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

The binary intermetallic clathrates K(8-x)Si(46) (x = 0.4; 1.2), Rb(6.2)Si(46), Rb(11.5)Si(136) and Cs(7.8)Si(136) were prepared from M(4)Si(4) (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb(11.5)Si(136) was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si(20) dodecahedra, was found in all compounds. SPS preparation of Na(24)Si(136) 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 K(7.6)Si(46) and K(6.8)Si(46) 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.