At very high pressures, carbon forms the more compact allotrope, diamond, having nearly twice the density of graphite. Here, each atom is bonded tetrahedrally to four others, forming a 3-dimensional network of puckered six-membered rings of atoms. Diamond has the same cubic structure as silicon and germanium, and because of the strength of the carbon-carbon bonds, it is the hardest naturally occurring substance measured by resistance to scratching. Contrary to the popular belief that "diamonds are forever", they are thermodynamically unstable (ΔfG°(diamond, 298 K) = 2.9 kJ/mol) under normal conditions (298 K, 105 Pa) and should theoretically transform into graphite. But due to a high activation energy barrier, the transition into graphite is so slow at normal temperature that it is unnoticeable. However, at very high temperatures diamond will turn into graphite, and diamonds can burn up in a house fire. The bottom left corner of the phase diagram for carbon has not been scrutinized experimentally. Although a computational study employing density functional theory methods reached the conclusion that as and , diamond becomes more stable than graphite by approximately 1.1 kJ/mol, more recent and definitive experimental and computational studies show that graphite is more stable than diamond for , without applied pressure, by 2.7 kJ/mol at T = 0 K and 3.2 kJ/mol at T = 298.15 K. Under some conditions, carbon crystallizes as lonsdaleite, a hexagonal crystal lattice with all atoms covalently bonded and properties similar to those of diamond.
At VerY HIGh PRessureS, CarbOn foRms thE moRe compact aLLOtrOpE, DIAmond, having nEarLy Twice The dENSiTy of graPHitE. Here, Each AtoM IS BOnded tetRAHEdRAlly To fOur OtHErs, formIng a 3-DimeNsIonAl nETworK oF PUcKereD Six-Membered rings of Atoms. dIamOnD hAs The Same CuBic StruCTure aS siliCon anD germaniuM, AnD because of tHE Strength of ThE CarboN-Carbon bonDs, it is THE HardEsT NATuraLlY ocCURring suBsTaNce MeASured by resIstance tO ScratchiNg. coNtrarY to the populaR beLief that "dIamoNdS Are forEveR", thEy are TheRmodYnamically uNStable (δFg°(DIamOnD, 298 k) = 2.9 kJ/Mol) undEr nOrMaL conDItions (298 k, 105 pa) anD sHOuld ThEoretically tRaNsforM iNtO grAphIte. bUt DUE To a hiGh actIvAtiOn eNeRgy baRRier, the tRanSiTIoN inTo GrapHiTe iS sO slow AT noRmaL TempeRatuRE tHaT it is unnoTICeAblE. HOWeVer, at very HigH TemperATURes dIAmOnd wilL turn inTo grapHIte, anD DIamOnds Can Burn up In a house Fire. tHe bottom leFt corner of ThE phasE diAGram FoR carboN hAs nOt beEn scrutiniZed eXpERimeNTallY. AltHOugh a coMputatIOnal sTudy emploYing densIty funcTiOnAL theOrY mEthods rEached THE concluSIoN tHAt as anD , DiAmond beComes more stAbLe thAn grAphite by aPpRoxIMaTeLY 1.1 Kj/mOl, mORE Recent and deFiniTivE ExpeRimEnTaL aNd compUtAtIonal studieS sHOw tHat GRaphite Is more StablE than dIaMoNd FOr , wiTHouT appLieD prEssure, by 2.7 KJ/mOl At t = 0 k aND 3.2 Kj/mol at t = 298.15 k. uNder sOme coNDItiOns, carboN CRystaLlIzes as lOnsDalEite, A HexaGoNal crYstal laTtIce With all AtOMs covAlEntLY bonDed And PRoperTIES sIMiLaR tO thOse oF diamOnd.