Molecular dynamics studies of the structure of pure molten ThF4 and ThF4–LiF–BeF2 melts

Abstract

The local structure around Th4+ ions in molten ThF4, and its mixtures with alkali and alkaline earth fluorides (LiF and LiF–BeF2) was investigated by molecular dynamics (MD) simulations with a polarizable force field. We found that the network formed determined to be almost exclusively composed of corner-sharing, F− linked [ThF8] in pure ThF4. The network became sparser when LiF was added to ThF4. In the ThF4–LiF–BeF2 system, both Be2+ and Th4+ ions formed a discrete coordination complex. The Be2+ ion is surrounded by four equidistant F anions and there are approximately 8 fluoride anions around Th4+. The Be–F coordination complex was more stable than that of Th–F. By adding Be2+ into the ThF4–LiF mixture, the thorium compound network can be made more compact. We also observed a higher coordination number and a smaller barrier in the ThF4–LiF–BeF2 melt. In addition, we calculated a smaller characteristic time from the cage-out correlation function than that calculated for the ZrF4–LiF–BeF2 system. We show that, in comparison with the ZrF4–LiF–BeF2 melt, the molten salt of ThF4–LiF–BeF2 has a loosely bound shell coordination structure.