β-Y₂Si₂O₇, a new thortveitite-type compound, determined at 100 and 280 K

A new form of Y₂Si₂O₇ (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc₂Si₂O₇, and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y³⁺ cation occupies a distorted octahedral site, with Y—O bond lengths in the range 2.239 (2)–2.309 (2) Å. The SiO₄ tetrahedron is remarkably regular, with Si—O bond lengths in the range 1.619 (2)–1.630 (2) Å. The bridging O atom of the Si₂O₇ pyrosilicate group shows a large anisotropic displacement perpendicular to the Si—O bond. Changes in lattice and structural parameters upon cooling are small with, however, a distinct decrease of the anisotropic displacement of the briding O atom. Structure solution and refinement in the non-centrosymmetric space group C2 are possible but do not yield a significantly different structure model. The Si—O—Si bond angle of the isolated Si₂O₇ groups is 179.2 (1)° at 280 K in C2 and 180° per symmetry in C2/m. The C2/m structure model is favoured.

S1. Comment

Thortveitite, Sc₂Si₂O₇, is one of the few scandium minerals representing silicates with isolated Si₂O₇ groups (sorosilicates). A unique feature of the thortveitite structure is the unusual Si—O—Si angle, which is reported to be 180° (Kimata et al., 1998; Bianchi et al., 1988; Smolin et al., 1973; Cruickshank et al., 1962). There has been considerable discussion about the correctness of this structure model, as the value of 180° is rather unusual for sorosilicates, where the corresponding angles are usually much smaller (130–140°; Liebau, 1986). This discussion is also concerned with the question of whether the correct space group for thortveitite is C2/m, C2 or Cm, all of which are possible on the basis of the diffraction symmetry. It was concluded, however, that the correct space group is C2/m, as it provides the most consistent bond lengths and angles, in spite of an Si—O—Si angle of 180° (Bianchi et al., 1988; Smolin et al., 1973; Cruickshank et al., 1962). The C2 structural model provided by Cruickshank et al. (1962) yields Si—O—Si angles of about 165°.

The present structure determination of Y₂Si₂O₇, (I), confirms the structural similarity of this particular polymorph to thortveitite. The structure of (I) consists of sheets of YO₆ octahedra perpendicular to the crystallographic c axis and separated by the Si₂O₇ pyrosilicate groups, which run parallel to the a axis. Fig. 1 shows an ellipsoid plot with the atomic nomenclature of (I) and Fig. 2 shows a polyhedral representation of the structure viewed along [001]. \sch

All the YO₆ octahedra in (I) share three edges with each other and form a distorted honeycomb arrangement. The Y—O bond lengths are in the range 2.239 (2)–2.309 (2) Å, with an average value of 2.256 Å. The octahedral site in (I) is significantly larger than that in Sc₂Si₂O₇ (mean Sc—O bond length 2.123 Å; Smolin et al., 1972), but comparable with that in Yb₂Si₂O₇ (mean Yb—O bond length 2.240 Å; Christensen, 1994). These differences in the average octahedral bond lengths reflect the differences in ionic radii of Y³⁺ (0.892 Å), Yb³⁺ (0.858 Å) and Sc³⁺ (0.72 Å) in octahedral coordination (Shannon & Prewitt, 1969). The deviations of the individual bond lengths from their mean value are small in (I) [BLD (bond-length distortion) 0.38%] but the quadratic variance of the octahedral angles (OAV; Robinson et al., 1971) is very high (OAV 219.2°), revealing that Y³⁺ occupies a very much distorted octahedral environment. Similar octahedral distortion parameters are also found for Yb₂Si₂O₇ (BLD 0.44%, OAV 215.2°) and Sc₂Si₂O₇ (BLD 0.58%, OAV 216.0°).

