Crystal Structure of Lithium at 4.2 K

The (heretofore unknown) crystal structure of lithium at low temperature is identified to be close-packed rhombohedral $9R$. The stacking order of the hexagonal layers is $ABCBCA-CAB$, a nine-layer repeat sequence. There are three atoms per primitive rhombohedral cell, and nine in the (nonprimitive) hexagonal cell. The only other element with this structure is samarium.