EARLY STAGES OF THE PYROLYTIC CRYSTALLIZATION IN AMORPHOUS NANOPOWDERS OF SILICON CARBONITRIDES SIXCYNZ BY COMBINED WIDE-ANGLE X-RAY AND NEUTRON DIFFRACTOMETRIES

The transition from the amorphous to the crystalline state has been fo llowed by coupled X-ray and neutron wide-angle diffraction in silicon carbonitride powders prepared by laser pyrolysis of hexamethyldisilaza ne. The short-range order (SRO) and the medium-range order (MRO) modif ications under heating have been observed using the drastic contrast e ffect of the scattering factors (or lengths) of N and Si for X-ray and neutrons respectively. In the amorphous state, the fit by models of t he first neighbour peak (SRO) of the radial distribution function reve als the existence of C-N bonds in addition to the Si-C and Si-N bonds existing in mixed tetrahedra Si[C1-xNx](4) with 0<x<1. Extended anneal ing for 24 h at 1400 degrees C modifies the MRO through a solid-state diffusion process increasing the chemical composition fluctuations in SixCyNz puckered layers, by changing their stacking mode from the alph a to the beta sequences encountered in the Si3N4 polytypes. The destru ction of the mixed tetrahedra yields nanocrystals of SiC and Si3N4 at 1550 degrees C. The amorphous-crystalline transition is simulated by m olecular dynamic calculations. The starting models of 200-400 atoms ar e the crystalline alpha and beta-Si3N4 polytypes in which C atoms are substituted for certain Si and N atoms. The comparison with experiment s confirm the anisotropic layer-like character of the amorphous state and the alpha-->beta transition at 1400 degrees C.