Stability of lithium in α-rhombohedral boron

The stability of lithium atoms in α-rhombohedral boron was studied by the density functional theory and Car-Parrinello molecular dynamics (MD) simulations. At a low Li concentration (1.03 at%), a Li atom at the center of the icosahedral B₁₂ site (the I-site) was found to be metastable, and the potential barrier was estimated at 775±25 K (=67±25 meV). Over 800 K, Li atoms began to escape from the B₁₂ cage and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site was also metastable below 1400 K, and Li at the O-site was energetically the most favorable. At a high Li concentration (7.69 at%), the I-site changed to an unstable saddle point. The T-site was still metastable, and the O-site was the most stable. Regardless of concentration, MD simulations showed that Li atoms at the O-site never jumped to other sites below 1400 K. The migration of Li would be very slow below this temperature.