First-Principles Investigation of the Thermal Vibration Properties of α-U

We present a study on the equilibrium atomic volume and thermodynamic properties of α-U using first-principles calculations, along with harmonic and quasi-harmonic approximations. Our spin-polarized calculation of the equilibrium atomic volume of α-U shows good agreement with experimental results. However, there are discrepancies in the isochoric heat capacity of α-U obtained through the harmonic approximation, which agrees with experimental measurements below 150 K but deviates significantly above this temperature. On the other hand, the quasi-harmonic approximation yields result consistent with experimental measurements below 150 K, but shows substantial discrepancies above this temperature. We have also taken into account the calculated thermal expansion coefficient of α-U and have identified the absence of higher-order force constants as a significant factor contributing to the discrepancies between the calculated and experimental values of both isochoric and isobaric heat capacities above 150 K. This should be taken into consideration in future calculations.