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.
Type
Thesis, PhD Doctor of Philosophy
Collections
Description of related resources
Investigating the use of first-principles calculations for NMR studies of disordered in the solid state (thesis data) Moran, R.F., University of St Andrews, 2019. DOI: https://doi.org/10.17630/24ae94ef-3de0-4d96-8ec9-045f60b9606bRelated resources
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