The isolated Si₂O₇ groups in (I) are packed in columns along the a axis. As mentioned above, the unusual feature of the thortveitite structure is the existence of collinear Si—O1—Si bonds, as the bridging O1 atom is located at a centre of symmetry at (0,0,1/2). Each terminal O atom (two O2 and one O3) of the SiO₄ tetrahedron is also part of two YO₆ octahedra. The triangular faces defined by the terminal O atoms in adjacent SiO₄ groups are oriented in opposite directions. These tetrahedra are markedly regular in (I). The Si—O bond lengths vary between 1.619 (2) and 1.630 (2) Å, with an average value of 1.627 Å, which is slightly larger than the average Si—O bond in Sc₂Si₂O₇ (1.624 Å; Smolin et al., 1973). The tetrahedral bond-length distortion in (I) is small (BLD 0.24%) and about half of the value calculated for Sc₂Si₂O₇ (BLD 0.55%; Smolin et al., 1973). Contrary to other thortveitite-type compounds, the Si—O1 bond in (I) (bridging the two pyro groups) is not the shortest; that is the Si—O2 bond. The Si—O1 bond length is about 0.01–0.015 Å longer in (I) than, for instance, in synthetic Sc₂Si₂O₇ (Si—O1 1.606 Å; Smolin et al., 1972) or in In₂Si₂O₇ (Si—O1 1.608 Å; Patzke et al., 2000). It is the longer Si—O1 bond which reduces the BLD in (I) compared with the other compounds investigated to date. In addition to the BLD, the SiO₄ tetrahedra in (I) are also more regular in terms of the tetrahedral angle variance (TAV; Robinson et al., 1971). The TAV is 4.63° in (I), compared with 8.3–14.1° in other natural and synthetic thortveitites. This difference is due to a smaller O2—Si—O3 angle in (I) and a larger (more ideal) O1—Si—O2 angle, which is 106.25 (10)° in (I) but ranges between 103.5 and 104.8° in natural (Bianchi et al., 1988; Kimata et al., 1998) and synthetic (Smolin et al., 1973) thortveitites.

A striking feature of (I) is the rather large anisotropic displacement parameter of the bridging O1 atom. There is a strong component of motion perpendicular to the Si—O bond, reflecting some displacement with respect to the linkage of the individual SiO₄ tetrahedra. For their natural thortveitite sample, (Sc_1.693Y_0.181Yb_0.095Fe_0.031)Si₂O₇, Kimata et al. (1998) found by bond-length-bond-strength calculations that the bridging O1 atom possesses an oversaturation of 2.12 valence units. The authors argued that large atomic displacements may arise from either overbonding or underbonding. Overbonding supposedly directs repulsive energy to the nearest neighbour atom, resulting in dynamic positional disorder. The correlation between overbonding of the bridging O1 atom and its large ansisotropic thermal motion is also valid for the four natural thortveitites studied by Bianchi et al. (1988) and for the synthetic thortveitite of Smolin et al. (1973). This correlation, however, does not hold true for Y₂Si₂O₇ where atom O1 is saturated (bond-valence sum Σs 1.98), atom O2 is slightly overbonded (Σs 2.16) and atom O3 appears to be underbonded (Σs 1.83). [Bond-valence calculations were performed using the parameters of Brese & O'Keeffe (1991) and Brown & Altermatt (1985).]

The structure of (I) was also investigated at 100 K, revealing only very minor changes (i.e. one standard uncertainty or less) in lattice parameters and bond lengths. On the other hand, the anisotropic displacements of all atoms decreased significantly (20–50%) between 280 and 100 K. The most pronounced reduction in anisotropy occurs for the bridging atom O1 of the Si₂O₇ group, for which U₂₂ decreases from 0.0286 (19) Ų at 280 K to 0.0126 (15) Ų at 100 K, and the U₂₂/U₁₁ ratio correspondingly drops by almost a factor of 2.

