In chemistry a boride is a chemical compound between boron and a less electronegative element. This is a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term boride is also loosely applied to compounds such as B₁₂As₂ (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as icosahedral boride.

Ranges of compounds

The borides can be classified loosely as boron rich or metal rich, for example the compound YB₆₆ at one extreme through to Nd₂Fe₁₄B at the other. The generally accepted definition is that if the ratio of boron atoms to metal atoms is 4 : 1 or more the compound is boron rich, if it is less then it is metal rich.

Boron rich borides (B:M 4:1 or more)

The main group metals, lanthanides and actinides tend to form boron rich borides. The boron rich group is much larger than the metal rich. The properties of this group vary from one compound to the next, and includes examples of compounds that are semi conductors, superconductors, diamagnetic, paramagnetic, ferromagnetic or anti-ferromagnetic.^[1]. An example is LaB₆ an inert refractory compound.

Metal rich borides (B:M less than 4:1)

The transition metals tend to form metal rich borides. Metal-rich borides as a group are high melting and inert. Some are easily formed and this explains their use in making turbine blades, rocket nozzles etc. Some examples include AlB₂ and TiB₂. Recent investigations into this class of borides have revealed a wealth of interesting properties such as super conductivity at 39 K in MgB₂ and the ultra-incompressibility of OsB₂ and ReB₂

Boride structures

The boron rich borides contain 3-dimensional frameworks of boron atoms that can include boron polyhedra. The metal rich borides contain single boron atoms, B₂ units, boron chains or boron sheets/layers.

Examples of the different types of borides are :-

• isolated boron atoms, example Mn₄B
• B₂ units, example V₃B
• chains of boron atoms, example FeB
• sheets or layers of boron atoms CrB₂
• 3-dimensional boron frameworks that include boron polyhedra, example NaB₁₅ with boron icosahedra

See also

• lanthanum hexaboride (LaB₆)
• magnesium diboride (MgB₂)
• titanium boride (TiB₂)

See category for a fill list.

General References

• WebElements

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.

Footnotes

1. Lundstrom T Pure & Applied Chem. (1985) 57, 10,1383

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Boride". Image Credit.

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