Boride


Boride

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 form a wide variety of boron-rich borides, with metal:boron ratios up to YB66. 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. [ Lundstrom T Pure & Applied Chem. (1985) 57, 10,1383 ] . They are mostly stable and refractory.

Some metallic dodecaborides contain boron icosahedra, others (for example yttrium, zirconium and uranium) have the boron atoms arranged in cuboctahedra [cite journal |last=Matkovich |first=V.I.|coauthors= J Economy, R F Giese Jr, R Barrett|year= 1965 |title= The structure of metallic dodecaborides|journal= Acta Cryst.|volume=19 |pages=1056-1058 |url=http://journals.iucr.org/q/issues/1965/12/00/a04941/a04941.pdf |accessdate=August 28 2008 ] .

LaB6 is an inert refractory compound, used in hot cathodes because of its low work function for emission of electrodes; YB66 crystals are used as monochromators for low-energy synchrotron X-rays. [cite journal | last=Wong | first=Jo | coauthors=T Tanaka, M Rowen, F Schäfer, B R Müller, Z U Rek | journal=J Synchrotron Rad. | year=1999 |volume=6 |pages=1086-1095] describes the production of the crystals by a indirect-heating floating zone method, their characterisation and their use on the beamline.

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

ee also

* lanthanum hexaboride (LaB6)
* magnesium diboride (MgB2)
* titanium boride (TiB2)
* plutonium borides (PuB{1,2,4,6})

See for a full list.

General References

* [http://www.webelements.com/ WebElements] Greenwood&Earnshaw
Cotton&Wilkinson6th

Footnotes


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