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Elemental crystals with HCP structure:
[element: a (nm), c/a]
Be: 0.229 , 1.568;
Ti: 0.295 , 1.587;
Zn: 0.267, 1.856;
Tl: 0.346, 1.598;
Ho: 0.358, 1.57;
Mg: 0.321, 1.624;
Co: 0.251, 1.623;
Zr: 0.323, 1.593;
Gd: 0.363, 1.591;

Reference: Joel I. Gersten and Frederick W. Smith, "The physics and chemistry of materials", John Wiley & Sons, New York, 2001.

Si: a=5.43 Å
Ge: a= 5.65 Å
SiC: a= 3.08 Å
AlP: a= 5.46 Å
AlAs: a= 5.66 Å
Ga: a=5.45 Å
GaAs: a= 5.65 Å
GaN: a= 4.5 Å