Transformations to the aluminum coordination environment and network polymerization in amorphous aluminosilicates...

Area of Science:

• Materials Science
• Geochemistry
• Solid-State Chemistry

Background:

• Calcium aluminosilicate glasses are crucial in materials science and geology.
• Understanding their structural response to pressure is key to controlling material properties and geological processes.
• Previous studies have indicated pressure-induced structural changes, but detailed mechanisms remain unclear.

Purpose of the Study:

• To investigate the pressure-induced structural transformations in calcium aluminosilicate glasses.
• To elucidate the changes in aluminum coordination and their relationship with densification.
• To develop a self-consistent model for pressure-induced structural evolution in these glasses.

Main Methods:

• In situ high-pressure neutron diffraction.
• 27Al nuclear magnetic resonance (NMR) spectroscopy.
• Analysis of oxygen packing fraction and reduced density (ρ').

Main Results:

• Three distinct pressure regimes identified for aluminum coordination transformation from tetrahedral to octahedral.
• Aluminum speciation shows a common dependence on reduced density (ρ') in densified aluminosilicates.
• Formation of six-coordinated aluminum (Al(VI)) species increases rapidly beyond a reduced density threshold (ρ'thr ≈ 1.1).
• Network connectivity changes, with consumption of non-bridging oxygens and formation of bridging oxygens or triclusters, depending on network depolymerization.

Conclusions:

• A self-consistent model for pressure-induced structural change in calcium aluminosilicate glasses is proposed.
• The coordination of aluminum and the oxygen packing fraction are critical indicators of structural changes and densification.
• The role of six-coordinated aluminum (Al(VI)) as a network modifier or former is significant in pressure-induced structural evolution.