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

The binary intermetallic clathrates K_8-Si₄₆ (x = 0.4; 1.2), Rb_6.2Si₄₆, Rb_11.5Si₁₃₆ and Cs_7.8Si₁₃₆ 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 Rb_11.5Si₁₃₆ 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 K_7.6Si₄₆ and K_6.8Si₄₆ 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.