Boride
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In chemistry a '''boride''' is a chemical compound between boron and a less [[electronegativity|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 B<sub>12</sub>As<sub>2</sub> (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as [[boron arsenide|icosahedral boride]].
===Ranges of compounds===
The borides can be classified loosely as boron rich or metal rich, for example the compound YB<sub>66</sub> at one extreme through to Nd<sub>2</sub>Fe<sub>14</sub>B 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, [[lanthanide|lanthanides]] and [[actinide|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, [[diamagnetism|diamagnetic]], [[paramagnetism|paramagnetic]], [[ferromagnetism|ferromagnetic]] or [[antiferromagnetism|anti-ferromagnetic]].<ref> Lundstrom T Pure & Applied Chem. (1985) 57, 10,1383 </ref>. An example is [[lanthanum_hexaboride|LaB<sub>6</sub>]] 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 [[aluminum diboride|AlB<sub>2</sub>]] and [[titanium boride|TiB<sub>2</sub>]]. Recent investigations into this class of borides have revealed a wealth of interesting properties such as super conductivity at 39 K in [[magnesium_diboride|MgB<sub>2</sub>]] and the ultra-incompressibility of [[osmium diboride|OsB<sub>2</sub>]] and [[rhenium diboride|ReB<sub>2</sub>]]
===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, B<sub>2</sub> units, boron chains or boron sheets/layers.
Examples of the different types of borides are :-
*isolated boron atoms, example Mn<sub>4</sub>B
*B<sub>2</sub> units, example V<sub>3</sub>B
*chains of boron atoms, example FeB
*sheets or layers of boron atoms CrB<sub>2</sub>
*3-dimensional boron frameworks that include boron polyhedra, example NaB<sub>15</sub> with boron icosahedra
===See also===
* [[lanthanum hexaboride]] (LaB<sub>6</sub>)
* [[magnesium diboride]] (MgB<sub>2</sub>)
* [[titanium boride]] (TiB<sub>2</sub>)
See [[:Category:Borides|category]] for a fill list.
== General References ==
* [http://www.webelements.com/ WebElements]
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==Footnotes==
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