Presentation Information
[R3-07]Water incorporation mechanisms in SiO2 stishovite: Insights from first-principles calculation
*Xianyu XUE1 (1. Okayama University)
Keywords:
stishovite,water,first-principles calculation,NMR,vibrational spectroscopy
Stishovite is a potentially important nominally anhydrous mineral that may carry water in the subduction zone to the deep mantle. Despite SiO2 stishovite has been the subject of a number of studies, the interpretations of its water incorporation mechanisms (hydrogarnet substitution-like or molecular H2O) have been divided.
The ultimate aim of this study is to combine both first-principles calculations and comprehensive NMR and vibrational spectroscopic measurements to unambiguously clarify the issue. As a first step, this presentation reports the first-principles calculation results.
First-principles density functional theory (DFT) calculations of the enthalpy, NMR, and vibrational (IR/Raman) spectra of SiO2 stishovite containing 4H in Si vacancies ((4H)Si defects) were performed using the Quantum-ESPRESSO package (v. 7.0), similar to our recent studies on MgSiO3 enstatite (e.g., Xue et al., 2024).
The two most favorable configurations were found have one H on each of the four basal oxygens forming strong hydrogen bonding with the two axial oxygens, with a point group of D2 and C2h, respectively, similar to those reported by Palfey et al (2023). Both show larger a, and slightly smaller c values than anhydrous stishovite, consistent with those observed (e.g., Spektor et al., 2016).
The calculated 1H chemical shifts are 10.7 and 11.0 ppm, in broad agreement with those experimentally observed (10.5–11.5 ppm). Kueter et al (2023) attributed the observed 1H NMR peaks to molecular H2O, as they claimed the observed spinning sideband pattern cannot be explained by the (4H)Si defects. However, this is most likely invalid because their assumed H-H distances for the (4H)Si defects were too long. Our simulated 1H-29Si CPMAS NMR spectra for the (4H)Si defects are also overall in agreement with those observed (Spektor et al, 2011).
Four distinct OH stretching frequencies are predicted for each model, which are broadly consistent with the experimentally observed multiple bands, like those reported by Palfey et al (2023). However, caution should be taken against one-to-one band assignment, because of possible Fermi resonance between the OH stretching and overtones of SiOH bending vibrations, which fall into a similar frequency range. Our calculation predicts decreasing OH stretching frequencies with increasing pressure, whereas the Raman measurement by Nisr et al (2017) reported the opposite trend. The discrepancy could be related to Fermi resonance.
In summary, the experimentally observed trend in cell constant variations, and NMR and vibrational spectroscopic features are consistent with those predicted for the (4H)Si defects in stishovite.
The ultimate aim of this study is to combine both first-principles calculations and comprehensive NMR and vibrational spectroscopic measurements to unambiguously clarify the issue. As a first step, this presentation reports the first-principles calculation results.
First-principles density functional theory (DFT) calculations of the enthalpy, NMR, and vibrational (IR/Raman) spectra of SiO2 stishovite containing 4H in Si vacancies ((4H)Si defects) were performed using the Quantum-ESPRESSO package (v. 7.0), similar to our recent studies on MgSiO3 enstatite (e.g., Xue et al., 2024).
The two most favorable configurations were found have one H on each of the four basal oxygens forming strong hydrogen bonding with the two axial oxygens, with a point group of D2 and C2h, respectively, similar to those reported by Palfey et al (2023). Both show larger a, and slightly smaller c values than anhydrous stishovite, consistent with those observed (e.g., Spektor et al., 2016).
The calculated 1H chemical shifts are 10.7 and 11.0 ppm, in broad agreement with those experimentally observed (10.5–11.5 ppm). Kueter et al (2023) attributed the observed 1H NMR peaks to molecular H2O, as they claimed the observed spinning sideband pattern cannot be explained by the (4H)Si defects. However, this is most likely invalid because their assumed H-H distances for the (4H)Si defects were too long. Our simulated 1H-29Si CPMAS NMR spectra for the (4H)Si defects are also overall in agreement with those observed (Spektor et al, 2011).
Four distinct OH stretching frequencies are predicted for each model, which are broadly consistent with the experimentally observed multiple bands, like those reported by Palfey et al (2023). However, caution should be taken against one-to-one band assignment, because of possible Fermi resonance between the OH stretching and overtones of SiOH bending vibrations, which fall into a similar frequency range. Our calculation predicts decreasing OH stretching frequencies with increasing pressure, whereas the Raman measurement by Nisr et al (2017) reported the opposite trend. The discrepancy could be related to Fermi resonance.
In summary, the experimentally observed trend in cell constant variations, and NMR and vibrational spectroscopic features are consistent with those predicted for the (4H)Si defects in stishovite.