Accidental Silicon-Containing Compounds: Crystal Structures of La3Al0.44Si0.93S7, BaSm4(SiO4)3Se, and Monoclinic and Orthorhombic Ln2(SiO4)Te (Ln=Nd and Sm)

Abstract

The compounds La3Al0.44Si0.93S7, BaSm4(SiO4)3Se, and monoclinic and orthorhombic Ln2(SiO4)Te (Ln=Nd and Sm) were obtained accidentally by reactions in fused-silica tubes. La3Al0.44Si0.93S7 crystallizes in the hexagonal space group P63 with two formula units in a cell of dimensions a=10.277(2), c=5.793(1) Å (T=153 K). The structure consists of chains of face-sharing AlS6 octahedra and isolated SiS4 tetrahedra. The LaS8 coordination polyhedron is a square antiprism. BaSm4(SiO4)3Se crystallizes in the hexagonal space group P63/m with two formula units in a cell of dimensions a=9.869(1), c=6.851(1) Å (T=153 K). The structure is formed by orthosilicate ([SiO4]4−) tetrahedra separated by Ba2+ and Sm3+ cations. The Ba2+ and Sm(1)3+ cations are disordered in an MS9 tricapped trigonal prism and the Sm(2)3+ cation is in a monocapped trigonal prism. Monoclinic (P21/c) Ln2(SiO4)Te (Ln=Nd and Sm) is a layered structure with four formula units in a cell of dimensions a=9.823(2), b=6.421(1), c=8.676(2) Å, β=94.60(3)° (T=153 K) for Ln=Nd, and a=9.760(2), b=6.357(1), c=8.601(2) Å, β=94.87(3)° (T=153 K) for Ln=Sm. The layer is formed by [SiO4]4− tetrahedra separated by Ln and Te atoms. The Ln(1)O7Te polyhedron is a square antiprism and the Ln(2)O4Te4 polyhedron is a bicapped trigonal prism. Orthorhombic Pbcm Ln2(SiO4)Te (Ln=Nd and Sm) is a layered structure with four formula units in a cell of dimensions a=6.279(1), b=7.189(1), c=11.168(2) Å (T=153 K) for Ln=Nd, and a=6.201(1), b=7.091(1), c=11.077(2) Å (T=115 K) for Ln=Sm. The layer is formed by [SiO4]4− tetrahedra separated by Ln and Te atoms. The Ln(1)O6Te3 polyhedron is a tricapped trigonal prism; the Ln(2)O6Te2 polyhedron is a dodecahedron. Orthorhombic Ln2(SiO4)Te, ρ=6.696 g/cm3 for Ln=Nd and ρ=7.096 g/cm3 for Ln=Sm, are denser than the monoclinic forms, ρ=6.188 g/cm3 for Ln=Nd and ρ=6.500 g/cm3 for Ln=Sm.