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.