材料科學(xué)基礎(chǔ)

FundamentalofMaterialsScience

LessonnineProf:TianMin-Bo,SunXiao-DanTel:62772851，62772977E-mail:

DepartmentofMaterialScienceandEngineeringTsinghuaUniversity.Beijing1000843.5SomeRulesGoverningtheIonicCompounds

Pauling’sthreerulesgoverningthestructureofioniccompounds.1.AnionCoordinationPolyhedronRule

Eachcationissurroundedbyananionpolyhedron,thedistancebetweenanionandcationisro,dependsonr+＋r-,whiletheCNdependsonr+/r-.

ro＞r+＋r－

ro＝r+＋r－=re

(equlibriumdistance)

stableioniccompound

ro＜r+＋r－

thereissomecovalentbondingstableunstableTheelectroncloudlap，andthereiscovalentbondbesidesionicbond.

ro

r-

r+anionpolyhedronCN＋()minhexahedron8>0.732octahedron6>0.414tetrahedron4>0.225trigonal3>0.1552.ValenceruleThereisadefiniterelationbetweenvalenceandCN.LetSbethestrengthofelectrostaticbondingbetweenapairofanionandcation.

ifS1＝S2＝……＝S,thenlookupr+、r-r+/r－CN+3.Asarule,thecoordinationpolyhedra(formedbytheclosepackingofanions)willsharecorners(apex),asopposedtofacesoredges.3.6CrystalStructureofCeramicMaterials

(compoundsformedbetweenmetaland

non-metalelements)Types:AB,AB2,A2B3,ABO3,AB2O4Contentsconcerned: (1)validationofPauling’srules; (2)latticetype; (3)descriptionofcrystalstructure; (4)othermaterialswiththesamestructure.Latticetype+structureunitAnion(cation)lattice+cation(anion)interstialPolyhedronanditsconnectionSuperlatticeorsuperstructureCoordinateofatoms

Waystodescribecrystalstructure3.6.1ABtype1.NaCltype(rocksalt)

r+/r－＝0.54∴octahedron＝>

CN+＝6

Z－＝1＝CN－(Z+/CN+)＝CN－×1/6∴CN－＝6Crystalsystem:cubicLatticetype:FCCStructureunit:Na-ClCl-:FCCNa+:octahedralinterstitialSuperlatticeorsuperstructure:Cl-(FCC)+Na+(FCC)

Othermaterialswiththisstructure:NaI,MgO,CaO,BaO,MnO,FeO,CoO,NiO,TiN,TiC,et.al.

2.CsCltype

r+/r－＝0.91

∴hexahedron＝>CN+＝8

Z－＝1＝CN－(Z+/CN+)＝CN－×1/8∴CN－＝8Latticetype:SC

Othermaterialswiththisstructure:CsBr、CsI、TlCl,NH4Cl,et.al.Cl-:SCCs+:bodycenter3.Zincblendetype(b-ZnS)CN+

＝4 CN－＝4<＝>tetrahedronLatticetype:FCCS-:FCCZn2+:halfof

tetrahedralinterstitial

Othermaterialswiththisstructure:

-SiC、GaAs、AlP,et.al.Zn2+:FCCS-:halfof

tetrahedralinterstitial4.Wurtzitetype(a-ZnS,ZnO)ThenumberofOandZnineachunitcell:n(O)＝1/8×8＋1＝2n(Zn)＝1/4×4＋1＝2Latticetype:simplehexagonalCN＋＝CN－＝4<＝>tetrahedron4.Wurtzitetype(a-ZnS,ZnO)S-:HCPZn2+:halfof

tetrahedralinterstitial

Othermaterialswiththisstructure:BeO、AlN,

et.al.3.6.2AB2type1.Fluorspar/fluorite(CaF2)

CN+

＝8CN-

＝4Infact,F-islargerthanCa2+.Latticetype:FCChexahedron

<＝>Ca2+:FCCF-:allof

tetrahedralinterstitial

Othermaterialswiththisstructure:UO2,CeO2,

BaF2,

et.al.ZrO2(Zirconia)similartofluorite

structureLattice:monoclinicFeaturesofthefluoriteandanti-fluoritestructure

：Bothfluoriteandanti-fluorite

havearelativelylargeintersticeintheirstructures,whichfavorsthemigrationofions.Therefore,theycanbeusedassolidelectrolytesorforfiltration.Anti-fluoritestructure（A2Btype）e.g.Li2O,Na2O,Li2S,Na2S,et.al.Structrualfeatures：

Thestructureofcationsandanionsis

oppositetothatoffluorspar.

AnionsoccupythelatticepointsofFCC,

whilecationsoccupythetetrahedralinterstitial.2.Rutiletype(TiO2)[TiO6]octahedronsshareedgestoformchainsalongcaxis,whilethechainsshareapices.<＝>octahedron2.Rutiletype(TiO2)Lattice:tetragonaltwoTi4＋：(0,0,0)、(?,?,?)fourO2－：(u,u,0)、(1-u,1-u,0)、(?+u,?-u,?)、(?-u,?+u,?)

u=0.31

Othermaterialswiththisstructure:MnO2、SnO2、PbO2、MoO2、GeO2,

et.al.CN-=3——OneO2-hasthreenearestneighborsTi4+.ionionvacancy3.6.3A2B3type()??????????????????*Latticetype:simplehexagonalO2-:HCPAl3+:2/3of

octahedralinterstitialOneO2-hasfournearestneighborsAl3+.ionionvacancyAl3+vacancyAlAlAlAlAlAlAlOneunitofstructureAl3+:O2-:6(Al2O3)Al3+vacancyO2-

Othermaterialswiththisstructure:Cr2O3、a-Fe2O3、V2O3,

et.al.3.6.4ABO3type(BaTiO3,CaTiO3)Latticetype:SCBa2+:SCO2-:SCTi4+:octahedralinterstitial“FCC”

Othermaterialswiththisstructure:PbTiO3、PbZrO3、SrTiO3,et.al.3.6.5AB2O4type(spinel,MgAl2O4)spinelLatticetype:FCCMg2+:diamondstructureO2-:tetrahedroninevery

tetrahedralinterstitialAl3+:formingsmallcubic

withO2-tetrahedron

inthetetrahedral

interstitialwithoutMg2+

Mg2+O2-Al3+O2-tetrahedronO2-/Al3+cubicO:4×8＝32Mg:4＋4＝8

8(MgAl2O4)Al:4×4＝16

spinel

Othermaterialswiththisstructure:A：Mg2+、Mn2+、Fe2+、Co2+、Ni2+、Zn2+,et.al.B：Al3+、Cr3+、Fe3+、Co3+,et.al.

anti-spinelstructure:Fe3O4：Fe2+Fe3+2O4LiMn2O4O2-:FCC

A2+:1/8tetrahedralinterstitialB3+:1/2octahedralinterstitialDiscussionThecharacterofCaF