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Structure and Phase Stability of CaC2 Polymorphs, Li2C2 and Lithium Intercalated Graphite: A Revisit with High Pressure Experiments and Metal Hydride–Graphite Reactions
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
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 [en]
acetylide carbides, high pressure, Raman spectroscopy, powder X-ray diffraction, Rietveld refinement
National Category
Inorganic Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-120109ISBN: 978-91-7649-247-5 (print)OAI: oai:DiVA.org:su-120109DiVA: diva2:850311
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
List of papers
1. Intercalation Compounds from LiH and Graphite: Relative Stability of Metastable Stages and Thermodynamic Stability of Dilute Stage I-d
Open this publication in new window or tab >>Intercalation Compounds from LiH and Graphite: Relative Stability of Metastable Stages and Thermodynamic Stability of Dilute Stage I-d
2015 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 7, 2566-2575 p.Article in journal (Refereed) Published
Abstract [en]

The intercalation of lithium into graphite was studied at temperatures between 400 and 550 degrees C by heating mixtures of LiH and graphite powders with molar ratios 4:1, 1:1, and 1:6 under dynamic vacuum for periods between 1 and 72 h. These conditions probe the formation and thermal stability of metastable staged Ligraphite intercalation compounds (Li-GICs) close to the competing formation of the thermodynamically stable carbide Li(2)C2. Li-GICs of stages I (LiC6, A alpha), IIa (Li0.5C6, A alpha A), IIb (Li similar to C-0.33(6), A alpha AB beta B), III (Li similar to C-0.22(6), A alpha AB), IV (Li similar to C-0.167(6)), and dilute stage lithium Id have been identified and characterized by powder X-ray diffraction and Raman spectroscopy. The rate and extent of intercalation (i.e., the achieved stage of Li-GIC) depends on LiH activity and temperature. Stage I was only observed for temperatures above 500 degrees C. At 400 degrees C, the highest intercalation corresponded to stage IIb, which was obtained after 2 and 24 h for 4:1 and 1:1 reaction mixtures, respectively. Lower-staged Li-GICs attained at temperatures below 500 degrees C deintercalate upon prolonged dwelling with the exception of stage IIa, which can be maintained for very long periods (several days) in the presence of LiH. At temperatures above 500 degrees C, the kinetically controlled formation of Li-GICs is followed by Li2C2 carbide formation. It is shown that the Li-GIC I-d coexists with Li2C2 at temperatures up to 800 degrees C and that the Li content of I-d (solubility of Li in graphite) increases between 550 and 800 degrees C. Consequently, I-d with a temperature-dependent homogeneity range should be added as a stable phase in the Li-C phase diagram. A sketch of a revised Li-C phase diagram is provided.

National Category
Chemical Sciences Materials Engineering
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-117731 (URN)10.1021/acs.chemmater.5b00235 (DOI)000353176100038 ()
Available from: 2015-06-09 Created: 2015-06-01 Last updated: 2017-12-04Bibliographically approved
2. The many phases of CaC2
Open this publication in new window or tab >>The many phases of CaC2
Show others...
2016 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 239, 204-213 p.Article in journal (Refereed) Published
Abstract [en]

Polymorphic CaC2 was prepared by reacting mixtures of CaH2 and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 degrees C under dynamic vacuum. These conditions provided a well controlled, homogeneous, chemical environment and afforded products with high purity. The products, which were characterized by powder X-ray diffraction, solid state NMR and Raman spectroscopy, represented mixtures of the three known polymorphs, tetragonal CaC2-I and monoclinic CaC2-II and -III. Their proportion is dependent on the nominal C/CaH2 ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC2-I, whereas high temperatures (above 1100 degrees C) and C-deficiency favored the formation of CaC2-I. From first principles calculations it is shown that CaC2-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC2-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC2-delta. It is proposed that monoclinic II is the ground state of CaC2 and polymorph III is stable at temperatures above 200 degrees C. Tetragonal I represents a metastable, heterogeneous, phase of CaC2. It is argued that a complete understanding of the occurrence of three room temperature modifications of CaC2 will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC2-IV, which is stable above 450 degrees C. The effect of high pressure on the stability of the monoclinic forms of CaC2 was studied in a diamond anvil cell using Raman spectroscopy. CaC2-II and -III transform into tetragonal CaC2-I at about 4 and 1GPa, respectively.

Keyword
Acetylide carbides, Polymorphism, Structural stability
National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-131909 (URN)10.1016/j.jssc.2016.04.030 (DOI)000377422000030 ()
Available from: 2016-07-06 Created: 2016-07-04 Last updated: 2017-11-28Bibliographically approved
3. Structural behavior of the acetylide carbides Li2C2 and CaC2 at high pressure
Open this publication in new window or tab >>Structural behavior of the acetylide carbides Li2C2 and CaC2 at high pressure
Show others...
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 22, 224507Article in journal (Refereed) Published
Abstract [en]

The effects of high pressure (up to 30 GPa) on the structural properties of lithium and calcium carbide, Li2C2 and CaC2, were studied at room temperature by Raman spectroscopy in a diamond anvil cell. Both carbides consist of C-2 dumbbells which are coordinated by metal atoms. At standard pressure and temperature two forms of CaC2 co-exist. Monoclinic CaC2-II is not stable at pressures above 2 GPa and tetragonal CaC2-I possibly undergoes a minor structural change between 10 and 12 GPa. Orthorhombic Li2C2 transforms to a new structure type at around 15 GPa. At pressures above 18 GPa (CaC2) and 25 GPa (Li2C2) Raman spectra become featureless, and remain featureless upon decompression which suggests an irreversible amorphization of the acetylide carbides. First principles calculations were used to analyze the pressure dependence of Raman mode frequencies and structural stability of Li2C2 and CaC2. A structure model for the high pressure phase of Li2C2 was searched by applying an evolutionary algorithm.

National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-87137 (URN)10.1063/1.4770268 (DOI)000312491400092 ()
Note

AuthorCount:5;

Available from: 2013-01-28 Created: 2013-01-28 Last updated: 2017-12-06Bibliographically approved
4. Structural transformations of Li2C2 at high pressures
Open this publication in new window or tab >>Structural transformations of Li2C2 at high pressures
Show others...
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.

National Category
Chemical Sciences
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-120033 (URN)10.1103/PhysRevB.92.064111 (DOI)000359858800001 ()
Available from: 2015-09-01 Created: 2015-09-01 Last updated: 2017-12-04Bibliographically approved

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