Editing Diamond (section)

=== Thermodynamics ===
[[File:Carbon-phase-diagramp.svg|thumb|Theoretically predicted [[phase diagram]] of carbon]]
The equilibrium pressure and temperature conditions for a transition between graphite and diamond are well established theoretically and experimentally. The equilibrium pressure varies linearly with temperature, between {{val|1.7|ul=GPa}} at {{val|0|u=K}} and {{val|12|u=GPa}} at {{val|5000|u=K}} (the diamond/graphite/liquid [[triple point]]).<ref name=Bundy>{{cite journal | vauthors =  Bundy P, Bassett WA, Weathers MS, Hemley RJ, Mao HK, Goncharov AF |title=The pressure-temperature phase and transformation diagram for carbon; updated through 1994 |journal=Carbon |date=1996 |volume=34 |issue=2 |pages=141–153 |doi=10.1016/0008-6223(96)00170-4|bibcode=1996Carbo..34..141B }}</ref><ref>{{cite book| vauthors = Wang CX, Yang GW |chapter=Thermodynamic and kinetic approaches of diamond and related nanomaterials formed by laser ablation in liquid| veditors = Yang G |title=Laser ablation in liquids: principles and applications in the preparation of nanomaterials |date=2012 |publisher=Pan Stanford |isbn=978-981-4241-52-6 |pages=164–165}}</ref> However, the phases have a wide region about this line where they can coexist. At [[standard temperature and pressure]], {{convert|20|C|K}} and {{convert|1|atm|MPa}}, the stable phase of carbon is graphite, but diamond is [[metastable]], with a significant kinetic energy barrier that the atoms must overcome in order to reach the lower energy state,<ref name=baird>{{cite web | title=Why do diamonds last forever? | website=Science Questions with Surprising Answers|first=Christopher S.|last=Baird|publisher=West Texas A&M University| date=17 December 2013 | url=https://www.wtamu.edu/~cbaird/sq/2013/12/17/why-do-diamonds-last-forever/}}</ref> and its rate of conversion to graphite is negligible, with a timescale of millions to billions of years.<ref name=ChemThermo/><ref name=baird/> However, at temperatures above about {{val|4500|u=K}}, diamond rapidly converts to graphite. Experiments have found that diamond in the presence of {{chem2|H2O}} passes through an intermediate linear carbon phase.<ref>{{cite journal | last1=O'Bannon | first1=E. | last2=Xia | first2=G. | last3=Shi | first3=F. | last4=Wirth | first4=R. | last5=King | first5=A. | last6=Dobrzhinetskaya | first6=L. | title=The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase | journal=Diamond and Related Materials | volume=108 | date=2020 | doi=10.1016/j.diamond.2020.107876 | doi-access=free | page=107876 | bibcode=2020DRM...10807876O | url=https://www.osti.gov/servlets/purl/1631913}}</ref>

Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at {{val|2000|u=K}}, a pressure of {{val|35|u=GPa}} (about 350,000 standard atmospheres) is needed.<ref name=Bundy/>

Above the graphite–diamond–liquid carbon triple point, the melting point of diamond increases slowly with increasing pressure; but at pressures of hundreds of GPa, it decreases.<ref>{{cite journal | vauthors = Wang X, Scandolo S, Car R | title = Carbon phase diagram from ab initio molecular dynamics | journal = Physical Review Letters | volume = 95 | issue = 18 | pages = 185701 | date = October 2005 | pmid = 16383918 | doi = 10.1103/PhysRevLett.95.185701 | bibcode = 2005PhRvL..95r5701W }}</ref> At high pressures, [[silicon]] and [[germanium]] have a BC8 [[Cubic crystal system|body-centered cubic]] crystal structure, and a similar structure is predicted for carbon at high pressures. At {{val|0|u=K}}, the transition is predicted to occur at {{val|1100|u=GPa}}.<ref>{{cite journal | vauthors = Correa AA, Bonev SA, Galli G | title = Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 5 | pages = 1204–1208 | date = January 2006 | pmid = 16432191 | pmc = 1345714 | doi = 10.1073/pnas.0510489103 | doi-access = free | bibcode = 2006PNAS..103.1204C }}</ref>

Results published in ''[[Nature Physics]]'' in 2010 suggest that, at ultra-high pressures and temperatures (about 10&nbsp;million atmospheres or 1&nbsp;TPa and 50,000&nbsp;°C), diamond melts into a metallic fluid. The extreme conditions required for this to occur are present in the [[ice giant]] [[planet]]s [[Neptune]] and [[Uranus]], both of which are made up of approximately 10 percent carbon and could hypothetically contain oceans of liquid carbon. Since large quantities of metallic fluid can affect the magnetic field, this could serve to explain why the geographic and magnetic poles of the two planets are not aligned.<ref>{{cite news |title=Diamond oceans possible on Uranus, Neptune | vauthors = Bland E |newspaper=Discovery News |date=January 15, 2010| archive-url=https://web.archive.org/web/20120311163132/http://news.discovery.com/space/diamond-oceans-jupiter-uranus.html | archive-date=March 11, 2012|url=http://news.discovery.com/space/diamond-oceans-jupiter-uranus.html |access-date=January 16, 2010}}</ref><ref>{{cite journal |title=Diamond: Molten under pressure |vauthors=Silvera I |journal=Nature Physics |volume=6 |pages=9–10 |year=2010 |issue=1 |bibcode=2010NatPh...6....9S |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:9121282 |doi=10.1038/nphys1491 |s2cid=120836330 |access-date=November 9, 2020 |archive-date=July 30, 2022 |archive-url=https://web.archive.org/web/20220730040414/https://dash.harvard.edu/handle/1/9121282 |url-status=live }}</ref>