Temperature-induced phase transition and Li self-diffusion in Li2C2 : A first-principles study

Abstract

Lithium carbide, Li 2 C 2 , 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 (Fm3m) 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 CC 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.

Interaction between alkali metals and diamond: Etching and charge states of NV centers

“…The reaction of alkali metals with a diamond surface has not been systematically studied to the best of the authors' knowledge. Only Li 2 C 2 is known as an alkali metal carbide [54]. We found the comparison of the formation energy for Li 2 C 2 and hypothetical Na 2 C 2 and K 2 C 2 by DFT calculations from the alkali metals and graphite, which gives the values of 0.015eV, 0.241 eV, and 0.250 eV, respectively [47].…”

Exploration of AB3Si3 (A = Na/K/Rb/Cs) compounds under moderate pressure

“…Binary compounds consisting of alkali metals or alkaline-earth metals with element IVA (C or Si) 1–5 have attracted considerable attention due to their potential applications in solar cells, 6 lithium-ion batteries, 7 thermoelectric devices, 8 etc. LiC 12 has a positive formation enthalpy due to the occupation of electrons in anti-bonding states within −100 to 100 GPa, making it difficult to synthesize experimentally.…”