Boride

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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 B₁₂As (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as icosahedral boride. [1].

1 Ranges of compounds
2 Boron rich borides (B:M 4:1 or more)
3 Metal rich borides (B:M less than 4:1)
4 Boride structures
5 See also
6 General References
7 Footnotes

[edit] 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.

[edit] 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.

[edit] 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. Examples include TiB₂. Another example is MgB₂, this compound serves to illustrate the point that physical properties cannot be inferred from the formula for this is a superconductor at 35^oK!

[edit] 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

[edit] See also

See category for a fill list.

[edit] General References

WebElements

Greenwood, N. N.; A. Earnshaw (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.