Konar, Sumit ORCID: 0000-0003-3156-6536, Nylen, Johanna, Svensson, Gunnar, Bernin, Diana ORCID: 0000-0002-9611-2263, Eden, Mattias, Ruschewitz, Uwe ORCID: 0000-0002-6511-6894 and Haussermann, Ulrich (2016). The many phases of CaC2. J. Solid State Chem., 239. S. 204 - 214. SAN DIEGO: ACADEMIC PRESS INC ELSEVIER SCIENCE. ISSN 1095-726X

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Abstract

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. (C) 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Item Type: Journal Article
Creators:
CreatorsEmailORCIDORCID Put Code
Konar, SumitUNSPECIFIEDorcid.org/0000-0003-3156-6536UNSPECIFIED
Nylen, JohannaUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Svensson, GunnarUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Bernin, DianaUNSPECIFIEDorcid.org/0000-0002-9611-2263UNSPECIFIED
Eden, MattiasUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Ruschewitz, UweUNSPECIFIEDorcid.org/0000-0002-6511-6894UNSPECIFIED
Haussermann, UlrichUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
URN: urn:nbn:de:hbz:38-271818
DOI: 10.1016/j.jssc.2016.04.030
Journal or Publication Title: J. Solid State Chem.
Volume: 239
Page Range: S. 204 - 214
Date: 2016
Publisher: ACADEMIC PRESS INC ELSEVIER SCIENCE
Place of Publication: SAN DIEGO
ISSN: 1095-726X
Language: English
Faculty: Unspecified
Divisions: Unspecified
Subjects: no entry
Uncontrolled Keywords:
KeywordsLanguage
BRILLOUIN-ZONE INTEGRATIONS; AUGMENTED-WAVE METHOD; CALCIUM CARBIDE; CRYSTAL-STRUCTURE; PSEUDOPOTENTIALS; POLYMORPHISM; INFORMATION; METALSMultiple languages
Chemistry, Inorganic & Nuclear; Chemistry, PhysicalMultiple languages
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/27181

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