Lattice Dynamics And Lattice Thermal Conductivity Of Thorium Dicarbide

In the last decade, there has been renewed interest in thorium carbides since their potential application in Gneration-IV reactors. There are many advantages to the tho-rium carbides, such as the high melting point, the corrosion resistivity, the low ther-mal expansion coefficients and the large thermal conductivity. Meanwhile, thorium is thought to be three to four times more naturally abundant than uranium, and a thorium fuel cycle can be developed that produces negligible amounts of plutonium and fewer long-lived minor actinides than a uranium cycle.Due to these advantages, Thorium was choose as an important nuclear material of next generation nuclear energy system. Thorium has two main types of carbides:one is thorium monocarbide(ThC) and another is thorium dicarbide(ThC2). ThC has a rock-salt structure, while ThC2 exists as three different phases:at the low temperature(up to 1700 K) it shows a monoclinic structure, at the temperature from 1700 K to 1754 K it shows a tetragonal structure, and at the high temperature(T> 1754 K) it has a cubic structure.Currently, several first-principles studies have been performed to understand the structural, electronic, elastic, and thermodynamic properties of ThC, while the proper-ties of ThC2 have rarely been studied. To the best of our knowledge, only the structural and electronic properties of ThC2 have been studied by Shein et al..In this work, the elastic and thermodynamic properties of ThC2 (monoclinic phase) have been studied by means of density functional theory and direct force constant method. The calculated properties including the thermal expansion, the heat capaci-ty and the elastic constants are in a good agreement with experiment. Our results show that the vibrational property of the C2 dimer in ThC2 is similar to that of a free standing C2 dimer. This indicates that the C2 dimer in ThC2 is not strongly bonded to Th atom- s. The lattice thermal conductivity for ThC2 was calculated by means of the Debye-Callaway model. As a comparison, the conductivity of ThC was also calculated. Our results show that the ThC and ThC2 contributions of the lattice thermal conductivity to the total conductivity are 29% and 17%, respectively.