Structural transformations of Li2C2 at high pressures

TL;DR

This study investigates the high-pressure structural transformations of Li2C2 using synchrotron x-ray diffraction and computational methods, revealing new phases, their structures, and electronic properties under extreme conditions.

Contribution

The paper combines experimental and computational approaches to identify and characterize new high-pressure phases of Li2C2, including their structures and transition pressures.

Findings

Li2C2 transforms from Immm to Pnma structure around 15 GPa.

Pnma Li2C2 transitions to Cmcm phase at 32 GPa, predicted by calculations.

Cmcm Li2C2 becomes metallic above 40 GPa.

Abstract

Structural changes of Li2C2 under pressure were studied by synchrotron x-ray diffraction in a diamond anvil cell under hydrostatic conditions and by using evolutionary search methodology for crystal structure prediction. We show that the high-pressure polymorph of Li2C2, which forms from the Immm ground-state structure (Z = 2) at around 15 GPa, adopts an orthorhombic Pnma structure with Z = 4. Acetylide C2 dumbbells characteristic of Immm Li2C2 are retained in Pnma Li2C2. The structure of Pnma Li2C2 relates closely to the anticotunnite-type structure. C2 dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structure of Immm Li2C2. First-principles calculations predict a transition of Pnma Li2C2 at 32 GPa to a topologically identical phase with a higher Cmcm symmetry. The coordination of C2 dumbbell units by Li atoms is increased to 11. The structure of Cmcm Li2C2 relates closely to the Ni2 In-type structure. It is calculated that Cmcm Li2C2 becomes metallic at pressures above 40 GPa. In experiments, however, Pnma Li2C2 is susceptible to irreversible amorphization.