RR = radius of cation/radius of anion (usually oxygen)

Radius Ratio (RR) <0.155 0.155-0.225 0.225-0.414 0.414-0.732 0.732-0.99 1.0

Cation Coord. # 2 3 4 6 8 12

Configuration linear trigonal planar tetrahedral or square octahedral cubic close-packing (cubic or hexagonal)

Example CO2 CO32- SiO2 NaCl CsCl metals

Notes:

• Coordination numbers may vary for a particular element due to external influences such as pressure and temperature:
  • high pressure/low temperature - higher coordination number favored.
  • low pressure/high temperature - lower coordination number favored
    Example:

    RR (Si:O) = 0.3 --> ~middle of 4CN range; therefore,

    Si⁺⁴ - almost ALWAYS Coord. # = 4
    Exception - Stishovite (high pressure form of SiO₂) Coord. # = 6

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