Editing Carbide (section)

==Interstitial / Metallic carbides==
[[Image:Tungsten carbide.jpg|thumb|[[Tungsten carbide]] [[end mill|end mills]]]]
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as [[interstitial compound]]s.<ref name="Greenwood" /> These carbides have metallic properties and are [[refractory]]. Some exhibit a range of [[stoichiometries]], being a non-stoichiometric mixture of various carbides arising due to [[crystal defects]]. Some of them, including [[titanium carbide]] and [[tungsten carbide]], are important industrially and are used to coat metals in cutting tools.<ref name="Ettmayer">{{cite book|chapter=Carbides: transition metal solid state chemistry|author1=Peter Ettmayer |author2=Walter Lengauer |title=Encyclopedia of Inorganic Chemistry| editor=R. Bruce King |year=1994 |publisher=John Wiley & Sons|isbn=978-0-471-93620-6}}</ref>

The long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than approximately 135&nbsp;pm:<ref name="Greenwood" />
*When the metal atoms are [[close-packing|cubic close-packed]], (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the [[Rock-salt structure|rock salt structure]].<ref name=Zhu>{{Cite journal |last1=Zhu |first1=Qinqing |last2=Xiao |first2=Guorui |last3=Cui |first3=Yanwei |last4=Yang |first4=Wuzhang |last5=Wu |first5=Siqi |last6=Cao |first6=Guang-Han |last7=Ren |first7=Zhi |date=2021-10-15 |title=Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium |url=https://www.sciencedirect.com/science/article/pii/S0925838821016996 |journal=Journal of Alloys and Compounds |language=en |volume=878 |pages=160290 |doi=10.1016/j.jallcom.2021.160290 |issn=0925-8388}}</ref>
*When the metal atoms are [[close-packing|hexagonal close-packed]], (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI<sub>2</sub> structure.<ref name=Zhu/>

The following table<ref name="Greenwood" /><ref name="Ettmayer" /> shows structures of the metals and their carbides. (N.B. the body centered cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close-packed lattice.) The notation "h/2" refers to the M<sub>2</sub>C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice.

{| class="wikitable" style="text-align:center"
|-
! Metal
! Structure of pure metal
! Metallic <br />radius (pm)
! MC <br />metal atom packing
! MC structure
! M<sub>2</sub>C <br />metal atom packing
! M<sub>2</sub>C structure
! Other carbides
|-
| [[titanium]]
| hcp
| 147
| ccp
| rock salt
| 
|
|
|-
| [[zirconium]]
| hcp
| 160
| ccp
| rock salt
|
|
|
|-
| [[hafnium]]
| hcp
| 159
| ccp
| rock salt
|
|
|
|-
| [[vanadium]]
| [[Cubic crystal system|bcc]]
| 134
| ccp
| rock salt
| hcp
| h/2
| V<sub>4</sub>C<sub>3</sub>
|-
| [[niobium]]
| bcc
| 146
| ccp
| rock salt
| hcp
| h/2
| Nb<sub>4</sub>C<sub>3</sub>
|-
| [[tantalum]]
| bcc
| 146
| ccp
| rock salt
| hcp
| h/2
| Ta<sub>4</sub>C<sub>3</sub>
|-
| [[chromium]]
| bcc
| 128
|
|
| 
| 
|Cr<sub>23</sub>C<sub>6</sub>, Cr<sub>3</sub>C,<br /> Cr<sub>7</sub>C<sub>3</sub>, Cr<sub>3</sub>C<sub>2</sub>
|-
| [[molybdenum]]
| bcc
| 139
|
| hexagonal
| hcp
| h/2
| Mo<sub>3</sub>C<sub>2</sub>
|-
| [[tungsten]]
| bcc
| 139
|
| hexagonal
| hcp
| h/2
| 
|}

For a long time the [[non-stoichiometric]] phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.<ref>{{cite journal|title=Order and disorder in transition metal carbides and nitrides: experimental and theoretical aspects|author1=C.H. de Novion |author2=J.P. Landesman |journal=Pure Appl. Chem.|volume=57|issue=10|year=1985|page=1391|doi=10.1351/pac198557101391|s2cid=59467042 |doi-access=free}}</ref>

Iron forms a number of carbides, {{chem2|Fe3C}}, {{chem2|Fe7C3}} and {{chem2|Fe2C}}. The best known is [[cementite]], Fe<sub>3</sub>C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co and Ni are all hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.<ref name="Greenwood" />

Some metals, such as [[lead]] and [[tin]], are believed not to form carbides under any circumstances.<ref name="percy">{{cite book|author=John Percy|page=67|year=1870|url=https://archive.org/stream/metallurgyleadi01percgoog#page/n86/mode/2up/ |title=The Metallurgy of Lead, including Desiverization and Cupellation|publisher= J. Murray|place=London|access-date=2013-04-06|author-link=John Percy (metallurgist)}}</ref> There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.<ref>{{cite journal|author1=Y. C. Zhou |author2=H. Y. Dong |author3=B. H. Yu |year=2000|title=Development of two-dimensional titanium tin carbide (Ti2SnC) plates based on the electronic structure investigation|journal=Materials Research Innovations |volume=4|issue=1|pages=36–41|doi=10.1007/s100190000065|bibcode=2000MatRI...4...36Z |s2cid=135756713 }}</ref>