Boride

A boride is a compound between boron and a less electronegative element, for example silicon boride (SiB3 and SiB6). The borides are a very large group of compounds that are generally high melting and are covalent more than 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 if 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 include examples of compounds that are semi conductors, superconductors, diamagnetic, paramagnetic, ferromagnetic or anti-ferromagnetic.[1] 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.[2]

LaB6 is an inert refractory compound, used in hot cathodes because of its low work function which gives it a high rate of thermionic emission of electrons; YB66 crystals, grown by an indirect-heating floating zone method, are used as monochromators for low-energy synchrotron X-rays.[3]

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 inert and have high melting temperature. 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:

Formula CAS registry number density (g/cm3)[4] melting point (°C) electrical resistivity (10−8Ω·m) Knoop hardness (0.1 kp load)
TiB212045-63-5 4.38 3225 9–15 2600
ZrB2 12045-64-6 6.17 3050 7–101830
HfB2 12007-23-7 11.2 3650 10–12 2160
VB2 12007-37-3 5.10 2450 16–38 2110
NbB 12045-19-1 7.5 2270 - -
NbB212007-29-3 6.97 3050 12–65 2130
TaB 12007-07-7 14.2 2040 - -
TaB2 12007-35-1 11.2 3100 14–68 2500
CrB2 12007-16-8 5.20 2170 21–56 1100
Mo2B5 12007-97-5 7.48 2370 18–45 2180
W2B512007-98-6 14.8 2370 21–56 2500
Fe2B 12006-85-8 7.3 1389 -1800
FeB 12006-84-7 7 1658 30 1900
CoB 12006-77-8 7.25 1460 26 2350
Co2B 12045-01-1 8.1 1280 - -
NiB 12007-00-0 7.13 1034 23 -
Ni2B 12007-01-1 7.90 1125 - -
LaB6 12008-21-8 6.152715 15 2010
UB4 12007-84-0 9.32 2530 30 1850
UB2 12007-36-2 12.7 2430--

See also

References

  1. ↑ Lundstrom T (1985). "Structure, defects and properties of some refractory borides". Pure & Applied Chem (free download pdf) 57 (10): 1383. doi:10.1351/pac198557101383.
  2. ↑ Matkovich, V.I.; J Economy; R F Giese Jr; R Barrett (1965). "The structure of metallic dodecaborides" (PDF). Acta Crystallogr. 19 (6): 1056–1058. doi:10.1107/S0365110X65004954. Retrieved 2008-08-28.
  3. ↑ Wong, Jo; T Tanaka; M Rowen; F Schäfer; B R Müller; Z U Rek (1999). "YB66 – a new soft X-ray monochromator for synchrotron radiation. II. Characterization". J Synchrotron Rad. 6 (6): 1086–1095. doi:10.1107/S0909049599009000.
  4. ↑ Haynes, William M. (2010). Handbook of Chemistry and Physics (91 ed.). Boca Raton, Florida: CRC Press. ISBN 978-1439820773.

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