Single Crystal Growth of Type-II Si Clathrate Containing Fully Occupied Na and Ba from a Na-Based Metal Complex Solution

Single crystals of the Na–Ba–Si ternary clathrate were grown by heating a melt consisting of Na, Ba, Si, and Sn at 823 or 873 K and evaporating Na from the melt. The single crystals grown at 873 K had triangular {111} facets, with the length of one side of the triangle reaching up to 0.8 mm for crystals grown at 873 K. Both single crystals grown at 823 and 873 K exhibited a type-II clathrate structure (cubic, Fd3̅m), specifically Na16Ba7.71Si136 (a = 14.7008(3) Å) and Na16.72Ba7.28Si136 (a = 14.7007(3) Å), as determined by X-ray crystal structure analysis. Na and Ba are encapsulated in the Si20 and Si28 cages of type-II clathrate structure, respectively. Metallic electrical conduction was observed for the single crystal grown at 873 K with an electrical conductivity of 0.12 × 10–3 Ω cm at room temperature. The results of soft X-ray emission spectroscopy and the electronic density of states calculations also revealed that the Na–Ba–Si ternary clathrate with a type-II structure has a metallic electronic state.