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1、ElectronicStructureofMaterialsunderPressure.12N.E.ChristensenandD.L.Novikov1InstituteofPhysicsandAstronomy,AarhusUniversity,DK-8000AarhusC,Denmark2ArthurD.Little,Inc.AcornPark,Cambridge,MA02140-2390,USA(February10,1999)AbstractParameter-freecalculationsbasedonthedensity-functionalt
2、heoryareusedtoexaminehigh-pressurephasesofsolids.Fortheelementalsemiconductors,asrepresentedbySi,thehigh-pressurephasesareexaminedinsomedetail,andparticularattentionispaidtotheSi-VIorthorhombic(Cmca)structurewhichwasresolvedonlyveryrecently.ForII-Vsemiconductorstheoptimizationofthe
3、structuralparametersoftheCmcmandImm2phasesisdescribed.Thestructuralenergydi erencesareinseveralcasesverysmall,andinsometoosmalltoallowasafestructurepredictiononthebasisofthecalculations.Inthatcontextwealsodiscusswaystogobeyondthelocaldensityapproximation(LDA).Weshowthatthepredicted
4、high-pressurephasesmaybesigni cantlya ectedbyinclusionof(generalized)gradientcorrections(GGA).ElementalZn(hcp)isfurthertakenasanexamplewherewe ndthatthesimpleLDAleadstopoorresultsfortheequilibriumvolumeandaxialratio(c=a).Introducingcorrections,forexamplebyGGA,itis,however,possiblet
5、oachieveanaccuracythatallowsastudyofthestructuralchangesofZn(andCd)underpressure,andtoanalysetheseintermsofelectronictopologicaltransitions.1I.INTRODUCTIONTheoreticalstudiesofcohesive,structuralandvibrationalpropertiesofsemiconductorsunderpressurearenowroutinelybeingperformedbymean
6、sofabinitiocalculations.Theaccuracyoftotalenergiesobtainedwithinthedensity-functionaltheory,oftenevenusingthelocalapproximation(LDA),isinmanycasessucienttopredictwhichstructure,atagivenpressure,hasthelowestfreeenergy.Con gurational-aswellasvibrational-entropycontributionscanbeincl
7、uded[1]-[4],althoughmostcalculationsstillrefertozerotemperature.Whereasconventionalstructuraloptimizationsareperformedbycomparingthefreeenergiesofvariousguessedcrystalstructures,newabinitiomoleculardynamicsmethods[5]-[8]allowbetterdeterminationofthestructuresandunderstandingoftrans
8、formationmechanisms.Semico
