Ba₂Gd₂(Si₄O₁₃): a silicate with finite Si₄O₁₃ chains

The title compound, dibarium digadolinium(III) tetra­silicate, crystallized from a molybdate-based flux. It represents a new structure type and contains finite zigzag-shaped C₂-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.

S1. 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 M³⁺ cations (M = Sc, V, Cr, Fe, In, Y, 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 (SiO₄–M⁴⁺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; Wierzbicka-Wieczorek, 2007; see also Kolitsch et al., 2009). 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⁺ yielded so far the following three Gd silicates: Rb₃GdSi₈O₁₉ and Cs₃GdSi₈O₁₉, both isotypic with Cs₃ScSi₈O₁₉ (Kolitsch & Tillmanns, 2004), and BaKGdSi₂O₇, isotypic with the disilicates BaKREESi₂O₇ (REE = Y, Yb, Sc) (Kolitsch et al., 2009) and SrKScSi₂O₇ (Wierzbicka-Wieczorek, 2007).

The asymmetric unit in monoclinic (I) contains one Ba, one Gd, two Si and seven O atoms (Fig. 1). 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): 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.)

The atomic arrangement is based on finite zigzag-shaped Si₄O₁₃ chains and Gd₂O₁₂ dimers built of edge-sharing GdO₇ polyhedra (Fig. 2). 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 atomic arrangement, with Ba—O distances in the range 2.699 (3)–3.319 (3) Å.

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) 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) 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).

The configuration of the finite tetrasilicate group (Si₄O₁₃) in (I) has a zigzag shape, with Si—Si—Si angles of 99.88 (5)° (Table 1). The chain configuration in the title compound may be compared with those in the few other tetrasilicates reported so far (Fig. 3). The first of these was described only in 1979 (Ag₁₀[Si₄O₁₃]; Jansen & Keller, 1979). The Si₄O₁₃ unit in this compound is zigzag-shaped as well (Fig. 3b). A similar zigzag configuration is also shown by Na₄Sc₂[Si₄O₁₃] (Maksimov et al., 1980; Fig. 3c), Ba₂Nd₂[Si₄O₁₃] (Tamazyan & Malinovskii, 1985; Fig. 3d) and K₅Eu₂F[Si₄O₁₃] (Chiang et al., 2007; Fig. 3e). Two further reported tetrasilicates contain Si₄O₁₃ units but also additional silicate groups. Both Ag₁₈[SiO₄]₂[Si₄O₁₃] (Heidebrecht & Jansen, 1991a,b) and La₆[Si₄O₁₃][SiO₄]₂ (Müller-Bunz & Schleid, 2002; I-type modification of La₂Si₂O₇) contain isolated SiO₄ groups. The Ag silicate is characterized by an Si₄O₁₃ unit with a stretched zigzag configuration [Si—Si—Si = 125.9° (2×); Fig. 3f]. The La silicate has an unusual horseshoe-shaped Si₄O₁₃ unit (Si—Si—Si = 91.0 and 100.7°; Fig. 3g), probably because of the additional presence of two isolated SiO₄ tetrahedra.

Fig. 3 clearly demonstrates that the configuration in (I) (Fig. 3a) is most similar to that in Ag₁₀[Si₄O₁₃] (Fig. 3b) and least similar to those in La₆[Si₄O₁₃][SiO₄]₂ (Fig. 3g; horseshoe-shaped unit) and Na₄Sc₂Si₄O₁₃ (Fig. 3c; 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× 126.0°) 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 certainly 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) again also suggests that this compound 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).

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); 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), 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}. 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), Fe₂[V₄O₁₃] (Permer & Laligant, 1997) and ([(NH₃(CH₂)₃NH₂)Zn]³⁺₂[V₄O₁₃]^6- (Natarajan, 2003)}. If chromates are considered, one finds again a larger number of examples, such as K₂[Cr₄O₁₃] (Casari & Langer, 2005) and Cs₂[Cr₄O₁₃] (Kolitsch, 2004), all with a zigzag configuration.

S2. 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) and BaREE³⁺₂Si₃O₁₀ (REE = Gd, Er, Yb, Sc) (Wierzbicka-Wieczorek, 2007).

S3. Refinement

The highest residual electron-density peak in (I), 1.30 e Å^-3, is at a distance of 0.75 Å from the Gd1 site. The deepest hole in the difference map, -1.65 e Å^-3, is at a distance of 1.24 Å from the Ba1 site.

