Two new yttrium silicates have been synthesized by a high-temperature flux-growth technique in air. The phase BaKYSi2O7 is monoclinic, with space group m>Pm>21/m>nm>, m>am> 9.775(2), m>bm> 5.718(1), m>cm> 13.096(3) Å, β 104.61(3)°, m>Vm> 708.3(3) Å3, and m>Zm> = 4; Cs3YSi8O19 is orthorhombic, space group m>Pnmam>, with m>am> 11.476(2), m>bm> 7.059(1), m>cm> 26.971(5) Å, m>Vm> 2184.9(6) Å3, and m>Zm> = 4. The crystal structures were determined from single-crystal X-ray-diffraction data and refined to m>Rm>1(m>Fm>) = 2.0% (BaKYSi2O7) and m>Rm>1(m>Fm>) = 2.9% (Cs3YSi8O19). The phase BaKYSi2O7 represents a novel structure-type, and a compound that may also occur in nature. It has a mixed tetrahedron–octahedron framework structure containing Si2O7 groups and octahedrally coordinated Y atoms (<Y–O> = 2.256 Å). Voids in the framework host [9]-coordinated Ba atoms and [8]-coordinated K atoms. The YO6 octahedron shares each of its apices with oxygen atoms of the Si2O7 groups. The unusually small Si–O–Si angle of the disilicate group [124.51(9)°] is noteworthy. The diphosphates Na2CaP2O7, m>Mm>2SrP2O7 (m>Mm> = K, Rb, Cs) and K2CdP2O7, and the diarsenate K2CaAs2O7, have closely related structures. The phase Cs3YSi8O19 is isotypic with isoelectronic Cs3ScSi8O19 and the second representative of the microporous framework structure-type MCV–1. The silicate has a mixed octahedron–tetrahedron framework structure with a [m>Tm>O4]:[m>Mm>O6] ratio of 8:1. The structure contains isolated YO6 octahedra (<Y–O> = 2.251 Å) sharing corners with SiO4 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.
