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Acta Crystallogr C. 2010 March 15; 66(Pt 3): i29–i32.
Published online 2010 February 3. doi:  10.1107/S0108270110002842
PMCID: PMC2855576

Ba2Gd2(Si4O13): a silicate with finite Si4O13 chains

Abstract

The title compound, dibarium digadolinium(III) tetra­silicate, crystallized from a molybdate-based flux. It represents a new structure type and contains finite zigzag-shaped C 2-symmetric Si4O13 chains and Gd2O12 dimers built of edge-sharing GdO7 polyhedra. The [9+1]-coordinated Ba atoms are located in voids in the atomic arrangement. All atoms are in general positions except for one O atom, which lies on a twofold axis. The structure is compared with those of the few other known tetra­silicates.

Comment

In a comprehensive ongoing study we are focusing on the preparation, structural characterization and classification of novel microporous and small-pore mixed-framework silicates containing seven specific octa­hedrally coordinated M 3+ cations (M = Sc, V, Cr, Fe, In, Y and Yb). These poorly known members of the silicate family are expected to have useful properties and inter­esting technical applications, similar to the related mixed-framework (SiO4M 4+O6) titanosilicates and zirconosilicates. As part of our study, we have recently also included compounds containing the Gd3+ cation for comparison purposes, as this cation may either show octa­hedral coordination or have higher coordination numbers (7–8). The title compound, (I), was obtained as a by-product during the preparation of the Gd analogue of BaY2Si3O10 and isotypic Sc and lanthanide representatives (Kolitsch et al., 2006 [triangle]; Wierzbicka-Wieczorek, 2007 [triangle]; see also Kolitsch et al., 2009 [triangle]). It represents a novel structure type and is a new representative of a small class of tetra­silicates with finite Si4O13 chains (groups).

Additional flux-growth syntheses involving Gd3+ have so far yielded the following three Gd silicates: Rb3GdSi8O19 and Cs3GdSi8O19, both isotypic with Cs3ScSi8O19 (Kolitsch & Tillmanns, 2004 [triangle]), and BaKGdSi2O7, isotypic with the disilicates BaKREESi2O7 (REE = Y, Yb and Sc; Kolitsch et al., 2009 [triangle]) and SrKScSi2O7 (Wierzbicka-Wieczorek, 2007 [triangle]).

The asymmetric unit in monoclinic (I) contains one Ba, one Gd, two Si and seven O atoms (Fig. 1 [triangle]). All atoms are in general positions except O3, which lies on a twofold axis. Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O’Keeffe (1991 [triangle]): 2.01 (Ba), 3.05 (Gd), 3.99 (Si1), 3.85 (Si2), 2.06 (O1), 2.08 (O2), 2.27 (O3), 1.94 (O4), 1.84 (O5), 1.86 (O6) and 1.96 (O7) v.u. (valence units). Thus, these are all reasonably close to ideal valencies. (The oxygen ligands O3 and O4 represent bridging O atoms within the finite Si4O13 chain and are expected to have slightly high bond-valence sums. However, only atom O3 shows an elevated value, while O4, bonded also to Gd, shows a normal value.)

Figure 1
A view of the basic connectivity in Ba2Gd2(Si4O13), shown with displacement ellipsoids at the 50% probability level. [Symmetry codes: (i) −x + An external file that holds a picture, illustration, etc.
Object name is c-66-00i29-efi10.jpg, y + An external file that holds a picture, illustration, etc.
Object name is c-66-00i29-efi10.jpg, −z + An external file that holds a picture, illustration, etc.
Object name is c-66-00i29-efi10.jpg; (ii) −x, y + 1, ...

The atomic arrangement is based on finite zigzag-shaped Si4O13 chains and Gd2O12 dimers built of edge-sharing GdO7 polyhedra (Fig. 2 [triangle]). The latter are rather irregular, although they may be described as monocapped octa­hedra (the capping atom being O6). The Gd2O12 dimers are linked by SiO4 tetra­hedra to form a heteropolyhedral slab in the ab plane. These slabs are connected to adjacent slabs only via a bridging O3 atom of the Si4O13 chain. The Si1O4 tetra­hedron shares the O2–O4 edge with the GdO7 polyhedron. The [9+1]-coordinated Ba atoms are located in voids of the atomic arrangement, with Ba—O distances in the range 2.699 (3)–3.319 (3) Å.

Figure 2
A view of Ba2Gd2(Si4O13) along [010]. Dimers built of edge-sharing GdO7 polyhedra are linked via a zigzag-shaped finite Si4O13 chain (SiO4 tetra­hedra are marked with crosses). Ba atoms are shown as spheres. The unit cell is outlined.

