近代分析實(shí)驗(yàn)原理：第四課-XRD-2

四、X射線衍射-2

（Xraydiffraction（XRD））近代分析實(shí)驗(yàn)原理（Introductionofmodernanalyticalmethods）121.XRD設(shè)備（InstrumentationusedforX-rayDiffraction）Rigaku（日本理學(xué)）Smartlab331.1X光源（Xraysource）InanX-raytube,thehighvoltagemaintainedacrosstheelectrodesdrawselectronstowardametaltarget(theanode).X-raysareproducedatthepointofimpact,andradiateinalldirections.韌致輻射（Brakingradiation）：Theelectrons,acceleratedbyanelectricalfield,aresuddenlysloweddownwhentheycomeintocontactwiththetargetandlosesomeoftheirenergywhichisdispersedintheformofradiation.continuousCoolidgetubeafewhundredWattspersquaremillimeterpower4特征輻射（characteristicradiation）Theelectronstransmittheirenergytotheatomsofthetargetandcauseelectronictransitions,andtheemissionofXphotonswithnon-randomenergiesE=hν,equivalenttotheenergygapsbetweenthedifferentelectronshellsoftheatom.discontinuous5旋轉(zhuǎn)靶X射線管（rotatinganodetube）severalthousandWattspersquaremillimeterpowerathighspeeds(2,000to6,000rpm)Vacuum10-7Torrperformancesthedimensionsoftheelectronspottheanode’ssurfacespeed同步輻射（Synchrotronradiation）Achargedparticleisaccelerated,theelectromagneticradiationthatisemittediscalledsynchrotronradiation.veryhighintensityoftheX-raybeams6多色polychromatic單色monochromatic濾波片filter單色器monochromatorcrystal(ormonochromator)濾波片filterAbsorption：linearabsorptioncoefficientthelengthtraveledtoabsorbasmuchaspossibleoftheunwantedpeaksthickertobeastransparentaspossibletothewavelengthwewishtoselectthinnere-d7amolybdenumanodeandazirconiumfilter鉬靶鋯濾波片1028晶體單色器（Monochromatorcrystals）weak9彎晶單色儀curvedcrystalmonochromators同圓或等圓中，同弧或等弧所對(duì)的圓周角相等Radius=2Rsymmetrictiltingthediffractingcrystalplanesatanangleαinrelationtothesinglecrystal’ssurface101.2探測(cè)器（Detector）1.2.1 膠片（photographicfilm）ThedarkeningofthefilmisrelatedtotheintensityoftheX-raybeam.Linearregionsemi-quantitativemeasurementnotdirectlydigitalgradualdisappearanceoftheuseByR?ntgen111.2.2 氣體探測(cè)器（gasdetector）thediffractometerbuiltbytheBraggswasequippedwithagasdetectorNumberofion-electronpairs.ion-electronpairsrecombinetheelectronsarecollectedwithoutrecombination雪崩oneavalancheavalancheprocesswidespreadspontaneousandpermanentdischarge12ThedifferenttypesofgasdetectorsIonizationchambersatvoltagesinthesaturationareasaturationvoltagedependsontheintensityoftheradiationHigherradiationintensity,Highersaturationvoltage.heavygasisgenerallyusedGeiger-MüllercountersAvalancheprocesscannotbeusedforquantitativeintensitymeasurementsProportionalcountersHighcountingrate(107count/s)WavelengthselectiveLonglifetimesensitivityThissystemisrelativelysimpleandisusedasamobiledetector.currentlythemostusedinX-raydiffraction131.2.3 固體探測(cè)器（Soliddetectors）RadioluminescenceXradiationinducedluminescenceX-rayirradiationelectronswithahighenergyemissionofvisiblephotonsbombardtheneighboringatoms,causingelectronictransitionsDirectobservationoflightphotonsi)FluorescentscreensIntensityofdiffractedbeamismuchtoosmalltoyieldvisiblespotsvisualizetheprimarybeamforgeometricadjustmentsInX-raydiffraction,suchscreensarenotusedtodetectdiffractedbeams,sincetheirintensityismuchtoosmalltoyieldvisiblespots.Thesescreensareusedtovisualizetheprimarybeamwhilegeometricadjustmentsaremadetodiffractionapparatus.14iii)ScintillationdetectorsXphotonslightphotonsElectronsandelectricalsignalphotocathodeii)DetectorsthatuseCCDcameras（Chargecoupleddevice）responsetimeisextremelylow(≈0.2μS)ItissensitivetotheenergyofXraysbuthasapoorenergyresolution.15Indirectmethod:theimagingplateBaBrFdopedwitheuropiumlargesizes(upto25cmindiameter)aresolutionofupto100μm2Themaindrawbackisacertainremanenceoftheimageafterithasbeenerased.Directlymeasureallofthediffractionringsphosphorescentmaterial:16SemiconductordetectorsAvoltageappliedbetweenthetwofacesofthesemiconductorenablestheusertomeasuretheelectriccharge,whichisproportionaltotheenergyoftheincidentXphotons.Themainadvantageofthistypeofdetectorisitsenergyresolution.172.