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

✍ Scribed by Yuting Yang; James A. Ibers

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 224 KB
• Volume: 155
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2000.8946

✦ Synopsis

The compounds La 3 Al 0.44 Si 0.93 S 7 , BaSm 4 (SiO 4 ) 3 Se, and monoclinic and orthorhombic Ln 2 (SiO 4 )Te (Ln ‫؍‬ Nd and Sm) were obtained accidentally by reactions in fused-silica tubes. La 3 Al 0.44 Si 0.93 S 7 crystallizes in the hexagonal space group P6 3 with two formula units in a cell of dimensions a ‫؍‬ 10.277(2), c ‫؍‬ 5.793(1) A > (T ‫؍‬ 153 K). The structure consists of chains of face-sharing AlS 6 octahedra and isolated SiS 4 tetrahedra. The LaS 8 coordination polyhedron is a square antiprism. BaSm 4 (SiO 4 ) 3 Se crystallizes in the hexagonal space group P6 3 /m with two formula units in a cell of dimensions a ‫؍‬ 9.869( 1), c ‫؍‬ 6.851(1) A > (T ‫؍‬ 153 K). The structure is formed by orthosilicate ([SiO 4 ] 4؊ ) tetrahedra separated by Ba 2؉ and Sm 3؉ cations. The Ba 2؉ and Sm(1) 3؉ cations are disordered in an MS 9 tricapped trigonal prism and the Sm(2) 3؉ cation is in a monocapped trigonal prism. Monoclinic (P2 1 /c) Ln 2 (SiO 4 )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) A > , ‫؍‬ 94.60(3)3 (T ‫351؍‬ K) for Ln ‫؍‬ Nd, and a ‫؍‬ 9.760(2), b ‫؍‬ 6.357(1), c ‫؍‬ 8.601(2) A > , ‫؍‬ 94.87(3)3 (T ‫؍‬ 153 K) for Ln ‫؍‬ Sm. The layer is formed by [SiO 4 ] 4؊ tetrahedra separated by Ln and Te atoms. The Ln(1)O 7 Te polyhedron is a square antiprism and the Ln(2)O 4 Te 4 polyhedron is a bicapped trigonal prism. Orthorhombic Pbcm Ln 2 (SiO 4 )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) A > (T ‫؍‬ 153 K) for Ln ‫؍‬ Nd, and a ‫؍‬ 6.201(1), b ‫؍‬ 7.091(1), c ‫؍‬ 11.077(2) A > (T ‫؍‬ 115 K) for Ln ‫؍‬ Sm. The layer is formed by [SiO 4 ] 4؊ tetrahedra separated by Ln and Te atoms. The Ln(1)O 6 Te 3 polyhedron is a tricapped trigonal prism; the Ln(2)O 6 Te 2 polyhedron is a dodecahedron.

Orthorhombic Ln 2 (SiO 4 )Te, ‫؍‬ 6.696 g/cm 3 for Ln ‫؍‬ Nd and ‫؍‬ 7.096 g/cm 3 for Ln ‫؍‬ Sm, are denser than the monoclinic forms, ‫؍‬ 6.188 g/cm 3 for Ln ‫؍‬ Nd and ‫؍‬ 6.500 g/cm 3 for Ln ‫؍‬ Sm.