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MS115a Lect 05 10 05 2011

MS115a Lect 05 10 05 2011 - Metals Ionic Compounds anion...

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Metals
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cation anion Ionic Compounds
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Radius Ratio Rules CN (cation) Geometry min r c /R A ( f ) 2 linear none 3 trigonal planar 0.155 4 tetrahedral 0.225 6 octahedral 0.414 8 cubic 0.732 12 cubo-octahedral 1 if r c is smaller than f R A , then the space is too big and the structure is unstable common in ionic compounds sites occur within close-packed arrays
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Summary: Sites in HCP & CCP 2 tetrahedral sites / close-packed atom 1 octahedral site / close-packed atom sites are located between layers: number of sites/atom same for ABAB & ABCABC
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Common Ionic Structure Types Rock salt (NaCl) sometimes also ‘Halite’ Derive from cubic-close packed array of Cl - Zinc blende (ZnS) Derive from cubic-close packed array of S = Fluorite (CaF 2 ) Derive from cubic-close packed array of Ca 2+ Cesium chloride (CsCl) Not derived from a close-packed array Complex oxides Multiple cations
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Example: CaF 2 (Fluorite) F - ~ 1.3 Å; Ca 2+ ~ 1.0 Å; – r c /R A = 0.77 Ca 2+ is big enough for CN = 8 But there are no 8-fold sites in close-packed arrays Consider structure as CCP cations F - in tetrahedral sites – R A /
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