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

Abstract The local structure around Th 4 + ions in molten ThF 4 , and its mixtures with alkali and alkaline earth fluorides (LiF and LiF–BeF 2 ) 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 [ThF 8 ] in pure ThF 4 . The network became sparser when LiF was added to ThF 4 . In the ThF 4 –LiF–BeF 2 system, both Be 2 + and Th 4 + ions formed a discrete coordination complex. The Be 2 + ion is surrounded by four equidistant F anions and there are approximately 8 fluoride anions around Th 4 + . The Be–F coordination complex was more stable than that of Th–F. By adding Be 2 + into the ThF 4 –LiF mixture, the thorium compound network can be made more compact. We also observed a higher coordination number and a smaller barrier in the ThF 4 –LiF–BeF 2 melt. In addition, we calculated a smaller characteristic time from the cage-out correlation function than that calculated for the ZrF 4 –LiF–BeF 2 system. We show that, in comparison with the ZrF 4 –LiF–BeF 2 melt, the molten salt of ThF 4 –LiF–BeF 2 has a loosely bound shell coordination structure.