Early stages of the pyrolytic crystallization in amorphous nanopowders of silicon carbonitrides SixCyNz by combined wide-angle X-ray and neutron diffractometries

Abstract

Abstract The transition from the amorphous to the crystalline state has been followed by coupled X-ray and neutron wide-angle diffraction in silicon carbonitrides powders prepared by laser pyrolysis of hexamethyldisilazane. The short-range order (SRO) and the medium-range order (MRO) modifications under heating have been observed using the drastic contrast effect 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 the first neighbours peak (SRO) of the radial distribution function reveals the existence of C-N bonds in addition to the Si-C and Si-N bonds existing in mixed tetrahedra Si[C1−x N x ]4 with 0>x>1. Extended annealing for 24h at 1400°C modifies the MRO through a solid-state diffusion process increasing the chemical composition fluctuations in Si x C y N z puckered layers, by changing their stacking mode from the α to the β sequences encountered in the Si3N4 polytypes. The destruction of the mixed tetrahedra yields nanocrystals of SiC and Si3N4 at 1550°C. The amorphous-crystalline transition is simulated by molecular dynamic calculations. The starting models of 200–400 atoms are the crystalline α and β-Si3N4 poly types in which C atoms are substituted for certain Si and N atoms. The comparison with experiments confirm the anisotropic layer-like character of the amorphous state and the α → β transition at 1400°C.