Dynamics of an Excess Electron in Molten LiF, KF, MgF₂, and BeF₂

Ab initio molecular dynamics simulations suggest that the dynamics of an excess electron in different types of molten salts are not always the same. In molten LiF, KF, and MgF₂, the excess electron localizes in the cavity as a solvated electron for 10 ps, which agrees with the widely accepted theory of Pikaev. In molten BeF₂, the excess electron shows a different localization pattern: it mostly exists in localized states but also occurs in many delocalized states. This "localize-delocalize" pattern originates from the high viscosity of BeF₂ (16 000 cP at 900 °C), which will lead to slow ionic motion and finally result in slow solvent relaxation. Besides, the species formed by the localization of the excess electron in these four melts are also different. The spectral feature (broad peak in the vis-IR region) of the localized electron in molten alkaline halides was also observed in LiF, KF, MgF₂, and BeF₂. Both an excess electron and electrons in the bulk liquid could contribute to the spectra, but the excitation of the excess electron makes a bigger contribution to the broad vis-IR peak. Our predicted spectrum of molten LiF/KF qualitatively reproduces the major feature of the experimental spectrum, which partially validates our simulations.