High-yield synthesis of semiconductive type-II Si clathrates with low Na content

To develop an efficient synthesis of type-II Si clathrates with low Na content (NaxSi136: x = 0–24), various conditions for annealing the Zintl phase NaSi were examined. The addition of a pre-annealing process under vacuum at 250 °C following the preparation of NaSi resulted in a decrease in the Na content of type-II Si clathrates from 4 to 2 when the pre-annealing duration ranged from 0 to 60 h, while the volume fraction of type-II Si clathrate crystals in the synthesized specimens (type-II/(type-I + type-II + d-Si)) deduced by powder X-ray diffraction and Rietveld analysis was maintained at approximately 85%. These preparation techniques that enable the high-yield synthesis of semiconductive type-II Si clathrates open opportunities for the application of these substances to semiconductor devices.

► Optimization work for synthesis of type II Si clathrate with low Na content. ► Temperature and duration for the NaSi annealing was modified. ► Longer annealing decreased Na contents and yields of type II Si clathrate. ► Pre-annealing resulted in decrease of Na contents while yield kept at 85 wt.%.