title: NMR crystallographic investigations of defect motifs in high-pressure minerals of the Earth's deep interior
Creators: name: Grüninger, Helen
Publisher: name: University of Bayreuth
Year: 2018
Subjects: subject: NMR spectroscopy
subject: DFT simulations
subject: point defects
subject: 500
subject: 540
subject: 550

description: In this work NMR crystallography was used to elucidate the defect chemistry of two high-pressure magnesium silicates, namely ringwoodite (γ-Mg2SiO4) and bridgmanite (MgSiO3), which make up major parts of the Earth’s mantle. In this way, we focus on understanding the various defect formation mechanisms and provide a framework through which to understand the effects on the mineral properties. The required high-pressure synthesis of the minerals results in low sample yields, which in combination with low defect concentrations renders the structural characterization of the defects challenging. 
We have, however, been able to investigate OH defect formation mechanisms within ringwoodite by combining high-resolution multidimensional solid-state 1H NMR spectroscopy and Density Functional Theory (DFT) simulations of multiple model OH defect types. Using these methods, we were able to link experimental and simulated 1H chemical shifts, and 1H 1H connectivities to individual OH defect motifs. The qualitative solution revealed a surprisingly rich OH defect chemistry of ringwoodite’s spinel-type structure, including the formation of isolated low-valent Mg2+ vacancies charge balanced by two protons (V_Mg''+2H**), as well as isolated high-valent Si4+ defects, either charge balanced by four protons (V_Si''''+4H****) or by one Mg2+ and two protons (Mg_Si''+2H**). Additionally, a coupled defect type with an Mg2+ on a Si site coupled to an octahedral vacancy (Mg_Si V_Mg''''+4H****) makes up a significant proportion. Inherent disorder of hydrogen bonds within the defects prevented a straight-forward quantification of different defect motifs. Therefore, a new approach to model local 1H disorder within point defects based on molecular dynamics (MD) simulations employing the DFT method and to describe its influence on two complementary 1H NMR observables was developed. Individual 1H chemical shift footprints and characteristic ensemble average 1H-1H double-quantum (DQ) buildup curves were derived from the modelled OH disorder for each of the four above mentioned OH defect types. These were then used as joint cost function for the refinement of the strongly overlapping experimental 1H MAS NMR spectrum and 1H-1H DQ buildup curves leading to a quantitative determination of the defect motif proportions of ~50% V_Mg, 18% V_Si, ~13% Mg_Si and 20% coupled Mg_SiV_Mg.
In order to evaluate the OH defect formation mechanisms as a function of the hydration level the feasibility of 1H NMR spectroscopy on nanoliter hydrous ringwoodite crystals was investigated. For three crystals with hydration levels between an equivalent of 0.3-2.0 wt% H2O the 1D 1H MAS NMR spectra were successfully recorded using a standard MAS NMR setup and a piggy-back µMAS setup, and were quantitatively refined with the previously determined 1H chemical shift footprints. The resulting defect type ratios were linked to the experimental unit cell volumes of the same crystals using DFT-predicted unit cell volume trends for each defect motif in order to derive a correlation between defect formation mechanism and hydration level. The 1H NMR and XRD data suggest that up to ~1wt% H2O, mainly Si defects (V_Si''''+4H****, Mg_Si''+2H**, Mg_Si V_Mg''''+4H****) form, while at higher hydration levels the formation of Mg vacancies seems favoured (V_Mg''+2H**). 
This qualitative and quantitative solution of ringwoodite’s defect chemistry as a function of hydration level opens up the possibility for modelling the thermodynamics of each hydration mechanism, as well as their interaction. Finally, this could lead to a reasonable model of the impact of ringwoodite’s defect chemistry on the transport properties of the transition zone of the Earth’s mantle. Furthermore, the NMR spectroscopic investigation of single-crystals, instead of powders, opens up the possibility for a comprehensive characterization of each crystal providing access to unique correlations between quantitative atomic pictures of defect motifs, absolute defect concentrations and their influence on the crystal structure on homogenous samples, which is of fundamental relevance for the investigation of the influence of defects on macroscopic properties. Therefore, the improvement of µNMR methods to gain sensitivity could pave the way for the routine examination of small crystals. 
Al defects in bridgmanite formed by two competing Al substitution mechanisms were investigated by electronprobe micro analysis and 27Al MAS NMR spectroscopy on Al-bearing bridgmanite samples with varying Al contents synthesized in MgO-rich or SiO2-rich synthesis conditions. These revealed that in MgO-rich systems the oxygen vacancy (OV) mechanism is dominant at low Al contents, while the charge-coupled (CC) mechanism increases linearly with increasing Al content and generally dominates in SiO2-rich systems. Furthermore, the multidimensional 27Al MAS NMR data in combination with the results of DFT calculations of multiple Al defect models demonstrate the formation of OV clusters probably due to a high mobility of OVs. This surprising OV mechanism leads to a roughly two times larger vacant crystallographic site than formed by an isolated OV. Such sites have been shown to be able to accommodate small noble gas atoms, such as neon, as well as OH groups, and this may therefore greatly change the understanding of the volatile cycling of noble gases and water into the Earth’s lower mantle.