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 nucl ear magnetic resonance (NMR), continuous-wave and pulse electron paramagnet ic resonance (EPR), and by one- and two-dimensional electron nuclear double resonance spectroscopy. In both compounds, boron is found to be coordinate d exclusively by nitrogen with close to trigonal planar geometry and close to equal bond lengths. Silicon is four-coordinated by nitrogen with the coo rdination tetrahedra being distorted to accommodate the coordination prefer ences of boron. REDOR measurements demonstrate that boron resides in the se cond coordination sphere of silicon. Carbon incorporation into the Si-BN ne twork does not lead to any observable changes in NMR parameters including t he average dipolar coupling between B-11 nuclei which depends on the averag e distance of the boron atoms. Only spin-lattice relaxation of the nuclei i s accelerated due to the generation of paramagnetic centers. The unpaired e lectrons appear to be delocalized over several carbon atoms and exhibit sig nificant hyperfine- couplings to boron, silicon, nitrogen, and some residua l protons. In contrast to electron spectroscopic imaging experiments: the m agnetic resonance results suggest formation of carbon clusters. (C) 1999 El sevier Science B.V. All rights reserved.