THE CRYSTAL STRUCTURES OF THREE NEW COMPLEX SILICATES OF SCANDIUM

Abstract

Three new scandium silicates with framework structures made of octahedra and tetrahedra, KSrScSi2O7, NaBaScSi2O7 and RbBaScSi3O9, were obtained through a high-temperature flux-growth technique. Their crystal structures have been determined from single-crystal intensity data (Mo K α radiation, CCD area detector, 293 K; R ( F ) = 3.30, 1.82, 2.00%, respectively). The disilicate KSrScSi2O7 is monoclinic, P 21/ n , with a 9.446(2), b 5.478(1), c 12.537(3) A, β 104.39(3)°, V 628.4(2) A3, Z = 4. The disilicate NaBaScSi2O7 forms monoclinic crystals with space-group symmetry P 21/ m , with a 6.845(1), b 5.626(1), c 8.819(2) A, β 109.33(3)°, V 320.47(10) A3, Z = 2. The cyclosilicate RbBaScSi3O9 crystallizes as pseudohexagonal triplets; it is also monoclinic, P 21/ n , with a 6.957(1), b 10.199(2), c 11.881(2) A, β 90.07(3)°, V 843.0(2) A3, Z = 4. The topologies of KSrScSi2O7 and NaBaScSi2O7 show clear similarities, corresponding to isolated Si2O7 groups, bound to octahedrally coordinated Sc3+. The pseudohexagonal arrangement in RbBaScSi3O9 is based on isolated ScO6 octahedra, each of which is corner-linked to Si3O9 rings. In all three compounds, the alkali and alkaline-earth atoms occupy voids in the resulting three-dimensional frameworks. Results of a Raman spectroscopic study of the two disilicates and of five related novel disilicates are also briefly presented, with a focus on the behavior of the Si2O7 group.