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2.3 Basic structure types

Pauling rules: understanding polyhedral structures

coordination anion polyhedron radius ratios cation

3 triangle 0.15-0.22 C

4 tetrahedron 0.22-0.41 Si, Al

6 octahedron 0.41-0.73 Al, Fe, Mg, Ca

8 cube 0.73-1.00 K, Na

12 close packing 1.00

(anti)cuboctahedron 2r(anion)

Example: Octahedron

Radius ratio
octahedral coordination
2r(anion)
+
2r
2  2r (anion )  (cation ) 2r(cation)
1 2r (anion )

r (cation )
2   . 0 414
1
r (anion )