Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization.

Yuri Grin,Igor Veremchuk,Matt Beekman,Iryna Antonyshyn,Walter Schnelle,Michael Baitinger,George Nolas

doi:10.3390/ma9070593

Abstract

The binary intermetallic clathrates K8-xSi46 (x = 0.4; 1.2), Rb6.2Si46, Rb11.5Si136 and Cs7.8Si136 were prepared from M4Si4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb11.5Si136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si20 dodecahedra, was found in all compounds. SPS preparation of Na24Si136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K7.6Si46 and K6.8Si46 in the temperature range 4–700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.

Highlights

Binary phases of alkali metals with silicon have been prepared by thermal decomposition of the monosilicides M4 Si4 (M = Na, K, Rb, Cs) for over six decades [1,2,3,4]
We report on the preparation and crystal structure of different clathrate phases (Table 1) as well as SPS densification of K7.6Si46 and K6.8Si46 structure of different clathrate phases (Table 1) as well as SPS densification of K7.6 Si46 and K6.8 Si46 powders by SPS that allow for the investigation of their transport properties over a large powders by SPS that allow for the investigation of their transport properties over a large temperature temperature range
As first described in reference [16], clathrates are formed during SPS redox-treatment by oxidation of Si44− at the anode while the alkali metal is reduced at the cathode (Figure 2a,b)

Summary

Introduction

Defect-free single crystals of alkali metal silicon clathrates large enough for structure investigations have only been obtained in exceptional cases [4]. Materials 2016, 9, 593 plasma treatment (SPS) [16,17] Another approach—the kinetically controlled thermal decomposition (KCTD) method—can provide relatively large single crystals [18,19]. The unique advantage of SPS for the synthesis of clathrates by producing scalable, compact bulk materials continues to be of great interest [4]. In this work we discuss both the preparation and consolidation of alkali-metal silicon clathrates by SPS. We report on the preparation and crystal and consolidation of alkali-metal silicon clathrates by SPS.

Results and Discussion

34 M4 Si4 Ñcombined

Synthesis of Precursors

SparkSPS

Sample Characterization

Conclusions