多晶X射線衍射（X-rayDiffractionbyPolycrystallineMaterials）(a)Focusingdiffractometers:SeemanandBohlindiffractometersThediffractedbeamsconvergetothefocusingcircle,thediameterofwhichisconstant,thuscausingthesample-detectordistancetovarywiththediffractionangle.DetectorDisadvantage:18(b)Bragg-BrentanodiffractometersConstantsample-detectordistanceapproximatefocusingsystems19fixedthesamplerotatesarounditsaxisataspeedωthedetectorismovingataspeed2ωalongthecircle,centeredonthesamplegoniometriccirclethemostcommonsystemamajordrawback缺點(diǎn):Thesampleisrotating,itcansometimesfall,ifitisinpowderform,whentheangleofrotationistoohigh.Theuseofthiskindofdevicewithaheatingsampleholderisdifficultforthesamereason.20Rotateintheoppositedirectionwiththesamespeed.Forbothofthesetwosystems,themovementsofthedifferentelementscanbeeithercontinuousorincremental.moreexpensiveItisdifficulttoachieveaperfectlycontrolledmovementoftheX-raytube,whichisquiteheavy.21Slitlimittheincidentbeam’sopeningonlythebeamswiththechosenwavelengthareseenbythedetectorSollerslitsLimitverticaldivergencehighlyabsorbingmaterialsheavychemicalelements22WithoutamonochromatorCannotselectKα1only,BothKα1andKα2components23DepthandirradiatedvolumeTheirradiatedvolumeisconstantThepenetrationdepthvarieswiththeangleθperfectlyhomogenouswithrespecttoitsthicknessrequirement24(c)ParallelgeometrydiffractometersThepositionofthesamplehaslittleinfluenceonthemeasurements,thusmakingitpossibletoproducediffractionpatternswithsamplesthathaveirregularsurfaces.Advantage:Disadvantage:cannotselecttheKα1peak,resultinginsplitpeaks25bestangularresolution263.試樣的準(zhǔn)備（Samplepreparation）InpowderorpolycrystallinediffractionItisimportanttohaveasamplewithasmoothplanesurface.Normallygrindthesampledowntoparticlesofabout0.002mmto0.005mmcrosssection.homogeneousandthecrystallitesarerandomlydistributedGenerally,thepowdersampleispressedintoasampleholdersothatwehaveasmoothflatsurface.Ifwehaveatrulyrandomsample,eachpossiblereflectionfromagivensetofh,k,lplaneswillhaveanequalnumberofcrystallitescontributingtoit.27284.應(yīng)用（Applications）IdentificationofanUnknownSamplebyX-rayDiffraction(HanawaltMethod)thepowderdiffractionpatternofasubstanceCharacteristicofthatsubstance,likeafingerprintsimplesumXRDpatternXRDpatternofcomponent1··XRDpatternofcomponentntheHanawaltmethod:thed-valuesofthethreestrongestdiffractionpeaksEachsubstanceBycomputerThedspacingandintensityofthethreestrongestpeaksItisalsoimportanttoacquireinformationinadvanceabouttypeofelementscontainedinasampleandthecompositionratiooftheelementscontained,forexample,byfluorescentX-rayanalysis.Database(di,Ii)ICDD,PDFcards2930(1)Iftheparticlesizeofapowdercrystallinesampleislargerthanseveral10μm,goodreproducibilityoftheintensitypatternisnotalwaysconfirmedandonecanfrequentlyfindadifferencemorethanseveral10%foreverymeasurement.(2)KeepinmindtheusedwavelengthofX-raysformeasurement(forexample:Cu-KαorMo-Kα),becausesomedifferencesmaybefoundintherelativeintensities,dependinguponthewavelengthduetoanomalousdispersionfactors.(3)Thereverseorderoftherelativeintensitiesmaybefoundinasample,whichiseasilyreceivedunderthepreferredorientation,likeclayminerals.(4)Whenimpuritiesareincludedinasample,acertainshifttowardthelowerangleinthepeakpositionsisquitelikelytoappear.(