Cambridge 7th to 9th September
John V Hanna*
article posted 17 June 2015
John V Hanna
The Combination of Experimental Solid State NMR and Computational First Principles MD and DFT: A New Approach for the Structure Determination of Oxide Glasses
John V. Hanna*
Department of Physics, University of Warwick, Gibbet Hill Rd., Coventry CV4 7AL, UK
CEA, IRAMIS, NIMBE, CEA/CNRS UMR 3685, 91191 Gif-sur-Yvette, France
The short-range information forthcoming from solid state NMR technique is acutely sensitive to the immediate chemical/atomic scale environment of the nucleus under study, and thus provides a unique tool for the study of disordered systems such as oxide glasses. Current state-of-the art solid state NMR offers a wide variety of experimental methods to characterise materials structure from atomic to nanometer length scales; this subsequently enables the separation of geometric and chemical contributions to disorder. However, the associated inhomogeneous line broadening of spectra from glass systems leads to partial or complete overlap of the NMR resonances of the different constituent sites which can significantly complicate the processing and interpretation of the NMR data. Additionally (and more importantly), much of the information inherent within this line broadening remains unexploited.
The NMR parameters characterising periodic solids systems can now be accurately and efficiently determined due to the introduction of the first principles GIPAW DFT method.[2-4] These first principles calculations aid both the interpretation and assignment of solid state NMR resonances. Combination of GIPAW calculations with molecular dynamics (MD) simulations has therefore recently emerged as a valuable tool to significantly improve the exploitation of solid state NMR data from disordered systems, and in particular oxide glasses. This computational approach (named MD-GIPAW) now offers new opportunities to address several fundamental issues confronting the interpretation of solid state NMR data from glassy materials; these include (a) the direct comparison of MD simulations with NMR experimentation, to complement standard techniques such as neutron or X-ray scattering, (b) deciphering the dependence of NMR interaction parameters on the detailed local geometry (such as bond distances or angles), and (c) new insights into NMR interaction parameter distributions in glass. This presentation will demonstrate that the last point is essential for the accurate processing and simulation of contemporary solid state NMR data. A brief discussion of some aspects of the practical implementation of the MD-GIPAW methodology will be illustrated through several applications to oxide (and in particular silicate) glasses. In the context of nuclear waste glasses, applications to key issues will be also presented such as the elucidation of the incorporation mechanisms of low solubility elements such as Mo and La.
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