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MATERIALS RESEARCH

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Molecular orbital calculations of chemical bonding states of solute elements in amorphous silicon nitride ceramics Katsuyuki Matsunaga, Yuji Iwamoto, and Hideaki Matsubara Fine Ceramics Research Association, Synergy Ceramics Laboratory, 2-4-1 Mutsuno Atsuta-ku, Nagoya, 456-8587 Japan (Received 28 January 1999; accepted 23 March 1999)

We performed ab initio Hartree–Fock molecular orbital calculations of solute elements in amorphous silicon nitride (Si–N) ceramics. To investigate effects of solute elements, X, such as boron, carbon, aluminum, silicon, and phosphorus, on stabilization of the Si–N network, we used model clusters representing local atomic structures in the Si–N network, and the solute elements were substituted for nitrogen. Bonding characteristics around the solute elements were analyzed, and bond energies of Si–X were also calculated using model clusters. It was found that, among these solute elements in amorphous Si–N, the Si–C bond is able to make the Si–N network more stable due to its high covalency.

I. INTRODUCTION

Amorphous silicon nitride (Si–N) ceramics can be synthesized by pyrolysis of polymer precursors such as polysilazane. The amorphous Si–N ceramics have excellent high-temperature stability, and their amorphous state can be maintained up to temperatures above 1000 °C.1–3 In addition, doping of additional elements into polysilazane makes it possible to prepare amorphous Si–C–N and Si–B–C–N ceramics in order to improve the hightemperature stability. Riedel et al. showed that Si–C–N and Si–B–C–N ceramics preserve their amorphous states up to 1450 and 1700 °C, respectively.3 Funayama et al. also prepared the Si–B–O–N ceramic fibers that retain the amorphous state up to 1500 °C. 4 The hightemperature stability of amorphous Si–N ceramics strongly depends on the incorporation of the additional C and B atoms. Finally, the amorphous ceramics crystallize into thermodynamically stable phases of Si3N4, graphite, SiC, or BN. The distinctive stability of amorphous Si–N ceramics may be closely related to their atomic structures in the amorphous state. A number of experimental structural investigations have been made in order to understand the atomic structure of amorphous Si–N and the structural changes during pyrolysis.5–9 Seher et al. reported that the atomic structure of amorphous SiC0.63N0.89 derived from poly(hydridochlorosilazane) is composed of elementary SiCxNy tetrahedra, where x + y ⳱ 4.5 Nuclear magnetic resonance (NMR) measurements also revealed that the local environments of Si atoms can be represented by SiCxNy tetrahedra, so that a local compositional mixture of C and N is present around each Si atom in amorphous Si–C–N.6 Monthioux et al. suggested that such a comJ. Mater. Res., Vol. 15, No. 2, Feb 2000

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positional mixture around Si in Si–C–N is disadvantageous to the transformation of the amorphous state into thermodynamically stable crystalline phases such as Si3N4 and SiC.7 It is lik