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

Abstract

Single crystals of a complex Zintl compound with the composition NaGe 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 sp 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 [Ge] tetrahedra. This compound is the same as "NaGe" reported previously, but the availability of single crystals allowed for more complete structural determination with a formula unit best described as NaGe(Ge). 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.

An Electrochemical Approach toward the Metastable Type II Clathrate Germanium Allotrope

“…α-Ge (9 mass %) was revealed as a byproduct, again. However, different from the completed anodic conversion, also Na 4 Ge 13 40 , 41 was detected (8 mass %). Not being present after the completed reaction, Na 4 Ge 13 and Na 24−δ Ge 136 consequently occur as intermediates of anodic conversion of Na 12 Ge 17 toward elemental Ge, which is in line with reports on thermal degradation of Na 4 Ge 4 42 and chemical gas–solid oxidation of Na 12 Ge 17 .…”

Zintl Phases as Reactive Precursors for Synthesis of Novel Silicon and Germanium-Based Materials

“…For example, thermal decomposition of Na 4 Si 4 often results in a micro or nanocrystalline mixture of clathrate-I Na 8 Si 46 , clathrate-II Na x Si 136 , as well as α -Si and amorphous material [75]. Thermal decomposition of Na 4 Ge 4 produces the zeolite-like Zintl phase Na 4 Ge 13 [78,79] or clathrate-II Na x Ge 136 depending on the decomposition conditions [80]. Recent improvements to thermal decomposition synthesis of these materials provide better control over the partial pressure of Na, improving the phase selectivity and access to phase pure macroscopic single crystals of Si-based clathrates (Figure 4b) [81,82].…”

Zintl Defects in Intermetallic Clathrates

“…For higher concentrations of the electropositive metal, intricate structure patterns form, e.g., in compounds like Ba 6 Ge 25 [4,5] or Na 4 Ge 13 . [6] Here, the Ba or Na atoms reside in zeolitic channels, which also accommodate isolated Ge 4 4polyanions in case of the sodium compound. Prototypic examples with essentially fourfold connectivity of the E14 elements are clathrate phases.…”

“…In the third branch, the group 14 elements form 3D networks with three and fourfold connectivity. For higher concentrations of the electropositive metal, intricate structure patterns form, e.g., in compounds like Ba 6 Ge 25 , or Na 4 Ge 13 . Here, the Ba or Na atoms reside in zeolitic channels, which also accommodate isolated Ge 4 4– polyanions in case of the sodium compound.…”