The two non-equivalent SiO4 tetra­hedra form a tetra­silicate group (Si4O13) which can also be described as a finite chain. The bond-length distortion of the SiO4 tetra­hedra is remarkable (Table 1 [triangle]) and more pronounced than in comparable di- and tri­silicates. The bond-angle distortion is also very strong; for the Si1O4 and Si2O4 tetra­hedra the distortion parameters σ(oct)2 (Robinson et al., 1971 [triangle]) are 38.60 and 22.09, respectively. The most distorted SiO4 units are the two Si1-centred ones in the centre of the finite chain. This can be explained by the electrostatic repulsion between Si1 and the neighbouring Si1 and Si2 atoms. A similarly strong distortion of SiO4 tetra­hedra is observed in other chemically related tetra­silicates such as Ba2Nd2(Si4O13) (range of Si—O bond lengths in the most distorted SiO4 unit is 1.582–1.670 Å; Tamazyan & Malinovskii, 1985 [triangle]).

Table 1
Selected geometric parameters (Å, °)

The configuration of the finite C 2-symmetric tetra­silicate group (Si4O13) in (I) has a zigzag shape, with Si(...)Si(...)Si angles of 99.88 (5)° (Table 1 [triangle]). The chain configuration in (I) may be compared with those in the few other tetra­silicates reported so far (Fig. 3 [triangle]). The first of these was only described in 1979 [Ag10(Si4O13); Jansen & Keller, 1979 [triangle]]. The Si4O13 unit in this compound is zigzag-shaped as well (Fig. 3 [triangle] b). A similar zigzag configuration is also shown by Na4Sc2(Si4O13) (Maksimov et al., 1980 [triangle]; Fig. 3 [triangle] c), Ba2Nd2(Si4O13) (Tamazyan & Malinovskii, 1985 [triangle]; Fig. 3 [triangle] d) and K5Eu2F(Si4O13) (Chiang et al., 2007 [triangle]; Fig. 3 [triangle] e). Two further reported tetra­silicates contain Si4O13 units but also additional silicate groups. Both Ag18(SiO4)2(Si4O13) (Heidebrecht & Jansen, 1991a [triangle],b [triangle]) and La6(Si4O13)(SiO4)2 (Müller-Bunz & Schleid, 2002 [triangle]; I-type modification of La2Si2O7) contain isolated SiO4 groups. The Ag silicate is characterized by an Si4O13 unit with a stretched zigzag configuration (Fig. 3 [triangle] f). The La silicate has an unusual horseshoe-shaped Si4O13 unit (Fig. 3 [triangle] g), probably because of the additional presence of two isolated SiO4 tetra­hedra.

Figure 3
An overview of the configurations of the Si4O13 unit in the known tetra­silicates. (a) Ba2Gd2(Si4O13), (b) Ag10(Si4O13), (c) Na4Sc2(Si4O13), (d) Ba2Nd2(Si4O13), (e) K5Eu2F(Si4O13), (f) Ag18(SiO4)2(Si4O13) and (g) La6(Si4O13)(SiO4)2.

Fig. 3 [triangle] clearly demonstrates that the configuration in (I) (Fig. 3 [triangle] a) is most similar to that in Ag10(Si4O13) (Fig. 3 [triangle] b) and least similar to those in La6(Si4O13)(SiO4)2 (Fig. 3 [triangle] g; horseshoe-shaped unit) and Na4Sc2Si4O13 (Fig. 3 [triangle] c; twisted unit). This similarity is also reflected in the Si(...)Si(...)Si angles in these silicates: the angle in (I), 99.88 (5)° (×2), is fairly similar to the Si(...)Si(...)Si angles in Ag10(Si4O13) (101.0 and 110.2°) and K5Eu2F(Si4O13) (102.8 and 104.3°), whereas the values in Ba2Nd2(Si4O13) are distinctly smaller (85.1 and 85.3°), and those in Na4Sc2(Si4O13) (125.7 and 126.0°) and Ag18(SiO4)2(Si4O13) (2 × 125.9°) are distinctly larger. The unusual horseshoe-shaped unit in La6(Si4O13)(SiO4)2 shows fairly small Si(...)Si(...)Si angles, 91.0 and 100.7°. The cation radii of the non-Si cations in these silicates appear to be negatively correlated with the Si(...)Si(...)Si angle: the silicate with the largest cations [Ba2Nd2(Si4O13)] is characterized by the smallest Si(...)Si(...)Si angles, whereas the silicate with the smallest cations [Na4Sc2(Si4O13)] exhibits the largest Si(...)Si(...)Si angles. Compound (I) and K5Eu2F(Si4O13) show an inter­mediate behaviour.

The connectivities in the crystal structures of Na4Sc2(Si4O13), K5Eu2F(Si4O13) and Ba2Nd2(Si4O13) are slightly similar to that in (I). The first silicate contains Sc2O10 dimers (composed of two ScO6 octa­hedra sharing one common edge), comparable with the Gd2O12 dimers in (I). In contrast with (I), no edge is shared between an Sc-centred polyhedron and an SiO4 tetra­hedron. K5Eu2F(Si4O13) contains Eu2O10F dimers composed of two corner-sharing EuO5F octa­hedra (the F atom is the shared atom). In Ba2Nd2(Si4O13), NdO8 polyhedra are edge-linked to each other, thereby forming an incomplete polyhedral layer parallel to (011). Despite the strong similarity of the respective chain units, the connectivity in Ag10(Si4O13) bears no similarity to that in (I), and this is most likely due to the irregular coordination environ­ments of the Ag atoms and a much higher metal:Si ratio.

