The tetragermanatesA2Ge4O9(A= Na, K and Rb)

Abstract

The crystal structures of three alkali tetragermanatesA2Ge4O9(A= Na, K and Rb) [namely disodium tetragermanate, Na2Ge4O9, dipotassium tetragermanate, K2Ge4O9, and dirubidium tetragermanate, Rb2Ge4O9] are trigonal (space groupP\overline{3}c1). 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 GeO6octahedraviacorner-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 theA-site cation is accommodated by twist deformations of the tetrahedral rings and alterations in the Ge—O—Ge angles. With increasing size of theA-site cation, both the tetrahedral and octahedral sites become more regular, with slightly decreasing 〈Ge—O〉 distances from Na2Ge4O9to Rb2Ge4O9. This goes hand-in-hand with a more uniform distribution of bonds around theA-site cation. All these observations make Rb2Ge4O9the most regular member of thisA2Ge4O9octahedral–tetrahedral framework structure series.