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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)^[4]

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:

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

Formula	CAS registry number	density (g/cm³)^[5]	melting point (°C)	electrical resistivity (10⁻⁸Ω·m)	Knoop hardness (0.1 kp load)
TiB₂	12045-63-5	4.38	3225	9–15	2600
ZrB₂	12045-64-6	6.17	3050	7–10	1830
HfB₂	12007-23-7	11.2	3250	10–12	2160
VB₂	12007-37-3	5.10	2450	16–38	2110
NbB	12045-19-1	7.5	2270	-	-
NbB₂	12007-29-3	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
W₂B₅	12007-98-6	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	6.15	2715	15	2010
UB₄	12007-84-0	9.32	2530	30	1850
UB₂	12007-36-2	12.7	2430	-	-

References

^ Lundstrom T (1985). "Structure, defects and properties of some refractory borides". Pure Appl. Chem. (free download pdf). 57 (10): 1383. doi:10.1351/pac198557101383.
^ 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.
^ 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.
^ 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.
^ Haynes, William M. (2010). Handbook of Chemistry and Physics (91 ed.). Boca Raton, Florida, USA: CRC Press. ISBN 978-1-43982077-3.

Books

Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5

[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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@@ Line 13: / Line 13: @@
 [[lanthanum hexaboride|LaB<sub>6</sub>]] is an inert [[refractory]] compound, used in [[hot cathode]]s because of its low [[work function]] which gives it a high rate of [[thermionic emission]] of electrons; YB<sub>66</sub> crystals, grown by an [[zone refining|indirect-heating floating zone]] method, are used as [[monochromator]]s for low-energy [[synchrotron]] X-rays.<ref>{{cite journal|last=Wong|first=Jo|author2=T Tanaka |author3=M Rowen |author4=F Schäfer |author5=B R Müller |author6=Z U Rek |journal= Journal of Synchrotron Radiation|year=1999 |volume=6 |pages=1086–1095|doi=10.1107/S0909049599009000|title=YB66 – a new soft X-ray monochromator for synchrotron radiation. II. Characterization|issue=6|doi-access=free}}</ref>
-===Metal rich borides (B:M less than 4:1)<ref>{{Cite journal |last=Chen |first=Hui |last2=Zou |first2=Xiaoxin |date=2020 |title=Intermetallic borides: structures, synthesis and applications in electrocatalysis |url=http://xlink.rsc.org/?DOI=D0QI00146E |journal=Inorganic Chemistry Frontiers |language=en |volume=7 |issue=11 |pages=2248–2264 |doi=10.1039/D0QI00146E |issn=2052-1553}}</ref>===
+===Metal rich borides (B:M less than 4:1)<ref>{{Cite journal |last1=Chen |first1=Hui |last2=Zou |first2=Xiaoxin |date=2020 |title=Intermetallic borides: structures, synthesis and applications in electrocatalysis |url=http://xlink.rsc.org/?DOI=D0QI00146E |journal=Inorganic Chemistry Frontiers |language=en |volume=7 |issue=11 |pages=2248–2264 |doi=10.1039/D0QI00146E |s2cid=216259662 |issn=2052-1553}}</ref>===
 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 [[aluminium 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>]].