A Novel Technique for Controlling Anisotropic Ion Diffusion: Bulk Single-Crystalline Metallic Silicon Clathrate

Na-free Si clathrates consisting only of Si cages are an allotrope of diamond-structured Si. This material is promising for various device applications, such as next-generation photovoltaics. The probable technique for synthesizing Na-free Si clathrates is to extract Na⁺ from the Si cages of Na₂₄ Si₁₃₆ . Vacuum annealing is presently a well-known conventional and effective approach for extracting Na. However, this study demonstrates that Na⁺ cannot be extracted from the surface of a single-crystalline type-II metallic Si clathrate (Na₂₄ Si₁₃₆ ) in areas deeper than 150 µm. Therefore, a novel method is developed to control anisotropic ion diffusion: this is effective for various compounds with a large difference in the bonding strength between their constituent elements, such as Na₂₄ Si₁₃₆ composed of covalent Si cages and weakly trapped Na⁺ . By skillfully exploiting the difference in the chemical potentials as a driving force, Na⁺ is homogeneously extracted regardless of the size of the single crystal while maintaining high crystallinity. Additionally, the proposed point defect model is evaluated via density functional theory, and the migration of Na⁺ between the Si cages is explained. It is expected that the developed experimental and computational techniques would significantly advance material design for synthesizing thermodynamically metastable materials.