Phase boundary between Na–Si clathrates of structures I and II at high pressures and high temperatures

摘要: Understanding of the covalent clathrate formation is a crucial point for design new superhard materials with intrinsic coupling superhardness and metallic conductivity. It has been found that silicon clathrates have archetype structures, which can serve an existent model compounds frameworks Si–B, Si–C, B–C C intercalated atoms (e.g., alkali metals or even halogens) assure properties. Here we report our in situ ex studies high-pressure stability Na8Si46 (structure I) Na24+x Si136 II). Experiments performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa 1500 K. We established chemical interactions Na–Si system transition between two structures occur at temperatures below melting. The strong sensitivity crystallization products sodium concentration observed. A tentative diagram transformations proposed. At least ~6 GPa, II) stable lower as compared I).