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Structural transformations of Li2C2 at high pressures
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 6, 064111Article in journal (Refereed) Published
Abstract [en]

Structural changes of Li2C2 under pressure were studied by synchrotron x-ray diffraction in a diamond anvilcell 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 ofImmm Li2C2 are retained in Pnma Li2C2. The structure of Pnma Li2C2 relates closely to the anticotunnite-typestructure. C2 dumbbell units are coordinated by nine Li atoms, as compared to eight in the antifluorite structureof Immm Li2C2. First-principles calculations predict a transition of Pnma Li2C2 at 32 GPa to a topologicallyidentical phase with a higher Cmcm symmetry. The coordination of C2 dumbbell units by Li atoms is increasedto 11. The structure of Cmcm Li2C2 relates closely to the Ni2 In-type structure. It is calculated that Cmcm Li2C2becomes metallic at pressures above 40 GPa. In experiments, however, Pnma Li2C2 is susceptible to irreversibleamorphization.

Place, publisher, year, edition, pages
2015. Vol. 92, no 6, 064111
National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-120033DOI: 10.1103/PhysRevB.92.064111ISI: 000359858800001OAI: oai:DiVA.org:su-120033DiVA: diva2:850164
Available from: 2015-09-01 Created: 2015-09-01 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Structure and Phase Stability of CaC2 Polymorphs, Li2C2 and Lithium Intercalated Graphite: A Revisit with High Pressure Experiments and Metal Hydride–Graphite Reactions
Open this publication in new window or tab >>Structure and Phase Stability of CaC2 Polymorphs, Li2C2 and Lithium Intercalated Graphite: A Revisit with High Pressure Experiments and Metal Hydride–Graphite Reactions
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Alkali (A) and alkaline earth (AE) metals can form carbides and intercalated graphites with carbon. The carbides mostly represent acetylides which are salt-like compounds composed of C22− dumbbell anions and metal cations. Both the acetylide carbides and intercalated graphites are technologically important. Superconductivity has been observed in several intercalated graphites such as KC8 and CaC6. Li intercalated graphites are a major ingredient in Li ion batteries. CaC2 is an important commodity for producing acetylene and the fertilizer CaCN2.

In spite of the extensive research on A–C and AE–C compounds, phase diagrams are largely unknown. The thermodynamic and kinetic properties of both carbides and intercalalated graphites are discussed controversially. Recent computational studies indicated that well-known carbides, like CaC2 and BaC2, are thermodynamically unstable. Additionally, computational studies predicted that acetylide carbides will generally form novel polymeric carbides (polycarbides) at high pressures. This thesis is intended to check the validity of theoretical predictions and to shed light on the complicated phase diagrams of the Li–C and the Ca–C systems.

The Li–C and the Ca–C systems were investigated using well-controllable metal hydride–graphite reactions. Concerning the Li–C system, relative stabilities of the metastable lithium graphite intercalation compounds (Li-GICs) of stages I, IIa, IIb, III, IV and Id were studied close to the competing formation of the thermodynamically stable Li2C2. The stage IIa showed distinguished thermal stability. The phase Id showed thermodynamic stability and hence, was included in the Li–C phase diagram. In the Ca–C system, results from CaH2–graphite reactions indicate compositional variations between polymorphs I, II and III. The formation of CaC2  I was favored  only  at  1100  ◦C or  higher  temperature  and  with  excess calcium, which speculates phase I as carbon deficient CaC2−δ .

To explore the potential existence of polycarbides, the acetylide carbides Li2C2 and CaC2 were investigated under various pressure and temperature conditions, employing diamond anvil cells for in situ studies and multi anvil techniques for large volume synthesis. The products were characterized by a combination of diffraction and spectroscopy techniques. For both Li2C2 and CaC2, a pressure induced structural transformation was observed at relatively low pressures (10–15 GPa), which was followed by an irreversible amorphization at higher pressures (25–30 GPa). For Li2C2 the structure of the high pressure phase prior to amorphization could be elucidated. The ground state with an antifluorite Immm structure (coordination number (CN) for C22− dumbbells = 8) transforms to a phase with an anticotunnite Pnma structure (CN for C22− dumbbells = 9). Polycarbides, as predicted from theory, could not be obtained.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2015. 80 p.
Keyword
acetylide carbides, high pressure, Raman spectroscopy, powder X-ray diffraction, Rietveld refinement
National Category
Inorganic Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-120109 (URN)978-91-7649-247-5 (ISBN)
Public defence
2015-10-09, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2012-2956
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2015-09-17 Created: 2015-09-01 Last updated: 2016-07-06Bibliographically approved

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