课件-结构化学chapter

ChapterofMetalsand8.1MetallicBondinginthesolidFormetals,theatomshavelowelectron-negativitiesandelectronionizationpotentials;thereforetheelectronsaredelocalizedoveralltheatoms.Wemaythinkofthestructureofametalasanarrangementofpositiveatomcoresinaseaofelectrons. izethebondinginametalitmakessensetopackasmanyatomsaroundeachotheras izethenumberofthenearestneighbors(CN)andminimizethevolume.8.1MetallicBondinginthesolid•Highelectrical•Highthermal•HighreflectivityofvisibleThe“free-electron”modelofthemetallicbond(Asimplemodelformetals)TheFreeElectronConsiderametalsuchasNa,MgorAltobeessentiallyaboxinwhichthevalenceelectronsofthemetalareThepotentialwithintheboxistakentobeuniformandmuchlowerthanthatinthesurroundingmedium.(V=0)ConsidertheinteractionbetweenelectronstobeTreatquantummechanicallyasaparticleinaSolutionstoathreedimensionalparticleinabox28 V（周期性平面波：(xl) 1(x,y,z)()2l

(nxxnyynzh2hE

n2)

n2h2

n22ml

2ml•where,n•where,nx,ny,nzcanbepositive,zero,xyz《结构化学基础 Solutionstoathreedimensionalparticleinabox

E2ml

(nx

nz)

2ml在微观范围内,能级分Therearefourquantumnumbernx,ny,n在微观范围内,能级分Wecanhavetwoelectronsperuniquecombinationofnx,nyandn20,简并态1,ms=1/2,1/2,2电子Foralargeboxtheenergylevelsaregoingtobeclosetogether,considerenergylevelsasformingacontinuousbandHowmanyenergylevelsdowehavewithenergylessthansomeFermienergylevelEFistheuppermostfilledenergyFermienergylevelEFistheuppermostfilledenergylevelofaconductorat0K.eg.MetalNa:EF=3.15eV EF=3.2••At0K,ElectronsfilltheorbitalsfromthelowestenergylevelfirstuntiltheFermienergylevelEF0K时，导体中电子所占据最高能级）•AllenergylevelslowerthanEFarefilled,andallthosehigherthanEFareempty.•WhenT>0K,somefreeelectronscanbeactivatedtotheenergylevelhigherthanEF.8.1.2Bondtheoryof(Theoryofsolidsusingmolecular••Considertheelectronsinmetalsmovinginapotentialfieldsofthemetalatoms(V≠0，V(x)=V(x+l

2V]•Considera•Considerasolidtobealargemoleculeandmolecularorbitaltheory.Bandsresultfromtheoverlapofatomicorbitals(LCAO-MO) Energiesofthe2s(valence)molecularorbitalsforclustersoflithiumatomsfilledwith1valenceelectronperatom:Forasolid,Nissolarge(>1020)thattheMOenergiesformacontinuousPartiallyfilledbondmeansnoenergygapbetweenthehighestfilledorbital(HOMO)andthelowestunfilledorbitalElectronscanbemovedeasilyfromfilledtoemptyorbitalsbyanelectricfield,solithiummetalconductselectricity.Electronsincomple yfilledbandscan’tbemovedbyelectricfields,andthereforearepoorelectricalconductors(insulatorsorsemiconductors)Berylliumhas2valenceelectronsperatom,justenoughto yfillthe2sband,soBeshouldbeanelectricalBemetalisanelectricalconductor!Wheredidthetheorygoconsiderboth2sand2p(valence)molecularorbitalsofberylliumTheoverlap 2sand2pbandsformasinglebandthatispartiallyfilled.ThusBemetalisanelectricalconductor.Band

Empty

Conduction

Filled••能带较宽个能带按能量高低排为能带结构每个能带具有一定的能量范 间轨 少的每个能带具有一定的能量范 较窄,轨 多的外层原子轨道形成

SiEg=1.1ev;GeEg=0.72evGaAsEg=1.4ScanningTunnelingMicroscope(STM)ImageofIroninthe(110)planeAnactualSTMimageofaNiNotethehexagonalarrangementofClosepackingspheresandthestructureofpureClosepackingofhard MetallicsolidscanbemodeledashardLimitationsforpackingarepurelyResultsinhighestpossibledensityofspheres(12nearestneighbors)Thetwomostcommonclose-packedABABstructureofhexagonalclosepacking(HCP,TypeABCABCstructureofcubicclosepacking orFCC,Typeabplanea

cplanebplaneaplanecplaneHexagonal/CubicClosePackingandunitABABAB…Closeab aa

Unit(0,0,0)(1/2,(0,1/2,1/2)ABCABC…ClosecHexagonalclosepacked(hcp,TypeUnit

FaceCenteredCubic(FCC) ,TypeA1)

bplaneaplanecplane8.2.2.PackingEfficiencyand Usingalittlegeometryandsomediagramswecanshowthat74%oftheunitcellofacubicclosepackedstructureisfilledwithhardspheres,26%isemptyEmpty TwopossibletypesofOctahedralCoordinationNumber(C.N.)isPackingefficiencyofFCCstructure a4R2

a3 2R)3

R) coefficient

16

OctahedralholesinFCC ,FoundbetweentwoparalleltrianglesofNspheresresultinNoctahedralWithinWithinaunit 12*1/4+1=4octahedralTetrahedralholesinFCCWithinaunit tetrahedralWithinaunit tetrahedralTwopossibletypesof-upand-NholesofeachtypesforNNspheresresultin2NtetrahedralHCPstructure: octahedralholesinHCP tetrahedralholesinHCPa2

