Cubic structure of sodium calcium germanate Na3.70Ca1.15Ge3O9

Mr=492.9, cubic, Pa3, a=15.626 (3) A, V=3815.4 A3, Z=16, Dx=3.43 g cm-3, Mo Kα, λ=0.71069 A, μ=106.8 cm-1, F(000)=3707.2, T=298 K, R=0.059, wR=0.064 for 1238 observed reflections. The structure of Na3.70Ca1.15Ge3O9, which was found in a flux melt, is characterized by having a 12-membered ring of GeO4 tetrahedra and contains pseudocubic subcells with an edge length of 3.9 A. The ring is more corrugated than those of K4SrGe3O9 and the silicate analogue Na4CaSi3O9, mainly owing to the difference in effective charge of the cations located around the centres of the rings. The trend is observed that the GE-O bond length shortens with increase of Ge-O-Ge bond angle, giving in a Ge-O versus Ge-O-Ge diagram a slope which is much steeper than those for the silicate and aluminate tetrahedra. (orig.)