View source for Boride

{{Short description|Class of chemical compounds}}
A '''boride''' is a compound between [[boron]] and a less [[electronegativity|electronegative]] element, for example [[silicon boride]] (SiB<sub>3</sub> and SiB<sub>6</sub>). 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 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 [[Yttrium borides|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]]s and [[actinide]]s form a wide variety of boron-rich borides, with metal:boron ratios up to [[Yttrium borides|YB<sub>66</sub>]].
The properties of this group vary from one compound to the next, and include examples of compounds that are semi conductors, superconductors, [[diamagnetism|diamagnetic]], [[paramagnetism|paramagnetic]], [[ferromagnetism|ferromagnetic]] or [[antiferromagnetism|anti-ferromagnetic]].<ref>{{cite journal|author=Lundstrom T |journal=Pure Appl. Chem.|year=1985|volume=57|issue=10|page=1383|title=Structure, defects and properties of some refractory borides|type = free download pdf|doi=10.1351/pac198557101383|doi-access=free}}</ref> They are mostly stable and refractory.
Some metallic dodecaborides contain boron [[Icosahedron|icosahedra]], others (for example [[yttrium]], [[zirconium]] and [[uranium]]) have the boron atoms arranged in [[cuboctahedron|cuboctahedra]].<ref>{{cite journal|author1=VI Matkovich |author2=J Economy |author3=R F Giese Jr |author4=R Barrett |year=1965 |title=The structure of metallic dodecaborides |journal= Acta Crystallographica|volume=19 |pages=1056–1058 |url=http://journals.iucr.org/q/issues/1965/12/00/a04941/a04941.pdf |accessdate=2008-08-28 |doi=10.1107/S0365110X65004954 |issue=6 |bibcode=1965AcCry..19.1056M |url-status=dead |archiveurl=https://web.archive.org/web/20141222104429/http://journals.iucr.org/q/issues/1965/12/00/a04941/a04941.pdf |archivedate=2014-12-22 }}</ref>
[[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|bibcode=1999JSynR...6.1086W }}</ref> VB<sub>2</sub> has shown some promise as potential material with higher energy capacity than lithium for batteries.<ref>{{Cite web |title=High Energy Density VB2/Air Batteries for Long Endurance UAVs {{!}} SBIR.gov |url=https://www.sbir.gov/node/414642 |access-date=2024-02-08 |website=www.sbir.gov}}</ref>
===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 [[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>]].<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|doi-access=free }}</ref>
===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
{| class="wikitable"
|-
! Formula !! [[Chemical Abstracts|CAS registry number]] !! density (g/cm<sup>3</sup>)<ref>{{CRC91}}</ref> !! melting point (°C)!! electrical resistivity (10<sup>−8</sup>Ω·m) !! Knoop hardness (0.1 kp load)
|-
|[[Titanium diboride|TiB<sub>2</sub>]]||12045-63-5|| 4.38|| 3225|| 9–15 ||2600
|-
|ZrB<sub>2</sub> ||12045-64-6|| 6.17|| 3050|| 7–10||1830
|-
|[[Hafnium diboride|HfB<sub>2</sub>]] ||12007-23-7 ||11.2 ||3250|| 10–12 ||2160
|-
|VB<sub>2</sub> ||12007-37-3 ||5.10 ||2450|| 16–38|| 2110
|-
|NbB ||12045-19-1 ||7.5 ||2270 ||- ||-
|-
|NbB<sub>2</sub>||12007-29-3|| 6.97|| 3050|| 12–65|| 2130
|-
|TaB|| 12007-07-7|| 14.2|| 2040|| -|| -
|-
|[[Tantalum boride|TaB<sub>2</sub>]] ||12007-35-1 ||11.2 ||3100 ||14–68 ||2500
|-
|CrB<sub>2</sub> ||12007-16-8|| 5.20|| 2170|| 21–56 ||1100
|-
|Mo<sub>2</sub>B<sub>5</sub>|| 12007-97-5|| 7.48|| 2370|| 18–45|| 2180
|-
|[[Tungsten borides|W<sub>2</sub>B<sub>5</sub>]]||12007-98-6 ||14.8|| 2370|| 21–56|| 2500
|-
|[[Iron boride|Fe<sub>2</sub>B]] ||12006-85-8 ||7.3 ||1389 ||-||1800
|-
|[[Iron boride|FeB]] ||12006-84-7|| 7 ||1658 ||30 ||1900
|-
|CoB ||12006-77-8 ||7.25 ||1460 ||26 ||2350
|-
|Co<sub>2</sub>B ||12045-01-1|| 8.1|| 1280|| -|| -
|-
|NiB ||12007-00-0 ||7.13|| 1034 ||23 ||-
|-
|Ni<sub>2</sub>B ||12007-01-1 ||7.90 ||1125 ||- ||-
|-
|[[Lanthanum hexaboride|LaB<sub>6</sub>]]|| 12008-21-8|| 6.15||2715 || 15|| 2010
|-
|UB<sub>4</sub> ||12007-84-0 ||9.32 ||2530 ||30 ||1850
|-
|[[Uranium boride|UB<sub>2</sub>]] ||12007-36-2|| 12.7|| 2430||-||-
|}
[[File:Unit Cell of RuB2.png|thumb|Unit cell of RuB<sub>2</sub>]]
==See also==
*[[Crystal structure of boron-rich metal borides]]
*[[Iron tetraboride]]
*[[Yttrium borides]] - a representative class of metal borides
*[[Magnesium diboride]] - a superconductor
==References==
{{Reflist}}
==Books==
*{{Greenwood&Earnshaw}}
*{{Cotton&Wilkinson6th}}
{{Borides}}
{{Monatomic anion compounds}}
{{Authority control}}
[[Category:Borides| ]]