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

Abstract

Five new mixed-framework trisilicates were synthesised using a high-temperature flux growth technique. Colourless, glassy plates of SrY 2 Si 3 O 10 crystallise in space group P 1, with a = 6.757(1), b = 6.885(1), c = 9.273(2) A, α = 72.42(3), β = 86.37(3), γ = 88.37(3)°, V = 410.38(12) A 3 , Z = 2. The main building units of the new structure type represented by SrY 2 Si 3 O 10 are slightly curved Si 3 O 10 trimers and Y 2 O 11 dimers (composed of YO 6 octahedra sharing an edge with YO 7 polyhedra), which are further edge-connected to adjacent dimers to form twisted zigzag chains parallel to [010]. BaREE 2 Si 3 O 10 (REE = Gd, Er, Yb, Sc) form colourless small prisms, pseudohexagonal plates or isometric crystals, and crystallise in space group P 2 1 / 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) A, 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) A 3 , Z = 2. BaREE 2 Si 3 O 10 (REE = Gd, Er, Yb, Sc) are isotypic with BaY 2 Si 3 O 10 . Their topology is characterised by horseshoe-shaped trisilicate (Si 3 O 10 ) groups and zigzag chains of edge-sharing distorted M O 6 octahedra ( M = Gd, Er, Yb, Sc). Correlations between β –, Si–Si–Si angle and unit-cell volume and REE 3+ ionic radii are discussed. The geometries of the Si 3 O 10 and T 3 O 10 groups ( T = Ge, P, As, Al, Ga, V) in non-silicates are briefly reviewed, with special focus on narrow Si–Si–Si angles.