Although the structural formula of the complex silicate NaBa3Nd3(Si2O7)(Si4O13) (Malinovskii et al., 1983 [triangle]) again suggests that this compound also contains finite chains, the Si4O13 unit is in fact a branched finite Si3O10 chain (insular tetra­mer). The finite chain anion (Si4O13)10− has not only been found in silicates, but also in acid aqueous solutions during the trimethyl­silylation of Ag10(Si4O13) (Calhoun et al., 1980 [triangle]).

Inter­estingly, tetra­silicates containing finite Si4O13 chains have not been reported yet in nature. Futhermore, only one germanate is known that contains finite Ge4O13 chains, namely Cu2Fe2(Ge4O13) (Masuda et al., 2003 [triangle]); it is characterized by a slightly curved chain (with all GeO4 tetra­hedra in an eclipsed orientation), a geometry completely different from those of the above silicates. A larger number of phosphates with finite P4O13 chains are known (see references cited in Alekseev et al., 2009 [triangle]), but only a single arsenate with a finite As4O13 chain was described very recently {Ag6[(UO2)2(As2O7)(As4O13)], with a zigzag configuration of the As4O13 unit; Alekseev et al., 2009 [triangle]}. Among vanadates, there are only three known examples containing finite V4O13 chains. All of these are characterized by a horseshoe-shaped configuration {Ba3[V4O13] (Gatehouse et al., 1987 [triangle]), Fe2[V4O13] (Permer & Laligant, 1997 [triangle]) and ([{NH3(CH2)3NH2}Zn]2 3+[V4O13]6− (Natarajan, 2003 [triangle])}. If chromates are considered, one finds again a larger number of examples, such as K2[Cr4O13] (Casari & Langer, 2005 [triangle]) and Cs2[Cr4O13] (Kolitsch, 2004 [triangle]), all with a zigzag configuration.

Experimental

The title compound was crystallized from a BaO–Rb2O–MoO3 flux containing dissolved precursor compounds of Ba, Rb, Gd and Si. The experimental parameters were: BaCO3 (1.0022 g), Rb2CO3 (0.5997 g), MoO3 (1.0020 g), Gd2O3 (0.1727 g) and SiO2 (0.1611 g); Pt crucible covered with a lid, T max = 1423 K, holding time = 3 h, cooling rate = 2 K h−1, T min = 1173 K, slow cooling to room temperature after switching off the furnace. The reaction products were recovered by dissolving the Rb–molybdate flux solvent in distilled water. Ba2Gd2(Si4O13) formed small colourless pseudotetra­gonal prisms, which were accompanied by tiny prisms of BaY2Si3O10 type (Kolitsch et al., 2006 [triangle]) and BaREE2 3+Si3O10 (REE = Gd, Er, Yb and Sc; Wierzbicka-Wieczorek, 2007 [triangle]).

Crystal data

  • Ba2Gd2(Si4O13)
  • M r = 909.52
  • Monoclinic, An external file that holds a picture, illustration, etc.
Object name is c-66-00i29-efi1.jpg
  • a = 12.896 (3) Å
  • b = 5.212 (1) Å
  • c = 17.549 (4) Å
  • β = 104.08 (3)°
  • V = 1144.1 (5) Å3
  • Z = 4
  • Mo Kα radiation
  • μ = 18.73 mm−1
  • T = 293 K
  • 0.08 × 0.08 × 0.02 mm

Data collection

  • Bruker APEXII CCD area-detector diffractometer
  • Absorption correction: multi-scan (SADABS; Bruker, 2006 [triangle]) T min = 0.316, T max = 0.706
  • 11887 measured reflections
  • 2625 independent reflections
  • 1910 reflections with I > 2σ(I)
  • R int = 0.032

Refinement

  • R[F 2 > 2σ(F 2)] = 0.026
  • wR(F 2) = 0.046
  • S = 1.04
  • 2625 reflections
  • 97 parameters
  • Δρmax = 1.30 e Å−3
  • Δρmin = −1.65 e Å−3

The highest residual electron-density peak is 0.75 Å from the Gd1 site and the deepest hole is 1.24 Å from the Ba1 site.

Data collection: APEX2 (Bruker, 2008 [triangle]); cell refinement: SAINT (Bruker, 2008 [triangle]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: ATOMS (Shape Software, 1999 [triangle]); software used to prepare material for publication: SHELXL97.

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S0108270110002842/lg3027sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S0108270110002842/lg3027Isup2.hkl

Acknowledgments

Financial support from the Austrian Science Foundation (FWF) (grant No. P15220-N06) is gratefully acknowledged.

Footnotes

Supplementary data for this paper are available from the IUCr electronic archives (Reference: LG3027). Services for accessing these data are described at the back of the journal.

References

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Articles from Acta Crystallographica Section C: Crystal Structure Communications are provided here courtesy of International Union of Crystallography