Thermo physical properties and pressure induced phase transition in thorium and uranium sesquicarbide (Th2C3, U2C3)

Carbides of actinide metals thorium and uranium are the potential candidates as a nuclear fuel for generation IV advanced reactors. With the aim to search for new probable structures under high pressures and to determine the thermo mechanical properties of two candidates Th2C3 and U2C3 of actinide–carbon system, the ab initio calculations in conjunction with an evolutionary structure search algorithm have been performed up to the pressure range of 200 GPa. Apart from reproducing the already established bcc structure at zero pressure, the calculations could predict the existence of some new high pressure phases such as orthorhombic structures (SG No. 69, Fmmm) and (SG No. 65, Cmmm) at 62 GPa and 120 GPa, respectively, for Th2C3 and triclinic phase (SG No. 2, P 1 ¯) at 57 GPa for U2C3. In U2C3, the effect of including the Hubbard potential (GGA + U = 3 eV) and spin–orbit coupling on structural, electronic, vibrational properties, and pressure induced phase transitions has also been studied. Furthermore, to substantiate the stability of the high pressure structures so predicted by comparing the enthalpies, the elastic and lattice dynamic stability analysis has also been carried out. Additionally, the high temperature thermo-physical properties of Th2C3 and U2C3 in the cubic phase derived from the present lattice dynamic calculations within quasiharmonic approximations have been compared with the experimental data reported in the literature.