Structure and thermal conductivity of Na1−Ge3+

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

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.

Zintl Ions within Framework Channels: The Complex Structure and Low-Temperature Transport Properties of Na4Ge13

Single crystals of a complex Zintl compound with the composition Na₄Ge₁₃ were synthesized for the first time using a high-pressure/high-temperature approach. Single-crystal diffraction of synchrotron radiation revealed a hexagonal crystal structure with P6/m space group symmetry that is composed of a three-dimensional sp³ Ge framework punctuated by small and large channels along the crystallographic c axis. Na atoms are inside hexagonal prism-based Ge cages along the small channels, while the larger channels are occupied by layers of disordered sixfold Na rings, which are in turn filled by disordered [Ge₄]^4- tetrahedra. This compound is the same as "Na_1-xGe_3+z" reported previously, but the availability of single crystals allowed for more complete structural determination with a formula unit best described as Na₄Ge₁₂(Ge₄)_0.25. The compound is the first known example of a guest-host structure where discrete Zintl polyanions are confined inside the channels of a three-dimensional covalent framework. These features give rise to temperature-dependent disorder, as confirmed by first-principles calculations and physical properties measurements. The availability of single-crystal specimens allowed for measurement of the intrinsic low-temperature transport properties of this material and revealed its semiconductor behavior, which was corroborated by theoretical calculations.