First-Principles Calculations and Analysis of29Si Nuclear Magnetic Resonance Chemical Shifts in Silicon Oxycarbide Ceramics

Abstract

We calculate 29Si NMR chemical shifts for silicon atoms in different chemical environments observed in amorphous silicon oxycarbide ceramics using a library of optimized structural models and the gauge-including projector augmented wave (GIPAW) method. For each type of mixed tetrahedral environment, {Si}O4, {Si}O3C, and {Si}O2C2, we develop linear angular correlation functions that relate the 29Si NMR shift of the central silicon atom to the Si–O–Si angles surrounding it. The bonding nature of carbon atoms, whether 4-fold (sp3) or 3-fold (sp2) connected, impacts the chemical shift of {Si}O3C units and can be used to distinguish between Si–C bonding in the “glassy” SiCO host structure and at the interface to so-called “free” carbon. With the derived correlation functions, we analyze two representative experimental 29Si NMR spectra and extract their angle distributions. Our results are in agreement with X-ray and neutron diffraction data. Probing for an interface between “free” carbon and “glassy” SiCO host in a C-rich SiCO material, we find no evidence of significant bonding between Si and three-connected (sp2) C. Rather, the material exhibits a large fraction of wide Si–O–Si angles, which are typical of cage-like and zeolitic structures.