractory bricks are used. Boron oxide is used as a binder, helping to hold them tightly together.
References
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Wikipedia. (2019). Boron trioxide. Recovered from: en.wikipedia.org
- PubChem. (2019). Boric oxide. Recovered from: pubchem.ncbi.nlm.nih.gov
- Rio Tinto. (2019). Borix oxide. 20 Mule Team Borax. Recovered from: borax.com
- A. Mukhanov, OO Kurakevich, and VL Solozhenko. (sf). On the Hardness of Boron (III) Oxide. LPMTMCNRS, Université Paris Nord, Villetaneuse, France.
- Hansen T. (2015). B 2 O 3 (Boric Oxide). Recovered from: digitalfire.com