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, 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.[4]

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
FormulaCAS registry numberdensity (g/cm3)[5]melting point (°C)electrical resistivity (10−8Ω·m)Knoop hardness (0.1 kp load)
TiB212045-63-54.3832259–152600
ZrB212045-64-66.1730507–101830
HfB212007-23-711.2325010–122160
VB212007-37-35.10245016–382110
NbB12045-19-17.52270--
NbB212007-29-36.97305012–652130
TaB12007-07-714.22040--
TaB212007-35-111.2310014–682500
CrB212007-16-85.20217021–561100
Mo2B512007-97-57.48237018–452180
W2B512007-98-614.8237021–562500
Fe2B12006-85-87.31389-1800
FeB12006-84-771658301900
CoB12006-77-87.251460262350
Co2B12045-01-18.11280--
NiB12007-00-07.13103423-
Ni2B12007-01-17.901125--
LaB612008-21-86.152715152010
UB412007-84-09.322530301850
UB212007-36-212.72430--
Unit cell of RuB2

See also

References
  1. Lundstrom T (1985). "Structure, defects and properties of some refractory borides". Pure Appl. Chem. (free download pdf). 57 (10): 1383. doi:10.1351/pac198557101383.
  2. VI Matkovich; J Economy; R F Giese Jr; R Barrett (1965). "The structure of metallic dodecaborides" (PDF). Acta Crystallographica. 19 (6): 1056–1058. doi:10.1107/S0365110X65004954. Archived from the original (PDF) on 2014-12-22. 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". Journal of Synchrotron Radiation. 6 (6): 1086–1095. doi:10.1107/S0909049599009000.
  4. Chen, Hui; Zou, Xiaoxin (2020). "Intermetallic borides: structures, synthesis and applications in electrocatalysis". Inorganic Chemistry Frontiers. 7 (11): 2248–2264. doi:10.1039/D0QI00146E. ISSN 2052-1553. S2CID 216259662.
  5. Haynes, William M. (2010). Handbook of Chemistry and Physics (91 ed.). Boca Raton, Florida, USA: CRC Press. ISBN 978-1-43982077-3.

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