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

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.