Nanoporous mixed-framework silicates are of particular interest because they show promising physico-chemical properties similar to zeolites. In the course of an ongoing project on new micro- and nanoporous M³⁺ silicates new di-, tri-and tetrasilicate compounds were synthesised. All syntheses were carried out using the flux-growth technique (MoO₃–based flux mixtures in Pt crucibles in air; T_max = 1150°C, cooling rate 2°C/h, T_min = 900°C). The crystal structures of all compounds have been determined from single-crystal X-ray intensity data (Mo Kα X-radiation, CCD area detector, 293 K).
The three isotypic compounds BaKYSi₂O₇ (Kolitsch et al., 2007), BaKYbSi₂O₇ and BaKScSi₂O₇ crystallise as small colourless isometric crystals in space group P2₁/n. BaKREESi₂O₇ (REE = Y, Yb, Sc) represents a novel tetrahedral-octahedral framework structure type. The structure is based on isolated Si₂O₇ groups with octahedrally coordinated REE³⁺ cations and [9]-coordinated Ba atoms and [8]-coordinated K atoms. The isolated REEO₆ octahedra share each of their apices with oxygen atoms of the Si₂O₇ groups. The connectivity results in a three-dimensional framework with Ba²⁺ and K¹⁺ cations in channels running parallel to [10-1]. By comparison to other disilicates, the Si-O-Si angle in BaKREESi₂O₇ is unusually small (124.51(9), 124.9(2) and 128.2(1)°, respectively). No isoelectronic or other related silicates are known, although the monoclinic diphosphates M₂SrP₂O₇ (M = K, Rb, Cs) (Trunov et al., 1991) have similar crystal structures.
A new dimorph of BaKYbSi₂O₇ crystallised together with P2₁/n-type BaKYbSi₂O₇ (see above); it is also monoclinic (space group Cc). Its crystal structure is closely related to the BaKREESi₂O₇ (REE = Y, Yb, Sc) type; the Si-O-Si angle is alsosimilar, 126.0(7)°.
The third new structure type based on disilicate groups is represented by BaNaScSi₂O₇ (Wierzbicka et al., 2007). It is also topologically similar to BaKREESi₂O₇. BaNaScSi₂O₇ crystallises as small colourless isometric crystals in P2₁/m. Its structure consists of ScO₆ octahedra that share each of their apices with O atoms of the Si₂O₇ groups and [9]-coordinated Ba atoms and [4+4]-coordinated Na atoms (Na-O = 2.457 – 2.907 Å). The Si-O-Si angle in BaNaScSi₂O₇, 123.12(12)°, is the smallest of all the mentioned new disilicates. The three above structure types differ in localisation of Ba²⁺, K¹⁺, and Na¹⁺ atoms and direction of the linkages between REEO₆ (REE = Y, Yb, Sc) and Si₂O₇ units.
In the trisilicate family, a large number of new compounds was obtained: BaREE₂Si₃O₁₀ (REE = Gd, Y, Er, Yb, Sc) (Kolitsch et al., 2006), SrY₂Si₃O₁₀ (Kolitsch et al., 2006) and BaRbScSi₃O₉ (Wierzbicka et al., 2007). All compounds formed small, colourless plates, with monoclinic or triclinic symmetry. BaREE₂Si₃O₁₀ crystallises in P2₁/m. Its crystal structure is based on zigzag chains parallel to [010] of edge-sharing distorted REEO₆-octahedra linked by horseshoe-shaped Si₃O₁₀ groups and [8]-coordinated Ba atoms, which occupy narrow channels extending parallel to [100]. The Si-Si-Si angle in Si₃O₁₀ units increase with decreasing REE³⁺ ionic radii (94.78(5), 96.12(4), 96.40(5), 97.09(5), 99.63(4)° for REE = Gd, Y, Er, Yb, Sc, respectively). The second trisilicate compound, SrY₂Si₃O₁₀, crystallises in P-1 (no. 2). The structure of SrY₂Si₃O₁₀ contains zigzag chains parallel to [100] of edge-sharing YO₇ polyhedra sharing further edges with YO₆ polyhedra. These decorated chains are linked by slightly curved Si₃O₁₀ groups (Si-Si-Si angle = 133.5°). It is noteworthy that one SiO₄ tetrahedron shares an edge with the YO₇ polyhedron. Voids in the resulting framework are occupied by Sr²⁺ cations in [8]-coordination (Kolitsch et al., 2006). BaRbScSi₃O₉ crystallises as small pseudohexagonal plates (drillings) in P₁/c. The topology of BaRbScSi₃O₉ is based on Si₃O₉ rings connected to the isolated ScO₆ octahedra. The [6-7]-coordinated Ba and approximately [7-8]-coordinated Rb atoms are both arranged along rows parallel to [010].
The single new tetrasilicate obtained, Ba₂Gd₂Si₄O₁₃, crystallises as colourless pseudotetragonal prisms in C2/c. The structure consists of finite Si₄O₁₃ chains connected with GdO₇ polyhedra. Two such polyhedra share a common edge creating isolated Gd₂O₁₂ groups, which form the basis of a heteropolyhedral slab in the ab plane. [8]-coordinated Ba atoms are located in channels running parallel to the b-axis. This compound shows structural similarities to Na₄Sc₂Si₄O₁₃ (Maksimov et al., 1980) and Ba₂Nd₂Si₄O₁₃ (Tamazyan et al., 1985).

Financial support by the Austrian Science Foundation (FWF) (Grant P17623-N10) and the ICDD (Grant 90-03ET) is gratefully acknowledged.

References

U. Kolitsch, M. Wierzbicka, E. Tillmanns, Can. Mineral. (submitted) (2007).

U. Kolitsch, M. Wierzbicka, E. Tillmanns, Acta Crystallogr, C62, 35 (2006).

U. Kolitsch, M. Wierzbicka, E. Tillmanns, 15^th Slovenian-Croatian Crystallographic Meeting, Jezersko, Slovenia, June 15-18, Book of Abstracts, 22 (2006).

B. A. Maksimov, O. K. Mel'nikov, T. A. Zhdanova, V. V. Ilyukhin, N. V. Belov, Dokl. Akad. Nauk SSSR, 251, 98 (1980).

R. A Tamazyan, Yu. A. Malinovskii, Dokl. Akad. Nauk SSR, 30, 907 (1985).

V. K. Trunov, Yu. V. Oboznenko, S. P. Sirotinkin, N. B. Tskhelashvili, Neorg. Mater., 27, 1993 (1991).

V. K. Trunov, Yu. V. Oboznenko, S. P. Sirotinkin, N. B. Tskhelashvili, Neorg. Mater., 27, 2370 (1991).

M. Wierzbicka, U. Kolitsch, E. Tillmanns, Z. Kristallogr. Suppl. (in press) (2007).

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