The tetragermanates A2Ge4O9 (A = Na, K and Rb).

The crystal structures of three alkali tetragermanates A2Ge4O9 (A = Na, K and Rb) [namely disodium tetragermanate, Na2Ge4O9, dipotassium tetragermanate, K2Ge4O9, and dirubidium tetragermanate, Rb2Ge4O9] are trigonal (space group P3c1). The main building units are a three-membered ring of tetrahedra, oriented within the (001) plane and forming tetrahedral sheets. These sheets are connected to each other by two different regular isolated GeO6 octahedra via corner-sharing to build up a tetrahedral-octahedral framework. The alkali cations are located in cavities within this framework and are sevenfold coordinated. The increasing size of the A-site cation is accommodated by twist deformations of the tetrahedral rings and alterations in the Ge-O-Ge angles. With increasing size of the A-site cation, both the tetrahedral and octahedral sites become more regular, with slightly decreasing distances from Na2Ge4O9 to Rb2Ge4O9. This goes hand-in-hand with a more uniform distribution of bonds around the A-site cation. All these observations make Rb2Ge4O9 the most regular member of this A2Ge4O9 octahedral-tetrahedral framework structure series.