2+8/4=WithinaWithinaunitcell:22octahedralNspheresresultinNoctahedralWithinaunitcell:2 tetrahedralNspheresresultin2Ntetrahedral8.2.3.NonClosePacked BCCand4R a3(4R)36433 33 2(4R3)8BCC(bcpor-Bodycentered

68%packing

Vsphere

8R3 64

a223a2BCCstructure(bcp,a223a2 Holes Withinaunit26distortedoctahedralholes(1/412+1/212distortedtetrahedralholesSCandSC, -Simplecubic52%packing

DC, -Thediamond34%packingMorecomplex,lesssymmetricstructuresaretypicallydistortionofthesepackingsystemsSCand4148Ca,Sr,Ba224Zn,Cd2168111 8.38.3StructuresofpureA.structuresofpure B.AtomicradiiofTheTheatomicradiiofthemetalswereobtainedbyhavingthedistanceofclosestcontactAtomicradiiofmetalschangeswithbondinmetalliccrystal,rMg=1.60inioniccrystal,rMg2+=0.78Atomicradiiofmetalschangeswithcoordination86421ThestructuresandpropertiesofAlloy:asubstancethatcontainsamixtureofelementsandhasmetallicproperties,maybedividedintothreemajorcategoriessubstitution:additivesdisplacelatticepointsInterstitial:additivesoccupyholes(填隙合金Completefillingofholes:beginstoformnewlatticeSolidsolutions（固溶体SignificantcommonCrystalAtomicradii radiiandcrystalstructurearerequiredsothatthelatticeisnotexcessivelydistortedElectronegativitysimilarityrequiredotherwiseintermetallicbondsform

structuraltypeofCu,Ag,Au,Ni,-Co,-Fe,Pt,Ir,bcp-Fe,V,Mo,OrderordisorderofthemetalliccF:ThestructureofsolidsolutioncanbecF:cP:a=3.753

a=b=3.960Åc=3.667ÅMetalliccompound(金属化合物SignificantdifferencesCrystalAtomicCharacteristicsofthemetallicThestructureisdifferentfromthepureEachtypeofmetaloccupyingoneofthe ExampleA:thestructureofthealloyMg:Mg:dcpackingCu:halftetrahedralElectronegativity:Mg CuRadius:Mg1.60;CuEachunitCu:DistanceDistancebetweenMg-几何上有效的填充空间是其稳定存在的(71.0%AB2laves (164种合金

•化学式主要为AB2•化学式主要为AB2positionof rΑ/rB:Radius:MgCu1.90;Ni1.91;ZnCu1.28;Zn1.39;NiExampleB:thestructureofthealloyatoms/unitatoms/unitCu:1/2Ca:1/8oneCaCu5/unitC.N.ofElectronegativity:Ca1.00;Cu1.90;Radius:Ca1.97;Cu1.28ElectronegativityLa1.10;Ni1.91;CoRadius:La1.82;Ni1.24;Co ExampleB:thestructureofthealloyLaNi5LaNi5,LaCo5,CeCo5havesamestructuresasCaCu561022~71022hydrogen>4.21022hydrogenatom(liquidOtherreservoirsforhydrogen:AlloyMn-Ti(lavesphase)HavecapacityHavecapacitytostoreInterstitialmetalliccompounds（金属填隙化合物•Interstitial•InterstitialAlloyformedwhensomeoftheholesinclosestpackedstructurearefilledwithsmallerRequiressmallatomadditivesthatfitwithinlatticemaybestoichiometricor dramaticallychangethephysicalpropertiesofthematerialExample:steelisaninterstitialalloyofironandcarbonrC/rFe=0.70/1.40=0.50>Thestructureandpropertiesofsteelandpurepureiron（<0.02%C） A2(bcp)structure:-（纯铁 -Feand-A1(fcc) -Steelisanalloysystemconsistingofironandcarbon。harderandstronger0.02%~2%C（0.25%低C2.0%C高C钢or铸铁castIron-soft,ductile,Steel-harder,stronger,lessductilethanpuremorecarbon,hardersteel(0.2%~StructureofFerrite(铁素体):asolidsolutionofCin-Fe(BCC).Sincethesizeoftheoctahedralholesisverysmallin-Fethesolubilityisverylow(thehighestsolubilityis0.02(pureiron纯铁)Austenite:aninterstitialsolidsolutionofCin-Fe(about0.8%byweight,atomratioFe:C=27:1,1Catom/6~7Feunit)Cementite(渗碳体 Fe3C orthorhombic:Martensite:asupersaturatedsolidsolutionofCin-Fe(about1.6%byweight,byfastcoolingofaustenite) Ferrite(0.02%Cin-Fe(BCC)

(0.8%Cin