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

The crystal structures of three alkali tetra­germanates A₂Ge₄O₉ (A = Na, K and Rb) are trigonal (space group Pc1). 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 polar phase of Li₂Ge₄O₉ at 298, 150 and 90 K

The structure of Li₂Ge₄O₉ has been determined to show the space group P2₁ca between 298 and 90 K, thus being in the polar state. A detailed description of structure and temperature variation is given.