reunit 5专业英语.docx
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reunit 5专业英语.docx
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reunit5专业英语
Unit5AnalysisMethodsandTechniques
1Generalcomments:
molecularandnon-molecularsolids
Thesimplestandmostobviousfirstquestiontoaskaboutaninorganicsubstanceis‘Whatisit?
’.Themethodsthatareusedtoanswerthiscomeintotwomaincategories,dependingonwhetherthesubstanceismolecularornon-molecular.Ifthesubstanceismolecular,whetheritbesolid,liquidorgaseous,identificationisusuallycarriedoutbysomecombinationofspectroscopicmethodsandchemicalanalysis.Ifthesubstanceisnon-molecularandcrystalline,identificationisusuallycarriedoutbyX-raypowderdiffractionsupplemented,wherenecessary,bychemicalanalysis.EachcrystallinesolidhasitsowncharacteristicX-raypowderpatternwhichmaybeusedasa'fingerprint'foritsidentification.ThepowderpatternsofmostknowninorganicsolidsareincludedinanupdatedversionofthePowderDiffractionFile;byusinganappropriatesearchprocedure,unknownscanusuallybeidentifiedrapidlyandunambiguously.
Oncethesubstancehasbeenidentified,thenextstagemaybetodetermineitsstructure,ifthisisnotknownalready.Formolecularmaterials,detailsofthemoleculargeometrymaybeobtainedfromfurtherspectroscopicmeasurements.Alternatively,ifthesubstanceiscrystalline,X-raycrystallographymaybeused,inwhichcaseinformationisalsoobtainedonthewayinwhichthemoleculespacktogetherinthecrystallinestate.Formolecularsubstances,thisusuallycompletesthestoryasfarasidentificationandstructuredeterminationareconcerned;attentionmaythenfocusonothermatterssuchaspropertiesorchemicalreactivity.
Fornon-molecularsubstances,however,theword‘structure’takesonawholenewmeaning.Obviously,weneedtoknowthecrystalstructure,asgivenbytheunitcellanditscontents.However,defectsandimpuritiesarealsooftenextremelyimportantandsometimescontrolproperties.Thus,thecolourandlasingactionofruby,Cr-dopedAl2O3,dependexclusivelyonthepresenceofCr3+impuritiesinthecorundumcrystalstructureofAl2O3.Insuchcases,thecrystalstructureoraveragestructureofthehostisimportantbutthelocalstructurecentredontheimpuritiesordefectscontrolstheproperties.
Onasomewhatlargerlengthscale,theopticalpropertiesofcolloids(ornanoparticles,togivethemanamethatiscurrentlymorefashionable)dependoncrystallitesize.Forinstance,thecolour,bandgapandphotoconductivityofCdSnanoparticlesdependontheparticlesizeandhence,ontheirnanostructure.
Onastilllargerscale,themechanicalandelectricalpropertiesofceramicsareoftendeterminedbythemicrostructure,whichcoversthesize,shapeanddistributionofcrystallinegrains,thebondingbetweengrainsandthesegregationofanyimpuritiestothesurfacesorintergranularregions.Togiveoneexample,ZnOceramicvaristorsareratherspecialmaterialswhoseelectricalpropertiesdonotobeyOhm'sLawandthisisassociatedwithcompositionalinhomogeneitiesintheceramic,inparticularthesegregationofdopantssuchasBiandCototheintergranularregions.Whenwerefertothestructureofinorganicmaterials,wemaybeinterestedinsomeoralloftheaboveaspectsofstructure,rangingfromlocalstructure,overdistancesofafewangstroms(1Å=0.1nm=10-4μm=10-7mm),tomicrostructureatthemicronlevel.Thiscontrastshugelywithmolecularsubstancesforwhichstructurerefersmainlytotheatomic-levelarrangementofatomsandoccasionally,tothepackingarrangementofmoleculesincrystals.Sinceinnon-molecularmaterialsweareinterestedinthestructureatseverallevels,awiderangeoftechniquesisneededtocharacterizethesolids.
Theprimereasonforthegreatdifferencebetweenmolecularandnon-molecularmaterialsliesintheverydifferentstatusofdefectsandimpuritiesinthetwocategoriesofmaterial.Inmolecularsubstances,defectsarenotallowed!
Ifacertainmoleculehasatomsmissing,orextraatomspresent,thentheresultingmoleculeisquitedifferenttotheparentmoleculeandcanbeseparatedbystandardpurificationmethods.Further,thepresenceofanysuch‘defective’moleculesismostunlikelytomodifythepropertiesoftheparent,non-defectivemolecules.Thus,moleculeshaveaccuratelyfixedformulaeorstoichiometriesandaredefect-free.
Innon-molecularmaterialsbycontrast,defectsandimpuritiesarealmostunavoidable.Theycannotbereadilyeliminatedandarealwayspresentfromthermodynamicconsiderations.Impuritiesgiverisetonon-stoichiometry,i.e.variablecompositionandmayinducedramaticchangesinpropertiesoftheparentstructure.
Anillustrationofthevastlydifferentchemistriesofmolecularandnon-molecularsubstancesisgiveninTable
Table1.Comparisonofamolecularsubstance,toluene,C6H5CH3,andanon-molecularsubstance,Al2O3
Toluene
Alumina
Stoichiometry
Impurities?
