Investigating the use of first-principles calculations for NMR studies of disorder in the solid state

Abstract

In this thesis, the use of first-principles calculations to assist solid-state NMR spectroscopic studies of disordered inorganic materials has been investigated, with emphasis placed on understanding the most relevant and efficient methods for computationally modelling a system. The first class of materials studied are oxide ceramics, more specifically Y₂SnₓTi₂₋ₓO₇, La₂SnₓZr₂₋ₓO₇, Y₂Zr₂O₇ and Y₂Hf₂O₇; the first two of which are pyrochlore solid solutions, while the latter adopt the disordered defect fluorite phase. Both pyrochlore systems exhibit configurational disorder in the form of B-site cation mixing, which leads to the overlapping and complex experimental

solid-state NMR spectra being challenging to assign. By considering several methods to generate structural models computationally, the site occupancy disorder (SOD) method was found to be particularly well suited to producing a set of structural models capable of representing the configurational disorder in these materials with the predicted ⁸⁹Y, ¹¹⁹Sn and ¹⁷O NMR parameters able to assist the assignment of the experimental NMR spectra and provide significant structural insight. Investigation into Y₂Zr₂O₇ and Y₂Hf₂O₇ defect fluorites proved considerably more challenging, with the high level of structural disorder preventing easy implementation of SOD-based approaches, and necessitating a less sophisticated and more manual modelling approach being employed. Although some understanding of the origin of the signals seen in the NMR spectra was able to be obtained in this way, the limited scope of this computational investigation prevents a more detailed and quantitative analysis.

The second class of materials investigated in this thesis are hydrous silicate minerals found in the inner-Earth, specifically, hydrous Fe-free wadsleyite (β-Mg₂SiO₄), a system that is challenging to study experimentally due to the positional disorder of the incorporated protons (and of the charge-balancing associated cation vacancies). In combination with experimental solid-state NMR spectra and the first-principles calculation of NMR parameters, the ab initio random structure searching (AIRSS) approach was used to probe the structure of hydrous wadsleyite, by identifying many possible protonation arrangements for semi- and fully-hydrous wadsleyite. Through this investigation, enthalpically stable protonation arrangements were identified for both semi- and fully-hydrous wadsleyite, with predicted NMR parameters for the AIRSS-generated structures used to assist assignment of the solid-state NMR spectra of a sample of wadsleyite containing ~3 wt% H₂O. By using the experimental NMR spectra to validate the accuracy and relevance of AIRSS-generated structural models, a new structural picture of the disorder in fully-hydrous wadsleyite was proposed, highlighting the success with which first-principles calculations can be used to assist the assignment of the solid-state NMR spectra of disordered inorganic materials.