Computing details

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

Figures

	Fig. 1. A view of the basic connectivity in Ba2Gd2[Si4O13], shown with displacement ellipsoids at the 50% probability level. [Symmetry codes: (i) -x + 1/2, y + 1/2, -z + 1/2; (ii) -x, y + 1, -z + 1/2; (iii) x, y + 1, z; (iv) x + 1/2, -y + 1/2, z + 1/2; (v) -x, y, -z + 1/2; (vi) x + 1/2, y + 1/2, z; (vii) -x, -y + 1, -z; (viii) -x, -y, -z.]
	Fig. 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.
	Fig. 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.

dibarium digadolinium(III) tetrasilicate

Crystal data

Ba2Gd2(Si4O13)	F(000) = 1600
Mr = 909.52	Dx = 5.280 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 11887 reflections
a = 12.896 (3) Å	θ = 1.0–35.6°
b = 5.212 (1) Å	µ = 18.73 mm^−1
c = 17.549 (4) Å	T = 293 K
β = 104.08 (3)°	Fragment, colourless
V = 1144.1 (5) Å^3	0.08 × 0.08 × 0.02 mm
Z = 4

Data collection

Bruker APEXII CCD area-detector diffractometer	2625 independent reflections
Radiation source: fine-focus sealed tube	1910 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
ϕ and ω scans	θmax = 35.6°, θmin = 2.4°
Absorption correction: multi-scan (SADABS; Bruker, 2006)	h = −20→20
Tmin = 0.316, Tmax = 0.706	k = −8→8
11887 measured reflections	l = −26→28

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.026	w = 1/[σ^2(Fo^2) + (0.015P)^2 + 4.2P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.046	(Δ/σ)max = 0.001
S = 1.04	Δρmax = 1.30 e Å^−3
2625 reflections	Δρmin = −1.65 e Å^−3
97 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.000166 (19)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ba1	0.162201 (16)	0.49502 (5)	0.332923 (11)	0.00826 (5)
Gd1	0.114286 (13)	0.50948 (4)	0.084515 (9)	0.00546 (4)
Si1	0.06246 (8)	0.04460 (18)	0.18398 (6)	0.00503 (18)
Si2	−0.11862 (8)	−0.03336 (19)	0.04351 (6)	0.00487 (18)
O1	0.0804 (2)	−0.2572 (5)	0.18914 (16)	0.0092 (5)
O2	0.1652 (2)	0.2257 (5)	0.19365 (15)	0.0074 (5)
O3	0.0000	0.1445 (7)	0.2500	0.0065 (7)
O4	−0.0187 (2)	0.1427 (5)	0.09994 (16)	0.0060 (5)
O5	−0.1569 (2)	0.0979 (5)	−0.04077 (16)	0.0084 (5)
O6	−0.2058 (2)	−0.0416 (5)	0.09656 (15)	0.0080 (5)
O7	−0.0710 (2)	−0.3241 (5)	0.03820 (16)	0.0072 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.00868 (10)	0.00808 (9)	0.00807 (9)	−0.00205 (9)	0.00215 (7)	−0.00115 (9)
Gd1	0.00517 (7)	0.00608 (8)	0.00523 (7)	0.00024 (7)	0.00141 (5)	0.00005 (7)
Si1	0.0061 (4)	0.0037 (5)	0.0054 (4)	0.0007 (3)	0.0018 (3)	−0.0003 (3)
Si2	0.0049 (4)	0.0044 (4)	0.0051 (4)	0.0000 (3)	0.0010 (3)	0.0000 (3)
O1	0.0165 (15)	0.0057 (11)	0.0057 (12)	0.0031 (10)	0.0032 (10)	−0.0002 (9)
O2	0.0045 (12)	0.0092 (12)	0.0083 (13)	0.0007 (9)	0.0015 (10)	0.0007 (9)
O3	0.0091 (18)	0.0074 (16)	0.0045 (16)	0.000	0.0045 (14)	0.000
O4	0.0050 (12)	0.0051 (11)	0.0066 (12)	−0.0003 (9)	−0.0012 (9)	−0.0008 (9)
O5	0.0083 (13)	0.0095 (12)	0.0069 (12)	−0.0006 (10)	0.0007 (10)	0.0017 (9)
O6	0.0065 (11)	0.0085 (13)	0.0102 (12)	0.0020 (9)	0.0043 (9)	0.0013 (9)
O7	0.0076 (12)	0.0050 (11)	0.0091 (13)	0.0006 (9)	0.0025 (10)	−0.0015 (9)

Geometric parameters (Å, º)

