The boron oxide or boric anhydride is an inorganic compound whose chemical formula is BtwoOR3. As boron and oxygen are elements of the p block of the periodic table, and even more heads of their respective groups, the electronegativity difference between them is not very high; therefore, it is to be expected that the BtwoOR3 is covalent in nature.
The BtwoOR3 It 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 approximately 300 ° C; or it can also be formed as a reaction product of diborane (BtwoH6) with oxygen.
Boron oxide can have a semi-transparent glassy, or crystalline appearance; the latter by grinding can be obtained in powder form (upper image).
Although it may not seem so at first glance, it is considered the BtwoOR3 as 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.
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The BtwoOR3 is a covalent solid, so in theory there are no B ions in its structure3+ nor Otwo-, but B-O links. Boron, according to the valence bond theory (TEV), can only form three covalent bonds; in this case, three B-O bonds. As a consequence of this, the expected geometry must be trigonal, BO3.
The BO molecule3 it is deficient in electrons, especially oxygen atoms; however, several of them can interact with each other to supply said deficiency. Thus, the triangles BO3 join together sharing an oxygen bridge, and are distributed in space as networks of triangular rows with their planes oriented in different ways.
An example of such rows with triangular units BO is shown in the image above.3. 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 the B is definedtwoOR3 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 the BtwoOR3 has two crystalline polymorphs: α and β.
The α-BtwoOR3 it 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-BtwoOR3, it is obtained at high pressures in the GPa range; therefore, its density must be greater than that of α-BtwoOR3.
BO networks3 they naturally tend to adopt amorphous structures; These are, they lack a pattern that describes the molecules or ions in the solid. As the B is synthesizedtwoOR3 its predominant form is amorphous and not crystalline; in correct words: it is a solid more glassy than crystalline.
It is then said that the BtwoOR3 it is vitreous or amorphous when its BO networks3 they are messy. Not only this, but they also change the way they come together. Instead of being arranged in a trigonal geometry, they end up looping 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 BtwoOR3 crystalline, and hexagonal to BtwoOR3 vitreous. Another way to refer to this amorphous phase is boron glass, or by a formula: g-BtwoOR3 (the 'g' comes from the word glassy, in English).
Thus, the g-B networkstwoOR3 are composed of boroxol rings and not BO units3. However, the g-BtwoOR3 can crystallize to α-BtwoOR3, which would imply an interconversion of rings to triangles, and would also define the degree of crystallization achieved.
It is a colorless, glassy solid. In its crystalline form it is white.
69.6182 g / mol.
Slightly bitter
-Crystalline: 2.46 g / mL.
-Vitreous: 1.80g / mL.
It does not have a fully defined melting point, because it depends on how crystalline or vitreous it is. The purely crystalline form melts at 450 ° C; however, the glassy form melts in a temperature range from 300 to 700ºC..
Again, the reported values do not match on this value. Apparently liquid boron oxide (molten from its crystals or its glass) boils at 1860ºC.
It must be kept dry, as it absorbs moisture to transform into boric acid, B (OH)3.
Boron oxide can be named in other ways, such as:
-Diboron trioxide (systematic nomenclature).
-Boron (III) oxide (stock nomenclature).
-Boric oxide (traditional nomenclature).
Some of the uses for boron oxide are:
From BtwoOR3 can be synthesized boron trihalides, BX3 (X = F, Cl and Br). 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..
A solid mixture with boric acid, BtwoOR3-B (OH)3, represents a formula that is used as a household insecticide.
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 BtwoOR3 used, as well as the technique, and the type of metal oxide, a rich variety of glasses (borosilicates), ceramics (nitrides and boron carbides), and alloys (if only metals are used).
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..
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..
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