Figure . 12. Comparison of the structures of the known Ba-Y-silicates emphasizing single yttrium-centred octahedra in Ba9Y2[SiO4]6 and Ba3Y2[Si2O7]2 (Phase B), dimers in BaY2Si3O10, chains of polyhedra in BaY4[Si2O7][Si3O10], condensed clusters in Ba5Y13[SiO4]8O8.5 (Phase A), and a 3D network of polyhedra in BaY16Si4O33. Crystallographic details of these compounds are given in Table . Silica tetrahedra and yttrium-centred polyhedra drawn in yellow (yellow/red for Phase B) and blue, respectively. Note that the coordination of yttrium atoms becomes more condensed with increasing yttrium content.