BaY₂Si₃O₁₀: a new flux-grown trisilicate

BaY₂Si₃O₁₀, barium diyttrium trisilicate, is a new silicate grown from a molybdate-based flux. The structure is based on zigzag chains, parallel to [010], of edge-sharing distorted YO₆ octa­hedra, linked by horseshoe-shaped trisilicate groups and Ba atoms in irregular eight-coordination. The layered character of the structure is caused by a succession of zigzag chains and trisilicate groups in planes parallel to (01). The Ba atoms occupy narrow channels extending parallel to [100]. The mean Y-O, Si-O and Ba-O bond lengths are 2.268, 1.626 and 1.633, and 2.872 Å, respectively. The two symmetry-equivalent terminal SiO₄ tetra­hedra in the Si₃O₁₀ unit adopt an eclipsed conformation with respect to the central SiO₄ tetra­hedron; the Si-O-Si and Si-Si-Si angles are 136.35 (9) and 96.12 (4)°, respectively. One Ba, one Si and two O atoms are located on mirror planes; all remaining atoms are in general positions. The geometry of isolated trisilicate groups in inorganic compounds is briefly discussed.

S1. Comment

As part of a comprehensive study of the synthesis (by hydrothermal and flux growth methods), crystal chemistry and topologies of M^I–(M^II–)M^III silicates (M^I = Na, K, Rb and Cs; M^II = Sr and Ba; M^III = Sc, V, Cr, Fe, In, Y and Yb) with mixed octahedral–tetrahedral frameworks (Wierzbicka et al., 2005), the title compound, (I), was prepared from a molybdate-based flux. Further new Ba–Y and Sr–Y silicates prepared by the flux method include BaKYSi₂O₇ (Kolitsch et al., 2006a), BaY₄Si₅O₁₇ (Kolitsch et al., 2006b) and SrY₂Si₃O₁₀ (Kolitsch et al., 2006c; Wierzbicka et al., 2006). The latter trisilicate is not isostructural with (I).

The asymmetric unit of (I) contains one Ba, one Y, two Si and six O atoms. The Ba, Si1, O1 and O2 atoms are located on mirror planes (y = 1/4); all remaining atoms are in general positions. The building units in the crystal structure of (I) are YO₆ octahedra, SiO₄ tetrahedra and eight-coordinated Ba atoms. The structure is based on zigzag chains parallel to [010] of edge-sharing distorted YO₆ octahedra, linked by horseshoe-shaped trisilicate (Si₃O₁₀) groups and Ba in irregular eight-coordination (Figs. 1 and 2); two further O neighbours (O6) of the Ba atom, each located 3.462 (2) Å away, are too remote to be considered as ligands.

A succession of octahedral zigzag chains and trisilicate groups in planes parallel to (101) results in the octahedral–tetrahedral layered character of (I) (Fig. 1). The strong linkage of these two different polyhedral units along [100] results in a heteropolyhedral slab parallel to (001) that is linked to adjacent slabs only via atoms O5 and Ba; the latter occupy narrow channels extending parallel to [100] (Fig. 1a). The mean Y—O, Si—O and Ba—O bond lengths are 2.268, 1.626 and 1.633, and 2.872 Å, respectively. The horseshoe-shaped geometry of the trisilicate unit is noteworthy. The two symmetry-equivalent terminal Si2O₄ tetrahedra in this unit show an eclipsed conformation with respect to the central Si1O₄ tetrahedron; the Si1—O—Si2 and Si2—Si1—Si2 angles are 136.35 (9) and 96.12 (4)°, respectively.

Bond-valence sums for all atoms were calculated using the bond-valence parameters of Brese & O'Keeffe (1991). The bond-valence sums are 1.76 (Ba1), 3.05 (Y1), 3.98 (Si1), 3.91 (Si2), 2.14 (O1), 2.02 (O2), 1.96 (O3), 1.88 (O4), 1.99 (O5) and 1.92 valence units (O6), and thus are all reasonably close to ideal valencies. Although the sum calculated for Ba may appear somewhat low, it is well known that Ba compounds in general tend to give poor bond-valence sums (Brown & Wu, 1976).

