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Temperature-induced phase transition and Li self-diffusion in Li2C2

S. Filippov, J. Klarbring, U. Häussermann, and S. I. Simak

Phys. Rev. Materials 3, 023602 (2019)



Lithium carbide, Li2C2, is a fascinating material that combines strong covalent and weak ionic bonding resulting in a wide range of unusual properties. The mechanism of its phase transition from the ground-state orthorhombic (Immm) to the high-temperature cubic (Fm¯3m) crystal structure is not well understood and here we elucidate it with help of first-principles calculations. We show that stabilization of the cubic phase is a result of a temperature-induced disorientation of the C-C dumbbells and their further thermal rotations. Due to these rotations rather large deviatoric stress, which is associated with the dumbbell alignment along one of the crystallographic axes, averages out making the cubic structure mechanically stable. At high temperature we observe a type-II superionic transition to a state of high Li self-diffusion involving collective ionic motion mediated by the formation of Frenkel pairs (see Fig. 1 below).

©2019 American Physical Society

Fig. 1: A 3D visualization of the diffusion events of selected Li ions. The C-C dumbbells are depicted in their starting positions of the molecular dynamics simulation. The green cloud around the lower-right dumbbell illustrates all orientation that the C-C dumbbell had due to rotation during the long simulation.

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Last updated: 02/21/19