Synthesis and solid-state characterization of RbNaCaSi 4 O 10 – the first phase in the quaternary system Rb 2 O–Na 2 O–CaO–SiO 2

Abstract Crystal growth involving the devitrification of glasses in the system Rb 2 O–Na 2 O–CaO–SiO 2 , prepared by quenching from 1,400 °C, was performed at 750 °C. The experiments resulted in the formation of the first known crystalline rubidium sodium calcium silicate, with a composition of RbNaCaSi 4 O 10 . Powder X-ray diffraction patterns of the bulk material of the synthesized product revealed that the material is almost phase pure, containing only very small amounts of an impurity whose identity remains to be determined. Solid-state characterization of this compound included chemical analysis, single-crystal and powder X-ray diffraction, as well as Raman and IR spectroscopy. The basic crystallographic data are as follows: triclinic symmetry, space group P 1 ̅ $̅{1}$ , a = 7.0713(3) Å, b = 8.2037(5) Å, c = 10.1792(5) Å, α = 103.504(5)°, β = 99.967(4)°, γ = 115.399(5)°, V = 492.95(5) Å 3 , Z = 2. Refinement calculations using anisotropic displacement parameters for all atoms converged to a residual of R (| F |) = 0.0385 for 1767 independent reflections. The compound is a synthetic member of the litidionite group of compounds and isostructural with KNaCaSi 4 O 10 . Furthermore, it is related to K 2 CaSi 4 O 10 . From a structural perspective, RbNaCaSi 4 O 10 belongs to the group of the tubular chain silicates. The fundamental building units are loop-branched dreier double chains running parallel to [100]. The Ca- and Na-ions are principally coordinated by five nearest oxygen neighbors in the form of distorted tetragonal pyramids. The pyramids are arranged in band-like elements by sharing common edges and corners. Charge compensation is achieved by the incorporation of the larger ten-fold coordinated rubidium ions into cavities of the heteropolyhedral network. A detailed comparison with isotypic KNaCaSi 4 O 10 is presented and the compositional strain induced by the Rb–K substitution is discussed.