Trisilicates such as (I) are uncommon representatives of the silicate family. In a first overview on natural and synthetic trisilicate compounds, Povarennykh et al. (1976) characterize only 17 compounds, and state `The scarcity of the minerals is due to the difficulty in forming stoichiometric compounds with such a large silicate anion and the instability of the Si₃O₁₀ group relative to Si₂O₇ and Si₃O₉ during mineral formation.' Few further examples of trisilicates have been described since then.

In the following we provide a brief overview of silicate compounds containing isolated Si₃O₁₀ units, including the protonated derivative Si₃O₈(OH)₂, where the two terminal SiO₄ groups are protonated. Trisilicate units show an notably variable geometry: They can appear nearly linear to slightly curved, with Si—Si—Si angles between about 160 and 150° {e.g. Na₂Ca₃[Si₃O₁₀] (Treushnikov et al., 1971) and isotypic Na₂Cd₃[Si₃O₁₀] (Simonov et al., 1977); ardennite, Mn₂(Mn,Ca)₂(Al,Mg)₆(OH)₆ [(As,V,Si)O₄][SiO₄]₂[Si₃O₁₀] (Donnay & Allmann, 1968, Pasero et al., 1994); high-pressure L-type REE₂Si₂O₇, where REE is a rare earth element (Fleet & Liu, 2004); B-type REE₄[Si₃O₁₀][SiO₄] (Felsche, 1972, Fleet & Liu, 2003, Hartenbach et al., 2003)].

More or less strongly curved units, with Si—Si—Si angles between about 134 and 103° occur in SrY₂[Si₃O₁₀] (Kolitsch et al., 2006c; Wierzbicka et al., 2006), in rosenhahnite, Ca₃[Si₃O₈(OH)₂] (Jeffery & Lindley, 1973; Wan et al., 1977), in Na₄Cd₂[Si₃O₁₀] (Simonov et al., 1968, 1978), and in the microporous compound MCV-2, Ba₂Na[Na_0.5,(H₂O)_0.5]₂Zr₂Si₆O₁₉·3H₂O (Si—Si—Si = 103.4°, Ferdov et al., 2005).

Finally, Si₃O₁₀ units with horseshoe-shaped geometries and Si—Si—Si angles ranging between 98.1 and 76.2° are encountered in kilchoanite, Ca₆[SiO₄][Si₃O₁₀] (Taylor, 1971), in the title compound, in kinoite, Ca₂Cu₂(H₂O)₂[Si₃O₁₀] (Laughon, 1971), in fluorthalenite-(Y)-type M₃F[Si₃O₁₀], where M = Y, Dy, Ho and Er (Yakubovich et al., 1988 or 1998; Schleid & Müller-Bunz, 1998; Müller-Bunz & Schleid, 2000), in K₃Y[Si₃O₈(OH)₂] (Maksimov et al., 1968), and in isotypic K₃Ho[Si₃O₈(OH)₂] (Ponomarev et al., 1988) and K₃Yb[Si₃O₈(OH)₂] (Filipenko et al., 1999).

It is conspicuous that the narrowest Si—Si—Si angles are predominantly shown by silicates containing either F atoms or OH groups (not bonded to Si) or [Si₃O₈(OH)₂]- groups. Preliminary studies of the several possible influences on the unusually variable geometry of Si₃O₁₀ groups in the mentioned compounds indicate that very narrow Si—Si—Si angles preferentially occur in structures where larger metal cations are octahedrally coordinated. A more detailed discussion, including a comparison with compounds containing Ge₃O₁₀ and Al₃O₁₀ groups, will be presented as part of the description of the crystal structure of SrY₂Si₃O₁₀ (Wierzbicka et al., 2006).

