Ab Initio Molecular Dynamics Calculation of Microstructure and Diffusion Behavior of Na-K Alloy

The sodium-potassium (Na-K) alloy is characterized by some exceptional physical and chemical properties and combines not only metallicity and fluidity, but also the strongest reduction potential, which makes it useful for many scientific and industrial applications. The Na-K alloy has garnered widespread attention in the development of advanced nuclear energy systems due to its excellent thermal conductivity and favorable passive safety characteristics as a liquid metal coolant. Understanding the microstructure and diffusion behavior of Na-K alloys from an atomic perspective is essential for elucidating their unique transport properties, such as heat transfer. This paper presented a systematic investigation of the effects of temperature and composition on the microstructure and diffusion behavior of Na-K alloys, conducted using ab initio molecular dynamics (AIMD) simulations. All the AIMD calculations were performed using the Quickstep module of the CP2K program. The Perdew-Burke-Ernzerhof (PBE) functional combined with D3BJ dispersion correction was employed at the Gamma point to account for electron exchange-correlation interactions. The core electrons of Na and K atoms were treated using the Goedecker-Teter-Hutter (GTH) norm-conserving pseudopotentials, while valence electrons were described using the double-ζ basis set DZVP-MOLOPT-SR-GTH. A cubic box containing a total of 200 Na and K atoms was constructed to investigate the effects of temperature and composition on the microstructure of Na-K alloys using three-dimensional periodic boundary conditions. After the system was fully equilibrated, a 20 ps production run was carried out using the NVT ensemble with the CSVR thermostat to generate trajectories for further statistical analysis. To validate the simulation approach, the AIMD results for density and structure factor S(k) were compared with reported experimental data at given temperatures and compositions. The essential consistency observed indicates that the AIMD method is effective in reflecting the physical and structural characteristics of Na-K alloys. The radical distribution functions (RDF), angle distribution functions (ADF) and S(k) were combined to gain insight to the local structure of Na-K alloys. The calculation results indicate that temperature and composition do not significantly affect the structural characteristics of the inner coordination sphere of Na atoms in Na-K alloys. The mean square displacement (MSD) and diffusion coefficients were also calculated to evaluate the diffusion characteristic. The results show that the diffusion coefficients are sensitive to the temperature and composition. And due to lower atomic mass compared to K, the Na atom exhibits a higher diffusion coefficient. The diffusion activation energies of Na atom (13.4 kJ/mol) in Na-K alloy are larger than the K atoms (11.0 kJ/mol), which means that the diffusion behaviors of Na atoms are more dependent on temperature. The overall diffusivity decreases with increasing Na content, which may be ascribed to the stronger interatomic interactions originating from Na’s smaller atomic radius, thereby imposing greater resistance to atomic migration. These results show that AIMD effectively captures the microstructure and diffusion behavior of liquid Na-K alloys and provides theoretical perspectives for the application of Na-K alloy as coolant in fast reactors.