Evolution of order in amorphous-to-crystalline phase transformation of MgF₂

Structural disorder and distortion play a significant role in phase transformations. Experimentally, electron diffraction in the transmission electron microscope offers the ability to characterize disorder via the pair distribution function (PDF) at high spatial resolution. In this work, energy-filtered in situ electron diffraction is applied to measure PDFs of different phases of MgF₂ from the amorphous deposit through metastable modifications to the thermodynamically stable phase. Despite the restriction of thick specimens resulting in multiple electron scattering, elaborate data analysis enabled experimental and molecular dynamics simulation data to be matched, thus allowing analysis of the evolution of short-range ordering. In particular, it is possible to explain the theoretically not predicted existence of a metastable phase by the presence of atomic disorder and distortion. The short-range ordering in the amorphous and crystalline phases is elucidated as three steps: (i) an initial amorphous phase exhibiting CaCl₂-type short-range order which acts as a crystallization nucleus to guide the phase transformation to the metastable CaCl₂-type phase and thus suppresses the direct appearance of the rutile-type phase; (ii) a metastable CaCl₂-type phase containing short-range structural features of the stable rutile type; and (iii) the formation of a large volume fraction of disordered inter­granular regions which stabilize the CaCl₂-type phase. The structure evolution is described within the energy landscape concept.