*Article* **A Mechanistic Study of Clustering and Diffusion of Molybdenum and Rhenium Atoms in Liquid Sodium**

**Zhixiao Liu 1,\*, Mingyang Ma 2,\*, Wenfeng Liang <sup>2</sup> and Huiqiu Deng <sup>3</sup>**

	- <sup>3</sup> School of Physics and Electronics, Hunan University, Changsha 410082, China; hqdeng@hnu.edu.cn
	- **\*** Correspondence: zxliu@hnu.edu.cn (Z.L.); mamingyang@caep.cn (M.M.)

**Abstract:** Liquid Na is widely used as the heat transfer medium in high-temperature heat pipes based on Mo-Re alloys. In this study, ab initio molecular dynamics are employed in order to understand the interactions between the Na solvent and Mo or Re solute in the liquid phase. Both the temperature and concentration effects on the clustering and diffusion behaviors of solute atoms are investigated. It is found that Mo2 and Re2 dimers can be stabilized in liquid Na, and the higher temperature leads to a stronger binding force. Pure Re and Mo-Re mixed solutes can form tetramers at the highest concentration. However, for the pure Mo solute, Mo4 is not observed. The diffusivities of a single solute atom and clusters are calculated. It is found that the Mo species diffuse faster than the Re species, and the diffusivity decreases as the cluster size increases.

**Keywords:** clustering; diffusivity; Na solvent; Mo or Re solute; ab initio molecular dynamics
