Synthesis and structural study of five new trisilicates, BaREE₂Si₃O₁₀ (REE = Gd, Er, Yb, Sc) and SrY₂Si₃O₁₀, including a review on the geometry of the Si₃O₁₀ unit

Five new mixed-framework trisilicates were synthesised using a high-temperature flux growth technique. Colourless, glassy plates of SrY₂Si₃O₁₀ crystallise in space group P1̅, with a = 6.757(1), b = 6.885(1), c = 9.273(2) Å, α = 72.42(3), β = 86.37(3), γ = 88.37(3)°, V = 410.38(12) ų, Z = 2. The main building units of the new structure type represented by SrY₂Si₃O₁₀ are slightly curved Si₃O₁₀ trimers and Y₂O₁₁ dimers (composed of YO₆ octahedra sharing an edge with YO₇ polyhedra), which are further edge-connected to adjacent dimers to form twisted zigzag chains parallel to [010]. BaREE₂Si₃O₁₀ (REE = Gd, Er, Yb, Sc) form colourless small prisms, pseudohexagonal plates or isometric crystals, and crystallise in space group P2₁/m, with respectively a = 5.435(1) / 5.389(1) / 5.377(1) / 5.273(1), b = 12.241(2) / 12.163(2) / 12.117(2) / 11.918(2), c = 6.932(1) / 6.840(1) / 6.790(1) / 6.591(1) Å, β = 106.26(3) / 106.47(3) / 106.50(3) / 107.06(3)°, V = 442.74(13) / 429.94(12) / 424.17(12) / 395.98(12) ų, Z = 2. BaREE₂Si₃O₁₀ (REE = Gd, Er, Yb, Sc) are isotypic with BaY₂Si₃O₁₀. Their topology is characterised by horseshoe-shaped trisilicate (Si₃O₁₀) groups and zigzag chains of edge-sharing distorted MO₆ octahedra (M = Gd, Er, Yb, Sc). Correlations between β–, Si–Si–Si angle and unit-cell volume and REE³⁺ ionic radii are discussed. The geometries of the Si₃O₁₀ and T₃O₁₀ groups (T = Ge, P, As, Al, Ga, V) in non-silicates are briefly reviewed, with special focus on narrow Si–Si–Si angles.