Quantum and isotope effects in lithium metal

The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state structure. Using synchrotron X-ray diffraction in diamond-anvil-cells and multiscale simulations with density functional theory and molecular dynamics we show that the previously accepted martensitic ground-state is metastable. The actual ground-state is face-centered-cubic. We find that isotopes of lithium, under identical thermal paths, exhibit a large difference in martensitic transition temperature. Lithium exhibits large quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum-effect-dominated structures, and the higher mass elements. By disentangling the quantum-kinetic complexities, we prove that fcc lithium is the groundstate, and we synthesize it by decompression.