Understanding precursor-derived amorphous Si-C-N ceramics on the atomic scale

The atomic structure of a precursor-derived amorphous ceramic with the composition Si₃₇C₃₂N₃₁ and a density of 2.4 g/cm³ was modeled using a density-functional-based molecular dynamics simulation. Three different model structures were generated, and their total structure factors and total pair correlation functions from x-ray and neutron diffraction were calculated. For two of them, these data were found to agree very well with the experimental results. The different atomic structures of these two models could only be distinguished by the calculated partial pair correlation functions and their infrared spectra. It could be shown that in the final ceramic a phase separation into amorphous Si₃N₄, amorphous SiC, and graphitelike amorphous carbon has to appear.