Theoretical and Experimental Investigation on 3d Transition Metal Anisotropic Diffusion in van Der Waals Layered Sb₂Te₃

Doping 3d transition metal (TM) into van der Waals (vdW) layered materials is an effective strategy to tailor microstructures, transport properties, magnetic properties, and functional properties. The anisotropic diffusion behavior of 3d TM atoms in vdW layered materials is critical to understand the interaction between TM atoms and the matrix. Here, the interstitial defect formation energy and anisotropic diffusion mechanism of 3d TM elements from Sc to Zn in a Sb2Te3 matrix were systematically investigated using density functional theory. Preferential occupancy of 3d TM atoms in different interstitial sites was evaluated by considering polyhedral distortion and bonding change. In-plane diffusion along the vdW gap has a consistently lower diffusion energy than out-of-plane diffusion across the quintuple layer, while the energy barrier is correlated to the atomic radius for both diffusion modes. Transquintuple-layer diffusion for Ni with the lowest out-of-plane diffusion barrier was confirmed and compared with the cases of Fe and Co using aberration-corrected scanning transmission electron microscopy. This work provides essential theoretical and experimental support for the design of novel 3d TM doped thermoelectric materials and quantum materials.