Phase Transitions in Silicon-Carbon-Nitride Compounds
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1040-Q09-26
Phase Transitions in Silicon-Carbon-Nitride Compounds Peter Kroll1, Jose Gracia2, and Ralf Riedel3 1 Chemistry and Biochemsitry, University of Texas at Arlington, 700 Planetarium Pl, Arlingtom, TX, 76019 2 Inorganic Chemistry, RWTH Aachen, Aachen, 52056, Germany 3 Institut fuer Materialwissenschaft, TU Darmstadt, Darmstadt, 64287, Germany ABSTRACT The crystal structures of α-Si(NCN)2 and Si2N2(NCN) undergo phase transformations upon compression. A new modification of HP-Si(NCN)2 exhibits all Si atoms in octahedral coordination to N and may be achievable below 30 GPa. In Si2N2(NCN) we find partial increase of coordination for Si, C, and N atoms at about 60 GPa. In both cases, the carbodiimide moiety is the source for the internal reaction that lead to compactification of the structure at elevated pressures. We present 29Si NMR chemical shifts that may help to identify the new coordination environments. INTRODUCTION The two refractory materials Si3N4 and SiC are virtually immiscible: the solubility of C in Si3N4 is below 1%. Nevertheless, ternary SiCN phases have been synthesized by several nonequilibrium routes, e.g. by deposition techniques or through a thermally induced ceramization of precursors (see Ref. [1] for a recent review). Synthesis of two crystalline SiCN compounds, SiC2N4 (also Si(NCN)2) and Si2CN4 (also Si2N2(NCN)), both comprising the carbodiimide N=C=N moiety, succeeded about a decade ago [2]. What makes these two compounds so interesting is that they are located at the (hypothetical) tie line between Si3N4 and C3N4, approaching the enigmatic carbon nitride material C3N4. Hence, they might offer a pathway to new hard materials that combine the properties of silicon nitride with that of one of the lowcompressible modifications of carbon nitride. Computations on SiCN compounds concentrated mostly on structures derived from αand β-Si3N4, but the pseudo-cubic and the willemite-II derived structure were also investigated [3,4,5]. A comprehensive study of SiC2N4 and Si2CN4 indicated the specific property of the SiN=C=N-Si structural fragment that both phases exhibit [6]. The relevance of the carbodiimide (N=C=N) moiety in high-pressure synthesis has been indicated explicitly in a study on hypothetical C3N4 [7]. It was shown that upon compression the N=C=N fragments in the structure undergo further reactions leading to higher coordination of atoms and densification. For the case of Si2CN4 this trend has subsequently been indicated as well. Overall, however, a systematic computational investigation of the high-pressure phase behavior of Si2CN4 and SiC2N4 has not been performed so far. In this contribution it is our aim to show that both structures Si(NCN)2 and Si2N2(NCN) exhibit a high-pressure phase transformation. The transformation proceeds via internal reaction of the NCN fragment and results in a higher average coordination. In case of Si(NCN)2 the
outcome is a new polymorph of SiC2N4 with all Si atoms octahedrally coordinated by NCN units. THEORY For the computation of structure and energ
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