Stable and Metastable Structures in Compressed LiC6: Dimensional Diversity

Abstract

Unbiased structural searching, together with ab initio calculations, was performed to investigate the crystalline structures of lithium-intercalated graphite compound (LiC6) under pressure. Four low-enthalpy phases are found as the thermodynamic ground states of LiC6 up to 240 GPa, including an experimentally observed P6/mmm phase and three predicted orthorhombic phases (Pmmn, Immm, and Cmmm). For carbon atoms, the 2D graphite layered structure is predicted to transform first to a 3D nanofoam structure, then into a 2D diamond-like strips structure, and ultalimately to a 3D network structure. The covalent bonding among carbon atoms exhibits sp2 and sp3 hybridization in the orthorhombic phases, and sp3 hybridization tends to increase with increasing pressure. Phonon calculations verify that the competitive phases are thermodynamically stable. These findings elucidate the phase transitions of LiC6 under high pressure, offering major implications as regards the high-pressure behaviors of graphite intercalatio...