I_4. Unit I_MEE 1005_Crystal System Metallic Crystal System

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crystal structures
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  School of Mechanical and Building Sciences, VIT University, Vellore ãA crystallinematerial isoneinwhichthe  atoms are situated in a repeating or periodic array over large atomic distances ;thatis,long-rangeorderexists,suchthatuponsolidification.ãPeriodicityisoneofthemostimportantpropertiesofcrystals.ãCrystalsare  highly symmetrical arrays of atoms whichsubstantiallysimplifiestheir analysis Crystal Structures -Fundamental concept  School of Mechanical and Building Sciences, VIT University, Vellore ãCrystallatticeisthe  periodic and systematic arrangement of atoms thatarefoundincrystalswiththeexceptionofamorphoussolidsandgases.ãSomeofthepropertiesofcrystallinesolidssuchascleavage,electronicbandstructureandopticaltransparencydependonthecrystalstructureofthematerial ã  Lattice – Arrangement of atoms/ions ã  Motif/Basis – A group of one/more atoms located in a particular way with respect to eachother and associated with lattice point  ã  Crystal Structure = Lattice + Motif/Basis Crystal Lattice  School of Mechanical and Building Sciences, VIT University, Vellore ãThe  basic structural unit or building block of the crystal structure anddefinesthecrystalstructurebyvirtueofitsgeometryandtheatompositionswithin.ãTheunitcellischaracterizedbyitslatticeparameterswhichconsistofthelengthofthecelledgesandtheanglesbetweenthem Unit Cell ãPrimitiveunitcellscontainonlyonelatticepoint,whichismadeupfromthelatticepointsateachofthecornersãNon-primitiveunitcellscontainadditionallatticepoints,eitheronafaceoftheunitcellorwithintheunitcell,andsohavemorethanonelatticepointperunitcell  School of Mechanical and Building Sciences, VIT University, Vellore ãTheunitcellgeometryisdefinedintermsofsixparameters : the three edge lengths a,b,andc,and  the three inter-axial angles α,β,andγandaretermedthe  lattice parameters or latticeconstants ofacrystalstructureãSevenpossiblecombinationsofa,b,c&α,β,γresultinginsevencrystalsystems Lattice Parameters ã  The size of the unit-cell is described in terms of its unit-cell parameters. These are the edgelengths and the angles of the unit-cell.
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