Single crystals of a complex Zintl compound with the composition Na4Ge13 were synthesized for the first time using a high-pressure/higherature 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 "Na1-xGe3+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 lowerature transport properties of this material and revealed its semiconductor behavior, which was corroborated by theoretical calculations.

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Journal Inorganic Chemistry
Stefanoski, S. (Stevce), Finkelstein, G.J. (Gregory J.), Ward, M.D. (Matthew D.), Zeng, T, Wei, K. (Kaya), Bullock, E.S. (Emma S.), … Strobel, T.A. (Timothy A.). (2018). Zintl Ions within Framework Channels: The Complex Structure and Low-Temperature Transport Properties of Na4Ge13. Inorganic Chemistry, 57(4), 2002–2012. doi:10.1021/acs.inorgchem.7b02914