ABSTRACT
In addition to magnesium, the crystal structures of many of the elements are approximated by hexagonal close-packed, hcp, structures. ese elements include helium, lithium, sodium, beryllium, strontium, titanium, cobalt, zinc, cadmium, and others. Other elements form in cubic close-packed, ccp, structures, among them the noble gases-neon,
argon, krypton, and xenon-and metals-lithium, calcium, manganese, iron, copper, gold, silver, aluminum, and lead. Some elements, like lithium, exhibit polymorphism. e crystal structure of lithium is hcp at 78 K (Wycko, 1963, pp. 7-83), is body-centered cubic at 6.6 GPa, 296 K and ccp at 9.8 GPa, 296 K (Olinger, 1983). For some interesting examples see The Pursuit of Perfect Packing (Aste and Weaire, 2008).
