Structural investigations on Rb 2 Ca 2 Si 8 O 19 – a double-layer sheet silicate in the system Rb 2 O–CaO–SiO 2

Abstract Single crystals of a previously unknown rubidium calcium silicate with composition Rb 2 Ca 2 Si 8 O 19 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) Å 3 , 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 R 1 = 0.038 for 3,296 reflections and 287 parameters. The compound belongs to the group of double-layer sheet silicates containing Q 3 - and Q 4 -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 [SiO 4 ] tetrahedra. The topology of the parent layers can be described using the following net symbol: (5.6.8) 2 (5 2 .6) 1 (5.8 2 ) 1 . Direct linkage between neighboring double layers is facilitated by the presence of ribbons of [CaO 6 ] 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 K 2 Ca 2 Si 8 O 19 and Rb 2 Ca 2 Si 8 O 19 is presented, and the compositional strain induced by the Rb–K substitution has been calculated.