Lithium's low-temperature phase transitions: Insights into quantum lattice dynamics and superconductivity

The large lattice dynamics of lithium, driven by its low atomic mass, results in energetically similar structures and significant isotope effects under pressure, posing challenges to current theoretical models. Above 20 GPa and at low temperatures, lithium's electronic properties deviate from simple metallic behavior, with superconductivity emerging in a complex, pressure-dependent manner, alongside an unusual isotope effect. The structural phases of $^{7}\mathrm{Li}$ reported under these conditions are inconsistent across studies, and the structures of $^{6}\mathrm{Li}$ remain unexamined. These gaps limit our understanding of the effects of pressure on lithium's electronic properties and the role of quantum lattice effects on its structural behavior under pressure. Here, we integrate experimental and theoretical approaches to investigate the low-temperature structural phase boundaries in lithium isotopes. We map the structural phase diagram of $^{7}\mathrm{Li}$ from 5 to 55 GPa and 15--75 K, identifying the sequence $\mathrm{fcc}\ensuremath{\rightarrow}hR1\ensuremath{\rightarrow}cI16$. A pronounced isotope effect is observed, with $^{6}\mathrm{Li}$ shifting the $\mathrm{fcc}\ensuremath{\rightarrow}hR1$ phase boundary to lower pressures at 15 K. Density functional theory calculations further clarify how these structural changes affect superconducting properties, particularly emphasizing the role of the $\mathrm{fcc}\ensuremath{\rightarrow}hR1$ transition in lithium's superconductivity. Our findings offer insights into the unique behavior of lithium isotopes under pressure.