Since the first structure determination of Sc₂Si₂O₇ by Zachariasen (1930), several silicate compounds belonging to the thortveitite structure type have been synthesized and their complete structural data reported. These include Yb₂Si₂O₇ (Christensen, 1994), Pr₂Si₂O₇ (Felsche, 1971) and In₂Si₂O₇ (Reid et al., 1977; Gaewdang et al., 1994; Patzke et al., 2000). For (I), the thortveitite structure type has not been described to date, although the compound has been described in the literature. Ito & Johnson (1968) noted that Y₂Si₂O₇ shows four polymorphic forms (α, β, γ and δ) with increasing temperature, following the sequence α → β (1498 K), β → γ (1718 K) and γ → δ (1808 K). The α, β, γ and δ polymorphs correspond to the B, C, D and E types defined by Christensen (1994). The γ phase is monoclinic, space group P2₁/m (Batalieva & Pyatenko, 1971). The δ phase is orthorhombic, with Pnam (Diaz et al., 1990) or Pna2₁ symmetry (Smolin & Shepelev, 1970; Christiansen, 1994). Diaz et al. (1990) noted that the γ phase transforms to an `α' phase with C2/m symmetry below 1718 K. This `α'-Y₂Si₂O₇ was described earlier by Batalieva et al. (1967), but no detailed structural data are available for this phase in the literature. It has to be noted here that, in both papers, the C2/m phase is wrongly identified as the α phase. Instead, this polymorph corresponds to the β phase, as shown by the the excellent match of its lattice parameter with those determined by Ito & Johnson (1968) for the β phase. Since the lattice parameters of (I) are very close to those given by Batalieva et al. (1967), with the same C2/m space group, it is concluded that (I) corresponds to β-Y₂Si₂O₇. On the other hand, α-Y₂Si₂O₇ is triclinic and, although no structural data have been reported to date, it is probably isotypic with triclinic α-Ho₂Si₂O₇ (Felsche, 1972) and Dy₂Si₂O₇ (Fleet & Liu, 2000). In contrast with the thortveitite structure, the structure of Dy₂Si₂O₇ contains linear triple tetrahedral [Si₃O₁₀] groups and isolated [SiO₄] tetrahedra, which are crosslinked by Dy³⁺ in one sixfold and three eightfold coordinated positions (Fleet & Liu, 2000). Therefore, the structural topologies of the α and β phases of Y₂Si₂O₇ seem to be quite distinct.

As mentioned before, β-Y₂Si₂O₇, (I) (C2/m), transforms to γ-Y₂Si₂O₇ (P2₁/m), and the structural topologies of these polymorphs are rather different from each other. In γ-Y₂Si₂O₇, the layers of YO₆ octahedra are still present, but the honeycomb arrangement observed in (I) is broken up. Instead, chains of cis-connected and edge-sharing YO₆ octahedra are formed parallel to the a axis. These chains are linked by common corners in the b direction via trans O atoms to form layers parallel to the ab plane. The γ-Y₂Si₂O₇ structure contains two different Y sites (within and between the octahedral chains), which are distinct in terms of polyhedral distortion (for the Y1 site, BLD 2.1% and OAV 352.4°; for the Y2 site, BLD 0.5% and OAV 262.3°) and which are both significantly underbonded (Σs 2.65 and 2.46 for Y1 and Y2, respectively). Another striking difference between the β and γ polymorphs of Y₂Si₂O₇ is the arrangement of the pyrosilicate groups. Whereas the Si—O—Si angle is 180° in the β phase, it is only 134° in the γ phase (Batalieva & Pyatenko, 1971). The latter contains two different tetrahedral sites, which are more distorted than in the β phase (for the Si1 site, BLD 1.2% and TAV 25.2°; for the Si2 site, BLD 0.9% and TAV 51.8°). Bond-valence calculations show that the bridging O atom of the pyrosilicate group in the γ phase is saturated (Σs 2.02), with adequate bond-valence sums around atoms Si1 (Σs 3.91) and Si2 (Σs 4.11). [It has been noted that the Inorganic Crystal Structure Database entry for γ-Y₂Si₂O₇ (28004) contains a typing error for the z coordinate of atom O5, which should read 0.736 instead of 0.786; ICSD reference?].

