Compressed MgCl₂ Reveals Multiple Pathways to Cotunnite Structures

Area of Science:

• Materials Science
• High-Pressure Chemistry
• Mineral Physics

Background:

• MeX2 compounds are crucial for understanding materials under extreme conditions.
• High pressure induces phase transitions in these materials, altering their structure and properties.

Purpose of the Study:

• To investigate the high-pressure phase transitions of magnesium dichloride (MgCl2).
• To identify new structural polymorphs and coordination geometries in MgCl2 under compression.
• To elucidate the transformation pathways of MeX2 compounds towards cotunnite-type structures.

Main Methods:

• Synthesis of anhydrous MgCl2 using direct reaction in laser-heated diamond anvil cells.
• High-pressure experiments conducted from 7 to 83 GPa.
• Single-crystal X-ray diffraction for structural identification.
• Ab initio calculations for stability, equations of state, and electronic properties.

Main Results:

• Observed the first trigonal prismatic coordination in MeX2 compounds, specifically in MgCl2.
• Identified two new high-pressure phases of MgCl2: orthorhombic oP72 and cotunnite-type oP12.
• Documented a pressure-induced structural transition from layered (hP3) to 3D frameworks (oP72 and oP12).
• Experimental data aligned well with theoretical ab initio calculations.

Conclusions:

• The study reveals complex structural evolution in MgCl2 under high pressure.
• New polymorphs and coordination behaviors were discovered, expanding knowledge of MeX2 phase transitions.
• The findings provide insights into transformation pathways relevant to materials science and high-pressure chemistry.