S2. Experimental

The title compound was grown from a BaO–Rb₂O–MoO₃ flux containing dissolved precursor compounds of Ba, Rb, Y and Si (experimental parameters: 1 g BaCO₃, 0.6 g Rb₂CO₃, 1 g MoO₃, 0.1614 g Y₂O₃, 0.1718 g SiO₂; Pt crucible covered with 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 furnace). The reaction products were recovered by dissolving the flux in distilled water. BaY₂Si₃O₁₀ formed small (up to about 0.10 mm) pseudohexagonal plates, accompanied by rectangular plates to thick tabular crystals of a silicate with the tentative chemical formula Rb₃YSi₈O₁₉, which, according to preliminary results, appears to be a superstructure variant of Cs₃ScSi₈O₁₉ (MCV-1; Kolitsch & Tillmanns, 2004) and isotypic Cs₃YSi₈O₁₉ (Kolitsch et al., 2006a). Preliminary results from single-crystal X-ray diffraction studies prove that an isotypic Yb analogue of the title compound can also be synthesized; this analogue has the following unit-cell parameters: a = 5.377 (1) Å, b = 12.117 (2) Å, c = 6.790 (1) Å, β = 106.50 (3)°) and V = 424.17 (12) ų.

S3. Refinement

The highest electron-density peak in (I) is at a distance of 0.98 Å from the O5 site. The deepest hole in the difference map is at a distance of 0.99 Å from the Ba1 site.

Computing details

Data collection: COLLECT (Nonius, 2005); cell refinement: SCALEPACK (Otwinowski et al., 2003); data reduction: DENZO (Otwinowski et al., 2003) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Shape Software, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The crystal structure of BaY2Si3O10, (a) projected along [100] and (b) projected along [001]. Zigzag chains parallel to [010] of edge-sharing distorted YO6 octahedra are linked by horseshoe-shaped trisilicate groups and Ba atoms (shown as spheres).
	Fig. 2. Connectivity in the structure of BaY2Si3O10, shown with displacement ellipsoids at the 50% probability level. [Symmetry codes: (i) x, −y + 1/2, z; (ii) x, y, z + 1; (iii) −x + 1, −y, −z + 1; (iv) −x, −y, −z + 1; (v) x − 1, y, z.]

barium diyttrium triscilicate

Crystal data

BaY2Si3O10	F(000) = 512
Mr = 559.43	Dx = 4.298 Mg m^−3
Monoclinic, P21/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yb	Cell parameters from 1640 reflections
a = 5.399 (1) Å	θ = 2.0–32.6°
b = 12.179 (2) Å	µ = 18.28 mm^−1
c = 6.852 (1) Å	T = 293 K
β = 106.37 (3)°	Fragment, colourless
V = 432.28 (14) Å^3	0.11 × 0.10 × 0.02 mm
Z = 2

Data collection

Nonius KappaCCD diffractometer	1636 independent reflections
Radiation source: fine-focus sealed tube	1578 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.013
ϕ and ω scans	θmax = 32.6°, θmin = 3.1°
Absorption correction: multi-scan (SCALEPACK; Otwinowski et al., 2003)	h = −8→8
Tmin = 0.238, Tmax = 0.711	k = −18→18
3139 measured reflections	l = −10→10

