Pathway for a martensitic quartz-coesite transition

Area of Science:

• Geophysics
• Materials Science
• Crystallography

Background:

• Quartz and coesite are polymorphs of silicon dioxide with different crystal structures.
• The transition between quartz and coesite typically occurs at high pressures.
• Understanding phase transitions is crucial for geology and materials science.

Purpose of the Study:

• To determine the atomistic mechanism of a strain-induced martensitic transition between quartz and coesite.
• To identify the minimal atomic displacements and transformation cell for this transition.
• To investigate the role of stress and pressure in coesite formation.

Main Methods:

• Atomistic pathway computation using density functional theory.
• Calculation of the reaction barrier for the quartz-coesite transition.
• Analysis of invariant shear planes during the martensitic transformation.

Main Results:

• A minimal transformation cell of 24 formula units was identified for the diffusionless martensitic transition.
• Two families of invariant shear planes, near {101} and {122}, were found, consistent with impact data.
• The reaction barrier (150 meV/atom) is pressure-invariant up to 5 GPa.
• Coesite formation in strained quartz is confirmed at pressures significantly below the hydrostatic equilibrium pressure.

Conclusions:

• Strain can induce the quartz-coesite martensitic transition at subsolidus conditions.
• The identified pathway and shear planes provide insights into defect structures in shocked quartz.
• The findings suggest that coesite can form under non-hydrostatic stress conditions at lower pressures than previously thought.