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/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.
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