First-Principles Calculations and Analysis of 29Si Nuclear Magnetic Resonance Chemical Shifts in Silicon Oxycarbide Ceramics
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- 作者：John P. Nimmo ; II ; Peter Kroll
- 刊名：Journal of Physical Chemistry C
- 出版年：2014
- 出版时间：December 26, 2014
- 年：2014
- 卷：118
- 期：51
- 页码：29952-29961
- 全文大小：490K
- ISSN：1932-7455
文摘
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鈥揙鈥揝i 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鈥揅 bonding in the 鈥済lassy鈥?SiCO host structure and at the interface to so-called 鈥渇ree鈥?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 鈥渇ree鈥?carbon and 鈥済lassy鈥?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鈥揙鈥揝i angles, which are typical of cage-like and zeolitic structures.