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)] = 0.016	w = 1/[σ^2(Fo^2) + (0.02P)^2 + 0.43P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.040	(Δ/σ)max = 0.001
S = 1.10	Δρmax = 0.97 e Å^−3
1636 reflections	Δρmin = −1.29 e Å^−3
80 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0116 (5)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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	0.76082 (3)	0.2500	0.01992 (2)	0.00954 (5)
Y1	0.15801 (3)	0.098858 (14)	0.68431 (2)	0.00637 (5)
Si1	0.58242 (13)	0.2500	0.48917 (10)	0.00570 (12)
Si2	0.30379 (9)	0.06205 (4)	0.21344 (7)	0.00579 (9)
O1	0.3723 (4)	0.2500	0.6158 (3)	0.0078 (3)
O2	0.8766 (3)	0.2500	0.6344 (3)	0.0069 (3)
O3	0.5483 (3)	0.14287 (12)	0.3397 (2)	0.0105 (2)
O4	0.4308 (3)	−0.05455 (11)	0.1907 (2)	0.0105 (2)
O5	0.1652 (3)	0.12347 (12)	0.0030 (2)	0.0107 (2)
O6	0.1002 (3)	0.05192 (12)	0.3511 (2)	0.0088 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba1	0.01005 (8)	0.01136 (8)	0.00778 (8)	0.000	0.00344 (5)	0.000
Y1	0.00760 (9)	0.00671 (9)	0.00475 (9)	−0.00103 (5)	0.00167 (6)	−0.00053 (5)
Si1	0.0050 (3)	0.0074 (3)	0.0048 (3)	0.000	0.0015 (2)	0.000
Si2	0.0062 (2)	0.0067 (2)	0.0046 (2)	0.00039 (16)	0.00165 (16)	0.00015 (15)
O1	0.0075 (8)	0.0071 (8)	0.0104 (8)	0.000	0.0050 (6)	0.000
O2	0.0058 (7)	0.0069 (8)	0.0068 (8)	0.000	−0.0004 (6)	0.000
O3	0.0087 (6)	0.0129 (6)	0.0105 (6)	−0.0034 (5)	0.0036 (5)	−0.0051 (5)
O4	0.0102 (6)	0.0090 (6)	0.0115 (6)	0.0020 (5)	0.0015 (5)	−0.0007 (5)
O5	0.0138 (6)	0.0127 (6)	0.0050 (6)	0.0047 (5)	0.0016 (5)	0.0015 (5)
O6	0.0094 (6)	0.0112 (6)	0.0070 (6)	−0.0038 (5)	0.0042 (4)	−0.0023 (4)

Geometric parameters (Å, º)

Si1—O1	1.611 (2)	Y1—O6	2.2883 (14)
Si1—O2	1.619 (2)	Y1—O1	2.2920 (11)
Si1—O3	1.6368 (14)	Y1—O2^v	2.3497 (12)
Si1—O3^i	1.6368 (14)	Ba1—O5^vi	2.7013 (14)
Si2—O4	1.6037 (15)	Ba1—O5^vii	2.7013 (14)
Si2—O5	1.6096 (15)	Ba1—O4^viii	2.8245 (14)
Si2—O6	1.6421 (14)	Ba1—O4^ix	2.8245 (14)
Si2—O3	1.6781 (15)	Ba1—O2^x	2.881 (2)
Y1—O5^ii	2.1935 (14)	Ba1—O1^x	2.965 (2)
Y1—O4^iii	2.2092 (15)	Ba1—O3^i	3.0392 (15)
Y1—O6^iv	2.2769 (14)	Ba1—O3	3.0392 (15)
O5^ii—Y1—O4^iii	85.04 (6)	O1—Si1—O2	112.73 (10)
O5^ii—Y1—O6^iv	93.14 (5)	O1—Si1—O3	111.41 (7)
O4^iii—Y1—O6^iv	111.25 (5)	O2—Si1—O3	107.61 (7)
O5^ii—Y1—O6	170.62 (5)	O1—Si1—O3^i	111.41 (7)
O4^iii—Y1—O6	99.51 (6)	O2—Si1—O3^i	107.61 (7)
O6^iv—Y1—O6	77.59 (5)	O3—Si1—O3^i	105.72 (11)
O5^ii—Y1—O1	103.05 (6)	O4—Si2—O5	115.34 (8)
O4^iii—Y1—O1	76.43 (6)	O4—Si2—O6	111.29 (8)
O6^iv—Y1—O1	162.79 (6)	O5—Si2—O6	109.39 (8)
O6—Y1—O1	86.05 (6)	O4—Si2—O3	105.88 (7)
O5^ii—Y1—O2^v	82.36 (6)	O5—Si2—O3	106.79 (8)
O4^iii—Y1—O2^v	141.78 (5)	O6—Si2—O3	107.73 (7)
O6^iv—Y1—O2^v	105.36 (5)	Si1—O3—Si2	136.35 (9)
O6—Y1—O2^v	98.62 (6)	Si2—Si1—Si2^i	96.12 (4)
O1—Y1—O2^v	71.59 (5)

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