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

Abstract

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 60h, 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.