Structure of sodium calcium germanate Na2CaGe2O6

Na2CaGe2O6, Mr=490.9, rhombohedral, Ranti 3m, a=10.788(2), c=13.46(2) A (hexagonal setting), V=1356.6 A3, Z=9, Dx=3.60 g cm-3, λ(Mo Kα)=0.71069 A, μ=113.5 cm-1, F(000)=1386, T=298 K, R=0.051, wR=0.055 for 708 observed reflections. The structure has a six-membered ring of GeO4 tetrahedra and contains pseudocubic subcells with an edge length of 3.8 A. These basic features are similar to those of the silicate analogue Na2CaSi2O6. The details of both structures are however, different. All the O atoms, for example, have statistical distributions near two-fold axes and mirror planes, while the silicate analogue has only one set of O atoms having a statistical distribution. In addition, all of the metal sites have disordered distributions, while some of them are ordered in the silicate analogue. (orig.)