Ba1—O2^i	2.669 (3)	Gd1—O1^iii	2.331 (3)
Ba1—O6^ii	2.710 (3)	Gd1—O2	2.382 (3)
Ba1—O1^iii	2.802 (3)	Gd1—O7^iii	2.485 (3)
Ba1—O2	2.827 (3)	Gd1—O4	2.625 (3)
Ba1—O5^iv	2.841 (3)	Si1—O1	1.589 (3)
Ba1—O3	2.890 (2)	Si1—O2	1.602 (3)
Ba1—O7^ii	2.947 (3)	Si1—O3	1.6481 (15)
Ba1—O4^v	3.041 (3)	Si1—O4	1.667 (3)
Ba1—O6^v	3.055 (3)	Si2—O5	1.595 (3)
Ba1—O1^ii	3.319 (3)	Si2—O6	1.625 (3)
Gd1—O6^vi	2.294 (3)	Si2—O7	1.646 (3)
Gd1—O5^vii	2.299 (3)	Si2—O4	1.692 (3)
Gd1—O7^viii	2.302 (3)
O2^i—Ba1—O6^ii	65.74 (8)	O3—Ba1—O1^ii	68.96 (7)
O2^i—Ba1—O1^iii	77.42 (8)	O7^ii—Ba1—O1^ii	54.83 (7)
O6^ii—Ba1—O1^iii	89.49 (8)	O4^v—Ba1—O1^ii	68.58 (7)
O2^i—Ba1—O2	83.41 (6)	O6^v—Ba1—O1^ii	118.61 (7)
O6^ii—Ba1—O2	142.42 (8)	O6^vi—Gd1—O5^vii	79.36 (9)
O1^iii—Ba1—O2	62.08 (8)	O6^vi—Gd1—O7^viii	92.71 (10)
O2^i—Ba1—O5^iv	69.62 (8)	O5^vii—Gd1—O7^viii	94.86 (9)
O6^ii—Ba1—O5^iv	75.87 (8)	O6^vi—Gd1—O1^iii	111.54 (10)
O1^iii—Ba1—O5^iv	147.00 (8)	O5^vii—Gd1—O1^iii	84.51 (9)
O2—Ba1—O5^iv	114.11 (8)	O7^viii—Gd1—O1^iii	155.08 (10)
O2^i—Ba1—O3	136.81 (6)	O6^vi—Gd1—O2	77.24 (9)
O6^ii—Ba1—O3	146.92 (7)	O5^vii—Gd1—O2	141.23 (9)
O1^iii—Ba1—O3	76.72 (8)	O7^viii—Gd1—O2	116.63 (9)
O2—Ba1—O3	53.92 (6)	O1^iii—Gd1—O2	76.02 (9)
O5^iv—Ba1—O3	129.90 (8)	O6^vi—Gd1—O7^iii	160.24 (9)
O2^i—Ba1—O7^ii	119.71 (8)	O5^vii—Gd1—O7^iii	82.47 (9)
O6^ii—Ba1—O7^ii	56.04 (7)	O7^viii—Gd1—O7^iii	81.11 (10)
O1^iii—Ba1—O7^ii	114.24 (8)	O1^iii—Gd1—O7^iii	74.09 (10)
O2—Ba1—O7^ii	156.13 (7)	O2—Gd1—O7^iii	122.31 (9)
O5^iv—Ba1—O7^ii	82.01 (8)	O6^vi—Gd1—O4	125.27 (8)
O3—Ba1—O7^ii	102.34 (6)	O5^vii—Gd1—O4	153.87 (9)
O2^i—Ba1—O4^v	162.15 (7)	O7^viii—Gd1—O4	76.99 (9)
O6^ii—Ba1—O4^v	116.07 (7)	O1^iii—Gd1—O4	92.67 (9)
O1^iii—Ba1—O4^v	119.73 (8)	O2—Gd1—O4	61.58 (9)
O2—Ba1—O4^v	99.94 (8)	O7^iii—Gd1—O4	71.82 (8)
O5^iv—Ba1—O4^v	93.24 (7)	O1—Si1—O2	118.16 (15)
O3—Ba1—O4^v	51.61 (5)	O1—Si1—O3	111.22 (16)
O7^ii—Ba1—O4^v	60.10 (7)	O2—Si1—O3	105.81 (14)
O2^i—Ba1—O6^v	113.84 (7)	O1—Si1—O4	113.91 (14)
O6^ii—Ba1—O6^v	129.33 (10)	O2—Si1—O4	103.76 (14)
O1^iii—Ba1—O6^v	141.14 (7)	O3—Si1—O4	102.49 (11)
O2—Ba1—O6^v	81.70 (7)	O5—Si2—O6	116.19 (15)
O5^iv—Ba1—O6^v	59.52 (7)	O5—Si2—O7	112.21 (15)
O3—Ba1—O6^v	70.39 (7)	O6—Si2—O7	109.14 (14)
O7^ii—Ba1—O6^v	92.90 (7)	O5—Si2—O4	109.27 (14)
O4^v—Ba1—O6^v	50.17 (7)	O6—Si2—O4	102.33 (14)
O2^i—Ba1—O1^ii	127.37 (7)	O7—Si2—O4	106.89 (14)
O6^ii—Ba1—O1^ii	77.96 (7)	Si1^v—O3—Si1	143.2 (2)
O1^iii—Ba1—O1^ii	65.04 (9)	Si1—O4—Si2	125.18 (16)
O2—Ba1—O1^ii	107.64 (8)	Si2—Si1—Si1^v	99.88 (5)
O5^iv—Ba1—O1^ii	136.80 (7)

Symmetry codes: (i) −x+1/2, y+1/2, −z+1/2; (ii) −x, y+1, −z+1/2; (iii) x, y+1, z; (iv) x+1/2, −y+1/2, z+1/2; (v) −x, y, −z+1/2; (vi) x+1/2, y+1/2, z; (vii) −x, −y+1, −z; (viii) −x, −y, −z.