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hcp fluorite
the hexagonal close packed analogue of the fluorite structure

Wikipedia: Calcium fluoride, Fluorite
Housecroft: p. 168
Shriver & Atkins: pp. 86–87
Smart & Moore: pp. 35 37
Greenwood & Earnshaw: pp. 117–118

The fluorite structure is the ideal structure of calcium fluoride, CaF2.

The structure can be envisaged as a cubic "close packed" array of cations with all tetrahedral holes filled by anions.

Ca2+ is cubically coordinated by eight F, whereas F is tetrahedrally coordinated by four Ca2+.

A hexagonal close packed array of cations with all tetrahedral holes filled by anions is the hcp analogue of the fluorite structure, but no solid has ever been found to adopt this structure.

The explanation usually given is that the hcp fluorite structure has face-sharing tetrahedra which give rise to strong electrostatic repulsion between ions of like charge at the centres of neighbouring tetrahedra. However, facing-sharing is known in structures such as NiAs, where it promotes Ni-Ni bonding.

Based on template by A. Herráez as modified by J. Gutow
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