At about 1808 K, γ-Y₂Si₂O₇ transforms to orthorhomic (Pnam) δ-Y₂Si₂O₇, which is also structurally distinct from both the γ and β phases (Diaz et al., 1990). The δ-Y₂Si₂O₇ structure contains only one symmetry-independent Y site which, in contrast with the other two polymorphs, is seven-coordinate and forms a net of edge- and corner-sharing YO₇ polyhedra. The δ phase also contains two symmetry-non-equivalent Si sites with similar tetrahedral angular distortions (for Si1, TAV 68.4°; for Si2, TAV 63.7°) but different bond-length distortions (BLD 2.1 and 0.5%, respectively). Within the pyrosilicate groups, the Si—O—Si angle is 157.3° and the bridging O atom is saturated (Σs 2.06). The bond-valence sum for the bridging O atom seems to increase slightly from the β to the γ and δ polymorphs, perhaps corresponding to the fact that this atom in δ-Y₂Si₂O₇ is not only bonded to the two Si atoms but also to the Y atom. Overall, among the three polymorphs of Y₂Si₂O₇, the thortveitite-type β phase contains the most regular YO₆ and SiO₄ polyhedra.

S2. Experimental

Single crystals of Y₂Si₂O₇, (I), were obtained while attempting to synthesize the clinopyroxene compound NaYSi₂O₆ using high-temperature solution (flux) growth methods. Na₂CO₃, Y₂O₃ and SiO₂ were mixed in the proportions corresponding to the chemical composition of NaYSi₂O₆. The carefully ground powders were mixed with Na₂MoO₄ as a high-temperature flux (nutrient:flux ratio of 1:10) and placed in a covered platinum crucible. The crucible was slowly heated to 1473 K, kept at this temperature for 24 h and then slowly cooled (2 K h⁻¹) to 673 K. As synthesis experiments have shown, the clinopyroxene phase NaYSi₂O₆ is not stable under these experimental conditions and, after dissolving the molybdate flux in boiling water, the product consisted of colourless cuboid-shaped crystals of (I), large thin platelets of Na₂Si₂O₅ and cuboid colourless crystals of an yttrium orthosilicate oxyapatite which will be described elsewhere.

S3. Refinement

Structure solution using the 280 K data was initially done in space group C2, as E statistics suggested a non-centrosymmetric space group (E²-1 = 0.745). The noncentrosymmetric structural model found with the Patterson method could be refined down to R1(all) = 2.5% and wR2(all) = 5.3%. However, the displacement parameters for two of the four O atoms became non-positive definite. Validation tests of the final C2 structure model using the ADDSYM option of PLATON (Spek, 2003) clearly showed the presence of additional symmetry (mirror plane), suggesting that C2/m was the correct space group. Thus a new structure solution was tried, revealing the known thortveitite structure type and yielding better final residual values (with fewer refined parameters) and positive definite displacement parameters for all atoms, even for the data before absorption correction. The differences between the C2/m and C2 structural models are very small. In C2, two different Y sites exist, Y1 and Y2. The Y1—O bond lengths vary between 2.231 (7) and 2.341 (7) Å (mean 2.274 Å), and the Y2—O bond lengths vary between 2.244 (7) and 2.285 (9) Å (mean 2.260 Å). The SiO₄ tetrahedron in the C2 structure is somewhat more distorted, with bond lengths between 1.591 (9) and 1.659 (9) Å (mean 1.625 Å). In contrast with the C2 model provided by Cruickshank et al. (1962), our C2 model contains an Si—O—Si angle of 179.2 (1)°. As in C2/m, the bridging O atom shows a large anisotropic displacement perpendicular to the Si—O bond.

