Rb₂Ca₂Si₂O₇: a new alkali alkaline-earth silicate based on [Si₂O₇]^6- anions

Synthesis experiments were conducted in the ternary Rb₂O-CaO-SiO₂ system, resulting in the formation of a hitherto unknown compound with the composition Rb₂Ca₂Si₂O₇, i.e. dirubidium dicalcium pyrosilicate. Single crystals of sufficient size and quality were recovered from a starting mixture with an Rb₂O:CaO:SiO₂ molar ratio of 2:1:3. The educts were confined in a lid-covered platinum crucible and gradually cooled from 1050 °C at a rate of 0.3 °C min^-1 to 800 °C before being finally quenched in air to ambient conditions. The crystal structure was investigated at -80 and 15 °C from single-crystal X-ray diffraction data, with structure determination performed using direct methods. The compound was found to be of orthorhombic symmetry, belonging to the space group Pmmn (No. 59), with a = 5.7363 (6), b = 13.8532 (12), c = 9.9330 (10) Å, V = 789.34 (13) ų and Z = 4 (at 15 °C). The final refinement calculations at ambient temperature converged at R1 = 0.030 and wR2 = 0.076 for 773 observed reflections with I > 2σ(I). The silicate anion is based on pyrosilicate units of composition [Si₂O₇]^6- with point-group symmetry m (C_s). Charge compensation is achieved by the incorporation of rubidium and calcium cations distributed among a total of five independent sites within the asymmetric unit. Two of the nontetrahedrally coordinated cation sites (M4 and M5) are exclusively occupied by calcium cations, which are surrounded by six O atoms in the form of octahedra or trigonal prisms, respectively. The rubidium cations on the M1-M3 sites show more complex coordination environments. The M2 site, for example, is characterized by a tricapped trigonal prism polyhedron. Notably, the M3 site exhibits a 50% population of Ca²⁺ and Rb⁺, respectively. The compound shows closer structural resemblances with K₂Ca₂Si₂O₇ and can be derived from a hexagonal aristotype with space-group symmetry P6₃/mmc by displacements of the atoms. The corresponding distortion modes can be classified by certain irreducible representations of the high-symmetry parent phase. Structural investigations were completed by determining the thermal expansion tensor for the temperature interval between -80 and 15 °C.