Ab Initio Calculation of Li-Sn System: Unraveling New Phases of Superconducting Materials with Increasing Compression

Abstract

Stoichiometry, crystal compound, electronic attributes and superconductivity of compressed lithium-tin composites have been thoroughly studied using quantum mechanical genetic algorithm approach and the first principles computations based on density functional theory. Our simulations at moderate pressure (5-20 GPa) predict a complex convex hull diagram, with the following stable Li‑rich phases: I4/mmm-Li₆Sn₂, -Li₇Sn₂, -Li₅Sn₂, Ama2-Li₄Sn₂, -Li₅Sn₂, -Li₆Sn₂, C2/m-Li₄Sn₁, P2₁/m-Li₆Sn₂, -Li₇Sn₂ and Cmcm-Li₄Sn₂. Careful examination at their independent elastic parameters reveals sufficient mechanical stability in them. These phases are metallic system, with reasonably high electron concentration near to Fermi level or N(E_F) that ranges from 0.6 to 2.4 states/eV cell. It is also interesting for us to observe soft modes and steep-flat energy bands at Fermi levels of Li₆Sn₂ structures which are stable throughout the pressure range. These features are prerequisites for superconducting behavior. Linear response function with Gaussian and tetrahedron methods reveals satisfactory superconducting transition temperature T_c (3.1 ~ 6.6 K) and T_c (2.1 ~ 2.4 K), respectively. Structural transition results for based elements Li and Sn agree well with literature thus signifying reliable prediction of intermediate phases.