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Title: The many phases of CaC{sub 2}

Abstract

Polymorphic CaC{sub 2} was prepared by reacting mixtures of CaH{sub 2} and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 °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 CaC{sub 2}-I and monoclinic CaC{sub 2}-II and -III. Their proportion is dependent on the nominal C/CaH{sub 2} ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC{sub 2}-I, whereas high temperatures (above 1100 °C) and C-deficiency favored the formation of CaC{sub 2}-I. From first principles calculations it is shown that CaC{sub 2}-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC{sub 2}-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC{sub 2-δ}. It is proposed that monoclinic II is the ground state of CaC{sub 2} and polymorph III is stable at temperatures above 200 °C. Tetragonal I represents a metastable, heterogeneous, phase of CaC{sub 2}. It is argued that a complete understanding of the occurrence of threemore » room temperature modifications of CaC{sub 2} will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC{sub 2}-IV, which is stable above 450 °C. The effect of high pressure on the stability of the monoclinic forms of CaC{sub 2} was studied in a diamond anvil cell using Raman spectroscopy. CaC{sub 2}-II and -III transform into tetragonal CaC{sub 2}-I at about 4 and 1GPa, respectively. - Graphical abstract: The famous tetragonal form of CaC{sub 2}, CaC{sub 2}-I, is dynamically unstable. Why does it exist? Display Omitted - Highlights: • Critical review of complex polymorphism of the commodity chemical CaC{sub 2}. • Tetragonal CaC{sub 2}-I is identified as dynamical instable and its existence attributed to disorder. • With pressure monoclinic CaC{sub 2}-II and –III transform into tetragonal CaC{sub 2}-I. • The high temperature form CaC{sub 2}-IV is assigned a key role for understanding the polymorphism of CaC{sub 2}.« less

Authors:
; ; ; ;  [1];  [2];  [1]
  1. Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm (Sweden)
  2. Department of Chemistry, University of Cologne, Greinstrasse 6, 50939 Cologne (Germany)
Publication Date:
OSTI Identifier:
22584162
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 239; Other Information: Copyright (c) 2016 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CALCIUM HYDRIDES; CARBIDES; DIAMONDS; GRAPHITE; GROUND STATES; MIXTURES; MONOCLINIC LATTICES; NUCLEAR MAGNETIC RESONANCE; POWDERS; RAMAN SPECTROSCOPY; SOLIDS; STABILITY; X RADIATION; X-RAY DIFFRACTION

Citation Formats

Konar, Sumit, Nylén, Johanna, Svensson, Gunnar, Bernin, Diana, Edén, Mattias, Ruschewitz, Uwe, and Häussermann, Ulrich, E-mail: Ulrich.Haussermann@mmk.su.se. The many phases of CaC{sub 2}. United States: N. p., 2016. Web. doi:10.1016/J.JSSC.2016.04.030.
Konar, Sumit, Nylén, Johanna, Svensson, Gunnar, Bernin, Diana, Edén, Mattias, Ruschewitz, Uwe, & Häussermann, Ulrich, E-mail: Ulrich.Haussermann@mmk.su.se. The many phases of CaC{sub 2}. United States. doi:10.1016/J.JSSC.2016.04.030.
Konar, Sumit, Nylén, Johanna, Svensson, Gunnar, Bernin, Diana, Edén, Mattias, Ruschewitz, Uwe, and Häussermann, Ulrich, E-mail: Ulrich.Haussermann@mmk.su.se. 2016. "The many phases of CaC{sub 2}". United States. doi:10.1016/J.JSSC.2016.04.030.
@article{osti_22584162,
title = {The many phases of CaC{sub 2}},
author = {Konar, Sumit and Nylén, Johanna and Svensson, Gunnar and Bernin, Diana and Edén, Mattias and Ruschewitz, Uwe and Häussermann, Ulrich, E-mail: Ulrich.Haussermann@mmk.su.se},
abstractNote = {Polymorphic CaC{sub 2} was prepared by reacting mixtures of CaH{sub 2} and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 °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 CaC{sub 2}-I and monoclinic CaC{sub 2}-II and -III. Their proportion is dependent on the nominal C/CaH{sub 2} ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC{sub 2}-I, whereas high temperatures (above 1100 °C) and C-deficiency favored the formation of CaC{sub 2}-I. From first principles calculations it is shown that CaC{sub 2}-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC{sub 2}-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC{sub 2-δ}. It is proposed that monoclinic II is the ground state of CaC{sub 2} and polymorph III is stable at temperatures above 200 °C. Tetragonal I represents a metastable, heterogeneous, phase of CaC{sub 2}. It is argued that a complete understanding of the occurrence of three room temperature modifications of CaC{sub 2} will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC{sub 2}-IV, which is stable above 450 °C. The effect of high pressure on the stability of the monoclinic forms of CaC{sub 2} was studied in a diamond anvil cell using Raman spectroscopy. CaC{sub 2}-II and -III transform into tetragonal CaC{sub 2}-I at about 4 and 1GPa, respectively. - Graphical abstract: The famous tetragonal form of CaC{sub 2}, CaC{sub 2}-I, is dynamically unstable. Why does it exist? Display Omitted - Highlights: • Critical review of complex polymorphism of the commodity chemical CaC{sub 2}. • Tetragonal CaC{sub 2}-I is identified as dynamical instable and its existence attributed to disorder. • With pressure monoclinic CaC{sub 2}-II and –III transform into tetragonal CaC{sub 2}-I. • The high temperature form CaC{sub 2}-IV is assigned a key role for understanding the polymorphism of CaC{sub 2}.},
doi = {10.1016/J.JSSC.2016.04.030},
journal = {Journal of Solid State Chemistry},
number = ,
volume = 239,
place = {United States},
year = 2016,
month = 7
}
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