Solution enthalpy calculation for impurity in liquid metal by first-principles calculations: A benchmark test for oxygen impurity in liquid sodium

The solution enthalpy of oxygen in liquid Na was calculated as a test case for the computational method to evaluate the solution enthalpy in liquid metal using first-principles calculations. To obtain the necessary thermodynamic quantities at high temperatures, (i) first-principles molecular dynamics for pure and O-including liquid Na systems, (ii) vibration analysis for an O2 molecule, and (iii) phonon-based quasi-harmonic approximation for solid Na and Na2O were conducted. The calculation results were compared with available experimental data to validate the method. Consequently, the O2 solution enthalpy was calculated to be −387.1 kJ/mol at 600 K and −374.0 kJ/mol at 1000 K, comparable to the experimental data of −375.7 kJ/mol at 600 K and −369.3 kJ/mol at 1000 K. The Na2O solution enthalpy was calculated to be 28.6 kJ/mol at 600 K and 38.2 kJ/mol at 1000 K, while the experimental data gave a temperature-independent value of 46.9 kJ/mol. The possible causes of errors in the calculations were discussed. This work shows that computational calculations can contribute to establishing a fundamental database on the solubility of impurities in liquid metals.