Structural transformations of Li sub(2) C sub(2) at high pressures

Structural changes of Li sub(2) C sub(2) 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 Li sub(2) C sub(2), which forms from the Immm ground-state structure (Z = 2) at around 15 GPa, adopts an orthorhombic Pnma structure with Z = 4. Acetylide C sub(2) dumbbells characteristic of Immm Li sub(2) C sub(2) are retained in Pnma Li sub(2) C sub(2). The structure of Pnma Li sub(2) C sub(2) relates closely to the anticotunnite-type structure. C sub(2) dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structure of Immm Li sub(2) C sub(2). First-principles calculations predict a transition of Pnma Li sub(2) C sub(2) at 32 GPa to a topologically identical phase with a higher Cmcm symmetry. The coordination of C sub(2) dumbbell units by Li atoms is increased to 11. The structure of Cmcm Li sub(2) C sub(2) relates closely to the Ni sub(2) In-type structure. It is calculated that Cmcm Li sub(2) C sub(2) becomes metallic at pressures above 40 GPa. In experiments, however, Pnma Li sub(2) C sub(2) is susceptible to irreversible amorphization.