24-layer structure of tricalcium germanate, Ca3GeO5

The crystal structure of a 24-layer polytype of tricalcium germanate (C3G), which was found in a flux melt, has been determined. The crystal data are: Msub(r)=272.8, rhombohedral, R3m, a=7.228(2), c=67.42(2) A, V=3050(2) A3, Z=24, Dsub(x)=3.56 g cm-3, MoKα, lambda=0.71069 A, μ=92.7 cm-1, F(000)=3168, T=298 K. The structure consists of two kinds of subunits, one is related to the structure of the 2-layer polytype and the other to that of the 9-layer polytype; these occur alternately in the structure. Half of the eight independent germanate tetrahedra show nearly perfect orientational disorder along the c axis. The positions of the Ge atoms of these tetrahedra are split, the separations being in the range 0.34(10) to 0.69(5) A. The coordination numbers of Ca, each calculated as the sum of the occupancies of the Ca-O bonds, are in the range 6.0 to 6.56, giving mean Ca-O bond lengths of 2.34 to 2.51 A. General principles of deriving polytypes for C3G are provided and, based on these, the stacking sequences of simple polytypes such as 4-layer, 5-layer and 6-layer structures have been predicted. The derivation of polytypes can be applicable to the case of tricalcium silicate (C3S). (orig.)