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Silylated carbodiimides in molecular and extended structures

Kroll, Peter and Riedel, R. and Hoffmann, R. (1999):
Silylated carbodiimides in molecular and extended structures.
In: Physical review, B 60APS, pp. 3126-3139, [Article]

Abstract

This work studies the ternary Si-C-N phases SiC2N4 and Si2CN4, exploiting an analogy between the NCN and O groups. Starting from the molecular model of N,N8-bis(trimethylsilyl)-carbodiimide and proceeding to extended models, we calculate that the energy hypersurface associated with the Si-N5C bond angle wN is very shallow, for both molecular and extended structures. We propose a crystal structure for the low-temperature modification a-SiC2N4 in space group P4322 ~95!, which is 40 meV (;4 kJ/mol) lower in energy than an ideal cubic arrangement in space group Pn3¯m. A second structure, b-SiC2N4 @space group P4¯ n2 ~118!#, is slightly higher in energy than a-SiC2N4, but still more stable than the cubic structure, and may be the high-temperature structure of SiC2N4. Both variants of SiC2N4 show a small bulk modulus of about 8 GPa (;0.13 Mbar), suggesting a high compressibility of these nonoxide covalently bonded materials. For Si2CN4 we refined the crystal structure of the compound within the experimentally determined space group Aba2 ~41!. We also found a second candidate nearly equal in energy, with space group Cmc21, differing only in the connection pattern of the SiN2 layered sheets. Both ternary compounds appear to be thermodynamically unstable with respect to decomposition into Si3N4, C, and molecular N2. (S0163-1829~99!02629-6)

Item Type: Article
Erschienen: 1999
Creators: Kroll, Peter and Riedel, R. and Hoffmann, R.
Title: Silylated carbodiimides in molecular and extended structures
Language: English
Abstract:

This work studies the ternary Si-C-N phases SiC2N4 and Si2CN4, exploiting an analogy between the NCN and O groups. Starting from the molecular model of N,N8-bis(trimethylsilyl)-carbodiimide and proceeding to extended models, we calculate that the energy hypersurface associated with the Si-N5C bond angle wN is very shallow, for both molecular and extended structures. We propose a crystal structure for the low-temperature modification a-SiC2N4 in space group P4322 ~95!, which is 40 meV (;4 kJ/mol) lower in energy than an ideal cubic arrangement in space group Pn3¯m. A second structure, b-SiC2N4 @space group P4¯ n2 ~118!#, is slightly higher in energy than a-SiC2N4, but still more stable than the cubic structure, and may be the high-temperature structure of SiC2N4. Both variants of SiC2N4 show a small bulk modulus of about 8 GPa (;0.13 Mbar), suggesting a high compressibility of these nonoxide covalently bonded materials. For Si2CN4 we refined the crystal structure of the compound within the experimentally determined space group Aba2 ~41!. We also found a second candidate nearly equal in energy, with space group Cmc21, differing only in the connection pattern of the SiN2 layered sheets. Both ternary compounds appear to be thermodynamically unstable with respect to decomposition into Si3N4, C, and molecular N2. (S0163-1829~99!02629-6)

Journal or Publication Title: Physical review
Volume: B 60
Publisher: APS
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Dispersive Solids
Date Deposited: 19 Nov 2008 16:04
Funders: The authors gratefully acknowledge the generous support by the National Science Foundation ~Grant No. CHE 94- 08455! and the Center for Materials Science at Cornell University., We also thank the Deutsche Forschungsgemeinschaft, Bonn, (Germany) for financial support through Contract No. Kr 1805/1-1. We thank the Cornell Theory Center for computer time., We acknowledge especially the theory group at the Fritz-Haber-Institut in Berlin/Germany for making their programs publicly available., R.R. thanks the Deutsche Forschungsgemeinschaft, Bonn, (Germany) for research Grant No. Ri 510/3-2 and the Fonds der Chemischen Industrie, Frankfurt, Germany, for financial support.
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