Natureofthematerial
Properties
Applications
Fixed,C6H5CH3
Notallowed
Volatileliquid
Solvent
Fixed,Al2O3
Readilydoped
Canbe:
powder,fibre,ceramic,singlecrystal,film
Dependonthenatureofthematerialanddopants/impurities
Abrasive(powder),thermalinsulator(saffilfibres),electricalinsulator(thinfilmorceramicsubstrate),rubygemandlaser(Crdoped),solidelectrolyte(Nabeta-alumina)
1fortwoapparentlysimplesubstances,oneineachcategory.Molecularchemistswilltellyouthattolueneisanextremelywell-understoodmoleculeandhasfewsurprisesleftaluminumoxide,bycontrast,showsarichdiversityofstructures,propertiesandapplicationsandisstillbeingactivelyresearched.
2Characterizationofsolids
Inorderforasolidtobewellcharacterized,oneneedstoknowabout:
(a)thecrystalstructure,asgivenbytheunitcell,itsdimensionsandthefractionalcoordinatesoftheatomspresentinthecell;
(b)thecrystaldefectsthatarepresent,theirnature,numberanddistribution;
(c)theimpuritiesthatarepresentandwhethertheyaredistributedatrandomorareconcentratedintosmallregions;
(d)forpolycrystallinesolids-powdersorceramics-thenumber,size,shapeanddistributionofthecrystallineparticles;
(e)thesurfacestructure,includinganycompositionalinhomogeneities,adsorbedsurfacelayersorstructuraldifferencesbetweensurfaceandinterior.
Nosingletechniqueiscapableofprovidingacompletecharacterizationofasolid.Rather,avarietyoftechniquesareusedincombination.Therearethreemaincategoriesofphysicaltechniquewhichmaybeusedtocharacterizesolids:
diffraction,microscopicandspectroscopictechniques.Inaddition,othertechniquessuchasthermalanalysisandphysicalpropertymeasurementsgivevaluableinformationincertaincases.
3X-raydiffraction
X-raydiffractionhasbeenusedfornearlyacenturyintwomainareas,forthefingerprintcharacterizationofcrystallinematerialsandfordeterminationoftheirstructure.Itistheprincipaltechniqueofsolidstatechemistryandaccordingly,isgivenmostspaceinthischapter.Abriefdescriptionisalsogivenofelectronandneutrondiffraction,tworatherspecializedtechniqueswhichhaveimportantapplications.
a)GenerationofX-rays
X-raysareelectromagneticradiationofwavelength~1Å(10-10m).Theyoccurinthatpartoftheelectromagneticspectrumbetweenγ-raysandtheultraviolet.X-raysareproducedwhenhigh-energychargedparticles,e.g.electronsacceleratedthrough30,000V,collidewithmatter.TheresultingX-rayspectrausuallyhavetwocomponents,abroadspectrumofwavelengthsknownaswhiteradiationandanumberoffixed,ormonochromaticwavelengths.Whiteradiationariseswhentheelectronsaresloweddownorstoppedbythecollisionandsomeoftheirlostenergyisconvertedintoelectromagneticradiation.Whiteradiationhaswavelengthsrangingupwardsfromacertainlowerlimitingvalue.ThislowerwavelengthlimitcorrespondstotheX-raysofhighestenergyandoccurswhenallthekineticenergyoftheincidentparticlesisconvertedintoX-rays.Itmaybecalculatedfromtheformula,λmin(Å)=12400/V,whereVistheacceleratingvoltage.
TheX-rayswhichareusedinalmostalldiffractionexperimentsareproducedbyadifferentprocessthatleadstomonochromaticX-rays.Abeamofelectrons,againacceleratedthrough,say,30kVisallowedtostrikeametaltarget,oftenCu.TheincidentelectronshavesufficientenergytoionizesomeoftheCu1s(Kshell)electrons,Fig.1(a).Anelectroninanouterorbital(2por3p)immediatelydropsdowntooccupythevacant1slevelandtheenergyreleasedinthetransitionappearsasX-radiation.ThetransitionenergieshavefixedvaluesandsoaspectrumofcharacteristicX-raysresults,Fig.1(b).ForCuthe2p→1stransition,calledKα,hasawavelengthof1.5418Åandthe3p→1stransition,Kβ,1.3922Å.TheKαtransitionoccursmuchmorefrequentlythantheKβtransitionandthismoreintenseKαradiationisusedindiffractionexperiments.Infact,theKαtransitionisadoublet,Kα1=1.54051ÅandKα2=1.54433Å,becausethetransitionhasaslightlydifferentenergyforthetwopossiblespinstatesofthe2pelectronwhichmakesthetransition,relativetothespinofthevacant1sorbital.InsomeX-rayexperiments,diffractionbytheKα1andKα2radiationsisnotresolvedandasinglelineorspotisobservedinsteadofadoublet(e.g.inpowderdiffractometryatlowangle).Inotherexperiments,separatediffractionpeaksmaybeobserved;ifdesired,thiscanbeovercomebyremovingtheweakerKα2beamfromtheincidentradiation.
ThewavelengthsoftheKαlinesofthetargetmetalscommonlyusedforX-raygenerationaregiveninTable2.Theyarerelatedtotheatomicnu
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