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Behavior of hydrogarnet-type defects in hydrous stishovite at various temperatures and pressures

Palfey, William R. and Rossman, George R. and Goddard III, William A. (2022) Behavior of hydrogarnet-type defects in hydrous stishovite at various temperatures and pressures. . (Unpublished)

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Dense polymorphs of silica have been demonstrated experimentally to incorporate from 1.5% to as much as 11.6% weight percent H₂O as OH groups, with implications for the hydrogen budgets of Earth and other planets. This OH is thought to enter the SiO₂ structure via a charge-balanced substitution in which silicon vacancies (V_(Si)) are compensated by protonating four of the surrounding six oxygen atoms, often referred to as a hydrogarnet-type defect. There are many possible configurations for this defect structure in dense silica, but the nature of these configurations and whether they can be distinguished experimentally is unknown. We present here density functional theory (DFT) calculations that systematically assess the possible configurations of a hydrogarnet-type defect in stishovite (rutile-type SiO₂), with direct comparisons to experimental vibrational spectroscopy data. We predict that stishovite synthesized at 450K and 10 GPa quenched to room temperature is dominated by a single defect type with tetrahedral geometry. This leads to OH stretching modes (2500-3000 cm⁻¹) and SiOH bending modes (~1400 to 1450 cm⁻¹) largely consistent with experimentally observed modes. One remaining issue is that our calculations produce results compatible with experimental data on H to D exchange, but do not explain why a considerable fraction of the 1420 cm⁻¹ mode shifts by only 40 cm⁻¹ in deuterated samples. At elevated pressures and temperatures, we find that a second square planar defect configuration also becomes favorable, leading to modes that should allow differentiation from the tetrahedral configuration.

Item Type:Report or Paper (Discussion Paper)
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URLURL TypeDescription Paper Information ItemJournal Article
Palfey, William R.0000-0002-9555-7877
Rossman, George R.0000-0002-4571-6884
Goddard III, William A.0000-0003-0097-5716
Additional Information:The authors thank Charles Musgrave for helpful discussions. W.A.G. was supported by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences,Fuels from Sunlight Hub, under Grant DE-SC0021266. G.R.R. is thankful for National Science Foundation Grant EAR-2149559. Open Research: All DFT calculations were carried out using the Vienna Ab Initio SimulationPackage (VASP, version, a licensed software (details at: Two Phase Thermodynamics (2PT) calculations (Lin et al., 2003; Lin et al., 2010; Pascal et al., 2011) were used to produce the phonon DoS from QM MD VASP calculations. Open Visualization Tool (OVITO), an open source visualization program, was used to convert QM MD output files into trajectories usable by 2PT (details at: Jmol: an open-source Java viewer for chemical structures in 3D ( was used to visualize the vibrational modes produced from DFPT calculations. Visualization for Electronic and STructural Analysis (VESTA, version 3.5.5 26Sep2020), an open source visualization software, was used to produce the structural figures used in this work (details at:
Group:Liquid Sunlight Alliance, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0021266
Record Number:CaltechAUTHORS:20220805-540001000
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:116157
Deposited By: George Porter
Deposited On:09 Aug 2022 19:48
Last Modified:01 Feb 2023 22:36

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