CRYSTAL CHEMISTRY AND TOPOLOGY OF TWO FLUX-GROWN YTTRIUM SILICATES, BaKYSi2O7 AND Cs3YSi8O19

Abstract

Two new yttrium silicates have been synthesized by a high-temperature flux-growth technique in air. The phase BaKYSi 2 O 7 is monoclinic, with space group P 2 1 / n , a 9.775(2), b 5.718(1), c 13.096(3) A, β 104.61(3)°, V 708.3(3) A 3 , and Z = 4; Cs 3 YSi 8 O 19 is orthorhombic, space group Pnma , with a 11.476(2), b 7.059(1), c 26.971(5) A, V 2184.9(6) A 3 , and Z = 4. The crystal structures were determined from single-crystal X-ray-diffraction data and refined to R 1( F ) = 2.0% (BaKYSi 2 O 7 ) and R 1( F ) = 2.9% (Cs 3 YSi 8 O 19 ). The phase BaKYSi 2 O 7 represents a novel structure-type, and a compound that may also occur in nature. It has a mixed tetrahedron–octahedron framework structure containing Si 2 O 7 groups and octahedrally coordinated Y atoms ( = 2.256 A). Voids in the framework host [9]-coordinated Ba atoms and [8]-coordinated K atoms. The YO 6 octahedron shares each of its apices with oxygen atoms of the Si 2 O 7 groups. The unusually small Si–O–Si angle of the disilicate group [124.51(9)°] is noteworthy. The diphosphates Na 2 CaP 2 O 7 , M 2 SrP 2 O 7 ( M = K, Rb, Cs) and K 2 CdP 2 O 7 , and the diarsenate K 2 CaAs 2 O 7 , have closely related structures. The phase Cs 3 YSi 8 O 19 is isotypic with isoelectronic Cs 3 ScSi 8 O 19 and the second representative of the microporous framework structure-type MCV–1. The silicate has a mixed octahedron–tetrahedron framework structure with a [ T O 4 ]:[ M O 6 ] ratio of 8:1. The structure contains isolated YO 6 octahedra ( = 2.251 A) sharing corners with SiO 4 tetrahedra to form an open framework with four-, six- and eight-membered rings. Large voids host two fully occupied Cs positions and four partly occupied and disordered Cs positions.