Structural evolution of magnesium difluoride: from an amorphous deposit to a new polymorph

Area of Science:

• Materials Science
• Crystallography
• Solid-State Chemistry

Background:

• Magnesium difluoride (MgF2) exists in different structural forms.
• Understanding its phase transitions is crucial for materials applications.

Purpose of the Study:

• To investigate the structural evolution of magnesium difluoride from amorphous deposits.
• To determine the phase transition temperatures and structures of MgF2 polymorphs.

Main Methods:

• In situ powder X-ray diffraction (XRPD) was used to analyze structural changes.
• Transmission electron microscopy (TEM) provided high-resolution structural imaging.
• Magnesium difluoride was deposited at varying substrate temperatures and vapor pressures.

Main Results:

• Amorphous MgF2 was formed below -100 °C substrate temperature.
• Upon warming, amorphous MgF2 transformed to a CaCl2-type structure (β-MgF2) at 70 °C.
• Further heating to 250 °C resulted in the stable rutile-type structure (α-MgF2).
• MgF2 deposited above -50 °C crystallized directly into the rutile structure.

Conclusions:

• The structural evolution of MgF2 is temperature-dependent, proceeding from amorphous to β-MgF2 and then to α-MgF2.
• A displacive order-disorder phase transition mechanism governs the transformation to the rutile structure.
• The study refined the orthorhombic CaCl2-type structure of β-MgF2.