L15 - SUMMARY FROM LAST CLASS Crystalline solids Packing...

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Dr. P. Lucas U of A MSE 110 Crystalline solids SUMMARY FROM LAST CLASS Packing density: Simple cubic: 52% Body Centered Cubic (BCC): 68% Faced Center Cubic (FCC) and Hexagonal Close Packed (HCP) : 74% Stacking dense layers generate octahedral and tetrahedral interstitial sites. Close packed structures have two tetrahedral site and one octahedral site for each atom: FCC : 4 atoms per unit cell, 8 tetra sites and 4 octa sites per cell. HCP : 6 atoms per unit cell, 12 tetra sites and 6 octa sites per cell Cubic sites are not present in close packed structure. The larger the number of atoms generating an interstitial site, the larger the volume of the site: Cubic>Octa>Tetra An interstitial atom must be at least as large as the site it occupies.
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Dr. P. Lucas U of A MSE 110 Crystal structure based on close packed lattices: CRYSTALLINE STRUCTURE The crystalline structure of many compounds can be described as a lattice of atoms with its interstitial sites filled with one or several different atoms. These arrangements (CaF 2 , ZnS, NaCl…) define some structure types that can be adopted by many other compounds of different compositions. FCC FCC
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U of A MSE 110 CRYSTALLINE STRUCTURE Crystal structure based on FCC: NaCl The NaCl structure can then be seen as two intertwined FCC lattices shifted by ½ cell parameter. The structure of sodium chloride is referred to as the rocksalt structure. It is a very important structure type adopted by many other compounds. The Na or Cl can sit as the unit cell corner atom equivalently as both atoms form an equivalent FCC lattice. It is based on a FCC lattice with all four octahedral site filled with another atom. Since there are four atoms in the FCC unit cell, the resulting composition is one to
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L15 - SUMMARY FROM LAST CLASS Crystalline solids Packing...

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