The Structure of Crystalline Solid

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1、TheStructureofCrystallineSolidSolidmaterialsmaybeclassifiedaccordingtotheregularitywithwhichatomsorionsarearrangedwithrespecttooneanother.Acrystallinematerialisoneinwhichtheatomsaresituatedinarespectingorperiodic(周期的)arrayoverlargeatomicdistances;thatis,long-rangeorder(长程有序)exists,suchthatuponsol

2、idification,theatomswillpositionthemselvesinarepetitivethree-dimensionalpattern,inwhicheachatomisbondedtoitsnearest-neighboratoms.Allmetals,manyceramicmaterials,andcertainpolymersformcrystallinestructuresundernormalsolidificationconditions.Forthosethatdonotcrystallize,thislong-rangeatomicorderisa

3、bsent.Someofthepropertiesofcrystallinesolidsdependonthecrystalstructureofthematerial,themannerinwhichatoms,ions,ormoleculesarespatiallyarranged.Thereisanextremelylargenumberofdifferentcrystalstructuresallhavinglong-rangeatomicorder.Whendescribingcrystallinestructures,atoms(orions)arethoughtofasbe

4、ingsolidsphereshavingwell-defineddiameters.Thisistermedtheatomichardspheremodelinwhichspheresrepresentingnearest-neighboratomstouchoneanother.Inthisparticularcasealltheatomsareidentical.Sometimesthetermlattice(晶格)isusedinthecontextofcrystalstructures;inthissense“lattice”meansathree-dimensionalarr

5、ayofpointscoincidingwithatomposition(orspherecenters).Theatomicorderincrystallinesolidsindicatesthatsmallgroupsofatomsformarepetitivepattern.Thus,indescribingcrystalstructures,itisoftenconvenienttosubdividethestructureintosmallrepeatentitiescalledunitcells(晶胞).Unitcellsformostcrystalstructuresare

6、parallelepiteds(平行六面体)orprisms(棱柱)havingthreesetsofparallelfaces;oneisdrawnwithintheaggregateofspheres,whichinthiscasehappenstobeacube.Aunitcellischosentorepresentthesymmetryofthecrystalstructure,whereinalltheatompositionsinthecrystalmaybegeneratedbytranslationsoftheunitcellintegraldistancesalong

7、eachofitsedges.Thus,theunitcellisthebasicstructuralunitorbuildingblockofthecrystalstructureanddefinesthecrystalstructurebyvirtueofitsgeometryandtheatompositionswithin.Convenienceusuallydictatesthatparallelepipedcorners