Structure of ZnCl2 Melt. Part I: Raman Spectroscopy Analysis Driven by Ab Initio Methods

The structure of molten ZnCl2 is investigated using a combination of computer simulation and experimental methods. Ab initio molecular dynamics (AIMD) is used to model the structure of ZnCl2 at 600 K. The structure factors and pair distribution functions derived from AIMD show a good match with those previously measured by neutron diffraction (ND). In addition, Raman spectroscopy is used to investigate the structure of liquid ZnCl2 and identify the relative fractions of constituent structural units. To ascertain the assignment of each Raman mode, a series of ZnCl2 crystalline prototypes are modeled and the corresponding Raman modes are derived by first-principles calculations. Curve fitting of experimental Raman spectra using these mode assignments shows excellent agreement with both AIMD and ND. These results confirm the presence of significant fractions of edge-sharing tetrahedra in liquid ZnCl2. The presence of these structural motifs has significant impact on the fragility of this tetrahedral glass-forming liquid. The assignment of Raman bands present in molten ZnCl2 is revised and discussed in view of these results.