Metastable, Unstable and ground-state crystal structure of lithium*

Lattice of light atomic nuclei remains restless at low temperature due to their quantum mechanical zero point energy. Combination of a quantum lattice and a free electron system is predicted to have exotic properties. Lithium is the lightest metal and has a nearly free electron system and therefore has been studied extensively for evidence of quantum mechanical contribution to its lattice. Here we present a detailed structural study of two lithium isotopes and present evidence that the thermodynamic ground state of the lithium lattice is face-centered-cubic (fcc). However, the known ambient pressure martensitic transition of body-centered-cubic (bcc) to 9R in Li, is a kinetically favored transition that prevails the transition to fcc structure. Most surprisingly, we find that unlike 7Li, 6Li does not show any evidence of a martensitic transition at low pressure and temperature (T ~ 20 K) and remains in bcc structure due to its lattice quantum-dynamics.