Structural transformations ofLi2C2at high pressures

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 PnmaLi2C2. The structure of Pnma-Li2C2 relates closely to the anticotunnite type structure. C2 dumbbell units are coordinated by 9 Li atoms, as compared to 8 in the antifluorite structure of ImmmLi2C2. 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 Ni2In 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.

Structural diversity in lithium carbides

“…Recent experimental work on Li 2 C 2 indicates a phase transition from Immm to a dumbbell-containing Pnma structure, and another dumbbell-type Cmcm structure was predicted at higher pressure [55]. In view of this work, we also did calculations for two C 2 dumbbell-containing structures proposed by Efthimiopoulos et al [55]. Our vdW calculations indicate both these Pnma and Cmcm structures are metastable( Fig.…”

“…Carbon nanoribbons are energetically favorable over carbon dimers in Li 2 C 2 at high pressures. Recent experimental work on Li 2 C 2 indicates a phase transition from Immm to a dumbbell-containing Pnma structure, and another dumbbell-type Cmcm structure was predicted at higher pressure [55]. In view of this work, we also did calculations for two C 2 dumbbell-containing structures proposed by Efthimiopoulos et al [55].…”

“…While disproportion is more favorable based on our calculations, this transition does represent the most thermodynamically favorable pathway amongst the dumbbell-type Li 2 C 2 structures, and can be understood by considering that room-temperature may not provide sufficient thermal energy to access the more energetically favorable Li 2 C and Li 3 C 4 phases. The experimentally observed amorphization transition above 25 GPa [51,55] can also be understood in this way and likely represents a frustrated transition that is kinetically hindered at room temperature. According to our computed formation enthalpies, LiC 6 (P 6/mmm) transforms to the Pmmn and P 6 3 /mcm structures, at pressures of 23.3 GPa and 48.4 GPa, respectively (Fig.…”

“…9). Under hydrostatic compression experiments [51,55], the ambient stable Li 2 C 2 (Immm) phase survived until 15 GPa, before transforming to Pnma structure. While disproportion is more favorable based on our calculations, this transition does represent the most thermodynamically favorable pathway amongst the dumbbell-type Li 2 C 2 structures, and can be understood by considering that room-temperature may not provide sufficient thermal energy to access the more energetically favorable Li 2 C and Li 3 C 4 phases.…”

“…Most of those previous studies focused on the molecular structure and the stability of molecular clusters. Of these reported compounds, only Li 2 C 2 [33,41,[49][50][51][52][53][54][55] and some graphite intercalation compounds (LiC 6 and LiC 12 , etc.) [47,53,[56][57][58][59][60] have been investigated experimentally in their crystal structures.…”