Theoretical investigation of metastable Al2SiO5 polymorphs

Area of Science:

• Materials Science
• Geophysics
• Computational Chemistry

Background:

• Aluminum oxysilicates (Al2SiO5) exhibit complex polymorphs like andalusite and sillimanite.
• Understanding their phase transitions under pressure is crucial for materials science and geophysics.

Purpose of the Study:

• To predict and analyze metastable phase transitions in low-pressure Al2SiO5 polymorphs (andalusite and sillimanite).
• To investigate the structural changes and the origin of five-coordinate Si and Al atoms in high-pressure phases.

Main Methods:

• Theoretical simulations using density functional theory (DFT) with the generalized gradient approximation (GGA).
• Semiclassical interatomic potentials within the ionic shell model were also employed for validation.
• Analysis of soft lattice modes and structural transformations.

Main Results:

• Predicted an isosymmetric phase transition in sillimanite at approximately 35 GPa, leading to a phase with five-coordinate Si and Al atoms.
• Identified an incommensurately modulated form of sillimanite at low temperatures and high pressures.
• Andalusite was predicted to transform directly into an amorphous phase around 50 GPa.
• A critical Si-O bond distance of 2.25 Å was identified, beyond which interactions are not considered bonding.

Conclusions:

• The study provides insights into the formation and nature of metastable crystal structures.
• The findings contribute to understanding the behavior of Al2SiO5 polymorphs under extreme conditions.
• The concept of a critical bonding distance offers a new perspective on interatomic interactions in materials.