SHORT-RANGE ATOMIC-STRUCTURE DESCRIPTION OF NANOMETRIC SI C/N POWDERSBY X-RAY-ABSORPTION SPECTROSCOPY/

We have investigated Si/C/N preceramics powders obtained from laser ae rosol interaction by x-ray-absorption spectroscopy at the Si K edge as a function of the C/N ratio. By combining extented x-ray-absorption f ine-structure spectroscopy (EXAFS), and a study of the x-ray-absorptio n near-edge structure we deduce that there is no chemical ordering aro und Si atoms in the as-prepared samples, and that the ratio of the par tial coordination numbers N-Si-C/N-Si-N follows the C/N atomic ratio. The Si-C and Si-N bond lengths are independent on the powder compositi on, and similar to the one found in references beta-SiC, and alpha and beta-Si3N4. The intermediate order was characterized in a first step by fitting the second peak of the Fourier transform of the EXAFS data. Finally we propose a structural model tested and compared to experime ntal data through multiple-scattering calculations using the FEFF code .