Computing details

For both compounds, data collection: X-AREA (Stoe & Cie, 19??); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997). Molecular graphics: DIAMOND (Brandenburg & Berndt, 1999) for 280K. Software used to prepare material for publication: WinGX (Farrugia, 1999) for 280K.

Figures

	Fig. 1. A view of the structure of (I) at 280 K, with 95% probability displacement ellipsoids [symmetry codes: (i) x, −y, z; (ii) −x, y, 1 − z; (iii) −x, −y, 1 − z; (iv) 1/2 − x, 1/2 − y, 1/2 − z; (v) 1/2 − x, 1/2 + y, 1/2 − z; (vi) x − 1/2, 1/2 + y, z − 1; (vii) x − 1/2, 1/2 − y, z; (viii) −x, y, −z].
	Fig. 2. The structure of (I) at 280 K viewed down the [001] direction, showing the distorted honeycomb arrangement of the YO6 octahedra.

(280K) diyttrium disilicate

Crystal data

O7Si2Y2	F(000) = 324
Mr = 345.98	Dx = 4.042 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 2329 reflections
a = 6.8691 (16) Å	θ = 3.8–32.1°
b = 8.960 (2) Å	µ = 20.72 mm^−1
c = 4.7168 (11) Å	T = 280 K
β = 101.730 (18)°	Cuboid, pale yellow
V = 284.26 (12) Å^3	0.17 × 0.15 × 0.14 mm
Z = 2

Data collection

STOE IPDS 2 diffractometer	497 reflections with I > 2σ(I)
rotation method scans	Rint = 0.039
Absorption correction: numerical via equivalents (X-SHAPE and X-RED; Stoe & Cie, 1996)	θmax = 32.2°, θmin = 3.8°
Tmin = 0.054, Tmax = 0.109	h = −10→9
1672 measured reflections	k = −13→11
520 independent reflections	l = −6→6

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0226P)^2 + 0.3813P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.019	(Δ/σ)max < 0.001
wR(F^2) = 0.045	Δρmax = 0.58 e Å^−3
S = 1.17	Δρmin = −0.71 e Å^−3
520 reflections	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
32 parameters	Extinction coefficient: 0.068 (3)

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.

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

	x	y	z	Uiso*/Ueq
Y	0.0	0.30621 (3)	0.0	0.00683 (12)
Si	0.21759 (11)	0.0	0.41320 (17)	0.00575 (16)
O1	0.0	0.0	0.5	0.0179 (7)
O2	0.3794 (3)	0.0	0.7153 (5)	0.0083 (4)
O3	0.2357 (2)	0.14921 (18)	0.2226 (3)	0.0093 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Y	0.00596 (15)	0.00616 (15)	0.00831 (16)	0.0	0.00131 (8)	0.0
Si	0.0053 (3)	0.0062 (3)	0.0057 (3)	0.0	0.0009 (2)	0.0
O1	0.0083 (13)	0.0286 (19)	0.0178 (16)	0.0	0.0050 (11)	0.0
O2	0.0083 (9)	0.0084 (9)	0.0071 (9)	0.0	−0.0009 (6)	0.0
O3	0.0097 (6)	0.0081 (7)	0.0103 (7)	0.0024 (5)	0.0029 (5)	0.0027 (5)

Geometric parameters (Å, º)

