weekendingPRL107,175504(2011)PHYSICALREVIEWLETTERS21OCTOBER2011AmorphousDiamond:AHigh-PressureSuperhardCarbonAllotropeYuLin,1,*LiZhang,2Ho-kwangMao,2PaulChow,3YumingXiao,3MariaBaldini,4JinfuShu,2andWendyL.Mao1,51DepartmentofGeologicalandEnvironmentalSciences,StanfordUniversity,Stanford,California94305,USA2GeophysicalLaboratory,CarnegieInstitutionofWashington,Washington,D.C.20015,USA3HighPressureCollaborativeAccessTeam,GeophysicalLaboratory,CarnegieInstitutionofWashington,Washington,D.C.20015,USA4HighPressureSynergeticConsortium,GeophysicalLaboratory,CarnegieInstitutionofWashington,Washington,D.C.20015,USA5PhotonScience,SLACNationalAcceleratorLaboratory,MenloPark,California94025,USA(Received14June2011;published19October2011)Compressingglassycarbonabove40GPa,wehaveobservedanewcarbonallotropewithafullysp3-bondedamorphousstructureanddiamondlikestrength.Synchrotronx-rayRamanspectroscopyrevealedacontinuouspressure-inducedsp2-to-sp3bondingchange,whilex-raydiffractionconfirmedtheperseveranceofnoncrystallinity.Thetransitionwasreversibleuponreleasingpressure.Usedasanindenter,theglassycarbonballdemonstratedexceptionalstrengthbyreaching130GPawithaconfiningpressureof60GPa.Suchanextremelylargestressdifferenceof>70GPahasneverbeenobservedinanymaterialbesidesdiamond,indicatingthehighhardnessofthishigh-pressurecarbonallotrope.DOI:10.1103/PhysRevLett.107.175504PACSnumbers:81.05.U,61.05.cf,62.50.p,83.80.AbAsthefourthmostabundantelementintheUniverse,carbon[16–18]andtetrahedralamorphouscarboncarbonformsavarietyofallotropeswithdramatically[19–24]usingsubplantationofincidentionshaveproduceddifferentphysicalandchemicalproperties[1–4].Thecar-thinfilmswithexceptionalpropertiesofhighhardness,bonatomsincrystallinediamondarecharacterizedbyinertness,transparency,andwide-bandgapsemiconduc-sp3-hybridizedorbitalswithallfourvalenceelectronstivityforapplicationssuchasprotectivecoatingsforopti-formingtetrahedrallycoordinatedbondstofouradjacentcal,electronic,mechanical,andbio