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

The structural evolution of magnesium difluoride from an amorphous deposit has been investigated by in situ powder X-ray diffraction (XRPD) and transmission electron microscopy (TEM). Crystalline MgF(2) was evaporated at different temperatures, which define the vapor pressures in the range from 4.7 × 10(-3) to 1.6 × 10(-1) mbar, and deposited onto various substrates. The temperature of the substrate was systematically varied from -228 to 25 °C. Magnesium difluoride was obtained as an amorphous sample when deposited on a substrate kept at a temperature below -100 °C. Upon warming, the deposit transforms via the CaCl(2) type of structure (β-MgF(2), at 70 °C) into the stable rutile type (α-MgF(2), 250 °C) by a displacive order-disorder phase transition. The new β-MgF(2) polymorph was refined assuming the orthorhombic CaCl(2) type of structure (Pnnm, No. 58) with the lattice constants a = 4.592(1) Å, b = 4.938(3) Å, and c = 2.959(3) Å. When deposited above -50 °C, samples crystallize directly in the rutile structure.