Crystal structures of new potassium silicates and aluminosilicates of Sm, Tb, Gd, and Yb and their relation to the armstrongite (CaZr(Si6O15)center dot 3H(2)O) structure
Silicates of composition K7.81Sm3Si12O32(OH)0.81·0.77H2O and K7Tb3Si12O32·1.36H2O, with the space group P1¯ and unit cell parameters of a=6.9218(3), b=11.4653(4), c=11.6215(4) Å, α=88.063(3)°, β=88.449(3)°, γ=79.266(3)°and a=6.872(3), b=11.440(5), c=11.542(6) Å, α=88.19(4)°, β=88.86(4)°, γ=79.65(4)°, respectively, were synthesized under hydrothermal conditions. Both crystal structures were determined from twinned crystals, and can be idealized to a composition of K7Ln3Si12O32 (KOH)x(H2O)(2-x) (Ln=Sm, Tb), which is closely related to K8Nd3Si12O32(OH). Crystals of the aluminosilicates K2GdAlSi4O12·0.25H2O and K2SmAlSi4O12·0.375H2O prepared by the same method possess monoclinic symmetry with the space group C2/c. The corresponding unit cell parameters are: a=26.67(1), b=7.294(3), c=14.835(6) Å, β=123.44(3)°; and a=26.7406(9), b=7.3288(2), c=14.8498 (6) Å, β=123.514(1)°, respectively. A new type of silicate anion that forms tubes was detected in the K4Yb2Si8O21 structure. K4Yb2Si8O21 is of monoclinic symmetry with the space group C2/c. The unit cell parameters are: a=17.440(2), b=11.786(1), c=12.802 (2) Å, and β=130.902(1)°. The structure is a mixed framework of tubes formed by silica-oxygen tetrahedra connected by pairs of edge sharing Yb-octahedra. The relation of the silicate layers and frameworks encountered in these compounds to the armstrongite silicate framework is discussed.