Crystal structure prediction with theoretical methods is particularly challenging when unit cells with many atoms need to be considered. Here we employ a symmetry-driven structure search (SYDSS) method and combine it with density functional theory (DFT) to predict novel crystal structures at high pressure. We sample randomly from all 1,506 Wyckoff positions of the 230 space groups to generate a set of initial structures. During the subsequent structural relaxation with DFT, existing symmetries are preserved, but the symmetries and the space group may change as atoms move to more symmetric positions. By construction, our algorithm generates symmetric structures with high probability without excluding any configurations. This improves the search efficiency, especially for large cells with 20 atoms or more. We apply our SYDSS algorithm to identify stoichiometric (H2O)n-(NaCl)m and CnOm compounds at high pressure. We predict a novel H2O-NaCl structure with Pnma symmetry to form at 3.4 Mbar, which is within the range of diamond anvil experiments. In addition, we predict a novel C2O structure at 19.8 Mbar and C4O structure at 44.0 Mbar with Pbca and C2/m symmetry respectively.PACS numbers:
The diffusion properties of noble
gases in minerals are widely
used to reconstruct the thermal histories of rocks. Here, we combine
density functional theory (DFT) calculations with laboratory experiments
to investigate controls on helium diffusion in quartz. DFT calculations
for perfect α-quartz predict substantially lower activation
energies and frequency factors for helium diffusion than observed
in laboratory experiments, especially in the [001] direction. These
results imply that no helium could be retained in quartz at Earth
surface temperatures, which conflicts with observations of partial
cosmogenic 3He retention over geologic time scales. Here,
we implement a model of helium diffusion in α-quartz modulated
by nanopore defects that disrupt energetically favorable diffusion
pathways. In this model, we find that laboratory-determined diffusivities
can be most closely reproduced when a helium atom encounters ∼70
nanopore sites per million interstitial sites. The results of our
model indicate that diffusion of helium in natural quartz, like other
noble gases in other minerals, can be significantly modulated by extended
defects.
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