Clinotobermorite-related Cs2Ca4Si6O17 – the first cesium calcium silicate

In the course of a systematic investigation of the ternary system Cs₂O-CaO-SiO₂, a phase with the chemical composition Cs₂Ca₄Si₆O₁₇ was discovered. Synthesis experiments were performed using a starting mixture with an oxide ratio of Cs₂O:CaO:SiO₂ = 2:1:3 contained in a lid-covered platinum crucible. The educts were heated to 1000 °C at 2 °C/min, annealed at maximum temperature for two hours, and then cooled to 700 °C at 0.3 °C/min before final quenching in air to ambient conditions. The crystal structure was determined from a single-crystal diffraction data set collected at 15 °C using direct methods. The previously unknown compound crystallizes in space group P 2₁/n and has the following basic crystallographic data: a = 11.1053(7) Å, b = 7.3138(4) Å, c = 22.2609(15) Å, β = 98.289(6)°, V = 1789.2(2) ų and Z = 4. The final refinement calculations converged to R1 = 0.045 and wR2 = 0.106, for 2967 observed reflections with I > 2σ(I). The silicate anions correspond to unbranched dreier double chains running along [010]. Charge compensation is achieved by the incorporation of cesium and calcium cations distributed over a total of six independent sites within the asymmetric unit. The calcium atoms are surrounded by six or seven oxygen atoms in the form of uncapped and monocapped trigonal prisms, respectively. The CaO₆- and CaO₇-polyhedra share common edges forming 7.5 Å wide sheets parallel (001), the presence of which is reflected in the pronounced platy morphology of the crystals. The cesium cations in turn have more irregular coordination environments involving seven and eight oxygen ligands. The crystal structure of Cs₂Ca₄Si₆O₁₇ shows a closer structural relationship with the MDO₁-polytype of clinotobermorite, which is discussed in detail. The structural investigations were complemented by the determination of the thermal expansion tensor for the temperature interval between - 80 °C and 15 °C.