Synthesis of Type II Ge and Ge-Si Alloyed Clathrates Using Solid-State Electrochemical Oxidation of Zintl Phase Precursors

Abstract

Germanium clathrates with the type II structure are open-framework materials that show promise for various applications, but the difficulty of achieving phase-pure products via traditional synthesis routes has hindered their development. Herein, we demonstrate the synthesis of type II Ge clathrates in a two-electrode electrochemical cell using Na₄Ge_4-ySi_y (y = 0, 1) Zintl phase precursors as the working electrode, Na metal as the counter/reference electrode, and Na-ion conducting β″-alumina as the solid electrolyte. The galvanostatic oxidation of Na₄Ge₄ resulted in voltage plateaus around 0.34-0.40 V vs Na/Na⁺ with the formation of different products depending on the reaction temperature. When using Na₄Ge₃Si as a precursor, nearly phase-pure, alloyed type II Ge-Si clathrate was obtained at 350 °C. The Na atoms in the large (Ge,Si)₂₈ cages of the clathrate occupied off-centered positions according to Rietveld refinement and density functional theory calculations. The results indicate that electrochemical oxidation of Zintl phase precursors is a promising pathway for synthesizing Ge clathrates with type II structure and that Si alloying of the Zintl phase precursor can promote selective clathrate product formation over other phases.