Structural transformations of Li2C2 at high pressures

Ilias Efthimiopoulos, Daryn E. Benson, Sumit Konar, Johanna Nylen, Gunnar Svensson, Ulrich Haeussermann*, Stefan Liebig, Uwe Ruschewitz, Grigory V. Vazhenin, Ingo Loa, Michael Hanfland, Karl Syassen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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 C-2 dumbbells characteristic of Immm Li2C2 are retained in Pnma Li2C2. The structure of Pnma Li2C2 relates closely to the anticotunnite-type structure. C-2 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 C-2 dumbbell units by Li atoms is increased to 11. The structure of Cmcm Li2C2 relates closely to the Ni-2 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.

Original languageEnglish
Article number064111
Number of pages8
JournalPhysical review B: Condensed matter and materials physics
Volume92
Issue number6
DOIs
Publication statusPublished - 19 Aug 2015

Keywords

  • CRYSTAL-STRUCTURE PREDICTION
  • BRILLOUIN-ZONE INTEGRATIONS
  • AUGMENTED-WAVE METHOD
  • PHASE-TRANSITION
  • CALCIUM CARBIDES
  • PSEUDOPOTENTIALS
  • ALGORITHM
  • ALKALI
  • USPEX
  • CAC2

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