Structural investigations on Rb2Ca2Si8O19 – a double-layer sheet silicate in the system Rb2O–CaO–SiO2

Abstract

Abstract Single crystals of a previously unknown rubidium calcium silicate with composition Rb2Ca2Si8O19 have been successfully grown by annealing a stoichiometric glass at 1,000 °C for 2 days. The highly crystalline product was characterized by powder and single-crystal X-ray diffraction. The basic crystallographic data are as follows: a = 7.4165(4) Å, b = 10.8089(6) Å, c = 12.4403(7) Å, α = 69.886(5)°, β = 82.331(4)°, γ = 87.399(4)°, V = 928.07(9) Å3, and Z = 2. The structure solution was based on Direct Methods in space group P 1 ̅ $̅{1}$ . Further least-squares refinements resulted in a convergence at R1 = 0.038 for 3,296 reflections and 287 parameters. The compound belongs to the group of double-layer sheet silicates containing Q3- and Q4-connected Si atoms in the ratio 3:1. Each one of the two parent layers forming the double layer contains five-, six-, and eight-membered rings of [SiO4] tetrahedra. The topology of the parent layers can be described using the following net symbol: (5.6.8)2(52.6)1(5.82)1. Direct linkage between neighboring double layers is facilitated by the presence of ribbons of [CaO6] octahedra that share common edges. Structurally speaking, there are two types of rubidium atoms which are coordinated by six or seven oxygen ligands. The first type is located in tunnel-like cavities inside the individual double layers. The other type is located between the columns of metal-centered octahedra and adjacent double layers. It is important to note that the Rb cations residing within the channels occupy two mutually exclusive split positions. The new compound is isostructural with the corresponding phase with potassium. A detailed comparison between K2Ca2Si8O19 and Rb2Ca2Si8O19 is presented, and the compositional strain induced by the Rb–K substitution has been calculated.