A magnetic resonance study on the structure of amorphous networks in the Si–B–N(–C) system

The amorphous networks Si₃B₃N₇ and ‘SiBN₃C' are studied by solid-state nuclear magnetic resonance (NMR), continuous-wave and pulse electron paramagnetic resonance (EPR), and by one- and two-dimensional electron nuclear double resonance spectroscopy. In both compounds, boron is found to be coordinated exclusively by nitrogen with close to trigonal planar geometry and close to equal bond lengths. Silicon is four-coordinated by nitrogen with the coordination tetrahedra being distorted to accommodate the coordination preferences of boron. REDOR measurements demonstrate that boron resides in the second coordination sphere of silicon. Carbon incorporation into the Si–B–N network does not lead to any observable changes in NMR parameters including the average dipolar coupling between ¹¹B nuclei which depends on the average distance of the boron atoms. Only spin–lattice relaxation of the nuclei is accelerated due to the generation of paramagnetic centers. The unpaired electrons appear to be delocalized over several carbon atoms and exhibit significant hyperfine couplings to boron, silicon, nitrogen, and some residual protons. In contrast to electron spectroscopic imaging experiments, the magnetic resonance results suggest formation of carbon clusters.