衍射花样指数化与应用概述.pptxVIP

衍射花样指数化

Thefirststepinanalysingunknownpowderpatternisoftenanattempttofindaunitcellthatexplainsallobservedlinesinthespectrum.Youdonotneedadditionalcrystallographicdata,althoughifitexistsitmakesforfasterandmorereliableresults.Thematerialtobeanalysedmustbesinglephasedandtheexperimentalmaterialmustbeveryaccurate.

Indexingprogramsuseonlythepositionalinformationofthepatternandtrytofindasetoflatticeconstants(a,b,c,a,b,g)andindividualMillerindices(hkl)foreachline.Theformofequationstosolveiscomplicatedforthegeneralcase(triclinic)indirectspacebutisstraightforwardinreciprocalspace.Inthelatterthesetofequationsis：

Q=h2A+k2B+l2C+hkD+hlE+klFwheretheQ-valuesareeasilyderivedfromthediffractionangleQ.Thissethastobesolvedfortheunknowns,A,B,C,D,E,F,whichareinasimplewayrelatedtothelatticeconstants.Findingthepropervaluesforthelatticeparameterssothateveryobservedd-spacingsatifiesaparticularcombinationofMillerindicesisthegoalofindexing.Itisnoteasyevenforthecubicsystem,butitisverydifficultforthetriclinicsystem.单击此处添加大标题内容

Therearetwogeneralapproachestoindexing,theexhaustiveandtheanalyticalapproach.Bothoftheseapproachesrequireveryaccurated-spacingdata.Thesmallertheerrors,theeasieritistotestsolutionsbecausethereareoftenmissingdatapointsduetointensityextinctionsrelatedtothesymmetryorthestructuralarrangementorduetolackofresolutionofthed-spacingthemselves.Theearliestapproacheswereoftheexhaustivetypeandweredonebygraphicalfittingornumericaltablefitting.

IndexingProgramsThemethodscurrentlyimplementedareshownbold.TheyareselectedthroughtheitemIndexinginthemainmenu.Program Author TypeITO Visser analyticalTREOR Werner exhaustivePOWDERDICVOLCUBIC

Theprogramsuseasetofcommonparameters,e.g.thewavele