Seeded Growth of Type-II Na24Si136 Clathrate Single Crystals

Haruhiko Morito,Kozo Fujiwara,Hisanori Yamane,Rie Y Umetsu

doi:10.3390/cryst11070808

Abstract

Type-II Na24Si136 clathrate octahedral single crystals surrounded by {111} facets were grown by evaporating Na from a molten mixture of Na4Si4 and Na9Sn4 at 823 K for 12 h. One of the obtained single crystals was used as a seed for the following single crystal growth of the type-II clathrate using the same method. The single crystal grown on the seed maintained the octahedral shape. The weight of the crystal grown with the seed was increased from 0.6 to 30.4 mg by repeating the seeded growth and was proportional to the surface area of the seed crystal.

Highlights

Clathrate compounds containing Na in Si cages are called Na–Si clathrates [1], and two types, type-I (Na8Si46) and type-II (NaxSi136, 0 < x ≤ 24), have been reported [2,3,4,5]
The type-II single crystals of about 300 μm were produced by another thermal decomposition method, in which Na was slowly removed from Na4Si4 heated at 938 K in a closed space surrounded with NaCl and graphite [12,13]
Crystal growth of Na–Si clathrates and seeded growth of type-II clathrate Na24Si136 were performed by heating Na4Si4 and Na9Sn4 at 823 K for 12 h

Summary

Introduction

Clathrate compounds containing Na in Si cages are called Na–Si clathrates [1], and two types, type-I (Na8Si46) and type-II (NaxSi136, 0 < x ≤ 24), have been reported [2,3,4,5]. Crystal growth of the type-II Na–Si clathrate was performed by thermal decomposition under pressure with spark plasma sintering (SPS) equipment [11]. In previous studies [15], single crystals of the type-I Na–Si clathrate with a size up to about 5 mm were prepared from a Na–Si–Sn solution at 773 K. In previous studies on the crystal growth of the Na–Si clathrates, Na, Na4Si4, and Na15Sn4 were used as the starting materials [14,15]. Since the Na–Si clathrates are dissolved into the melt of Na15Sn4, the seeded crystal growth could not be applied. Single crystals of the Na–Si clathrates were prepared using a Na9Sn4 melt as a flux, and the seeded crystal growth was attempted to grow a large Na–Si clathrate single crystal

Experimental

Findings

ConclusionS