Synthesis, Crystal Structure, and Vibrational Spectroscopy of K2Ca4Si8O21—An Unusual Single-Layer Silicate Containing Q2 and Q3 Units

Rb2Ca2Si3O9: the first rubidium calcium silicate

The crystal structure of Rb2Ca2Si3O9 has been characterized by X-ray diffraction techniques and Raman spectroscopy. Crystal growth was performed by the flux method in closed platinum capsules using a polycrystalline ceramic precursor as well as RbCl as a mineralizer. The crystal structure was solved from a single-crystal diffraction data set acquired at 23 °C and refined to a final residual of R(|F|)=0.022 for 1871 independent reflections. Basic crystallographic data are as follows: monoclinic symmetry, space group type P1n1, a=6.5902(3) Å, b=7.3911(3) Å, c=10.5904(4) Å, β=93.782(3)°, V=514.72(3) Å3, Z=2. With respect to the silicate anions the compound can be classified as a sechser single-chain silicate. The undulated chains run parallel to [-101] and are connected by Rb and Ca cations, which are distributed among four crystallographically independent sites. In a first approximation the coordination polyhedra of the two different calcium ions in the asymmetric unit can be described by distorted trigonal prisms and tetragonal pyramids, respectively. The two rubidium sites exhibit more irregular coordination spheres with eight to nine next neighbors. Structural investigations on the new phase are completed by solid state micro-Raman spectroscopy. DFT-calculations were employed for the interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species.