Y—O3^i	2.2386 (15)	Si—O2	1.619 (2)
Y—O3	2.2386 (15)	Si—O1	1.6280 (9)
Y—O2^ii	2.2479 (14)	Si—O3^viii	1.6301 (17)
Y—O2^iii	2.2479 (14)	Si—O3	1.6301 (17)
Y—O3^iv	2.3091 (17)	O1—Si^ix	1.6280 (9)
Y—O3^v	2.3091 (17)	O2—Y^ii	2.2479 (14)
Y—Y^vi	3.4729 (10)	O2—Y^x	2.2479 (14)
Y—Y^vii	3.5792 (8)	O3—Y^iv	2.3091 (17)
Y—Y^iv	3.5792 (8)
O3^i—Y—O3	102.13 (9)	O3^iv—Y—Y^vii	153.10 (4)
O3^i—Y—O2^ii	155.39 (7)	O3^v—Y—Y^vii	37.40 (4)
O3—Y—O2^ii	93.54 (6)	Y^vi—Y—Y^vii	106.347 (10)
O3^i—Y—O2^iii	93.54 (6)	O3^i—Y—Y^iv	115.19 (5)
O3—Y—O2^iii	155.39 (7)	O3—Y—Y^iv	38.79 (4)
O2^ii—Y—O2^iii	78.84 (8)	O2^ii—Y—Y^iv	88.87 (6)
O3^i—Y—O3^iv	117.13 (7)	O2^iii—Y—Y^iv	117.03 (6)
O3—Y—O3^iv	76.19 (6)	O3^iv—Y—Y^iv	37.40 (4)
O2^ii—Y—O3^iv	84.82 (7)	O3^v—Y—Y^iv	153.10 (4)
O2^iii—Y—O3^iv	79.81 (7)	Y^vi—Y—Y^iv	106.347 (10)
O3^i—Y—O3^v	76.19 (6)	Y^vii—Y—Y^iv	147.307 (19)
O3—Y—O3^v	117.13 (7)	O2—Si—O1	106.25 (10)
O2^ii—Y—O3^v	79.81 (7)	O2—Si—O3^viii	111.70 (7)
O2^iii—Y—O3^v	84.82 (7)	O1—Si—O3^viii	108.40 (6)
O3^iv—Y—O3^v	160.08 (8)	O2—Si—O3	111.70 (7)
O3^i—Y—Y^vi	128.93 (4)	O1—Si—O3	108.40 (6)
O3—Y—Y^vi	128.93 (4)	O3^viii—Si—O3	110.21 (12)
O2^ii—Y—Y^vi	39.42 (4)	Si—O1—Si^ix	180.000 (13)
O2^iii—Y—Y^vi	39.42 (4)	Si—O2—Y^ii	129.01 (4)
O3^iv—Y—Y^vi	80.04 (4)	Si—O2—Y^x	129.01 (4)
O3^v—Y—Y^vi	80.04 (4)	Y^ii—O2—Y^x	101.16 (8)
O3^i—Y—Y^vii	38.79 (4)	Si—O3—Y	130.17 (9)
O3—Y—Y^vii	115.19 (5)	Si—O3—Y^iv	123.22 (8)
O2^ii—Y—Y^vii	117.03 (6)	Y—O3—Y^iv	103.81 (6)
O2^iii—Y—Y^vii	88.87 (6)

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

(100K)

Crystal data

O7Si2Y2	F(000) = 324
Mr = 345.98	Dx = 4.042 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 2397 reflections
a = 6.8667 (16) Å	θ = 3.8–32.1°
b = 8.959 (2) Å	µ = 20.72 mm^−1
c = 4.7167 (11) Å	T = 100 K
β = 101.724 (18)°	Cuboid, pale yellow
V = 284.11 (12) Å^3	0.17 × 0.15 × 0.14 mm
Z = 2

Data collection

STOE IPDS 2 diffractometer	499 reflections with I > 2σ(I)
rotation method scans	Rint = 0.042
Absorption correction: numerical via equivalents (X-SHAPE and X-RED; Stoe & Cie, 1996)	θmax = 32.2°, θmin = 3.8°
Tmin = 0.057, Tmax = 0.111	h = −9→10
1660 measured reflections	k = −13→10
518 independent reflections	l = −6→6

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0237P)^2 + 0.6763P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.021	(Δ/σ)max < 0.001
wR(F^2) = 0.048	Δρmax = 0.75 e Å^−3
S = 1.12	Δρmin = −0.81 e Å^−3
518 reflections	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
32 parameters	Extinction coefficient: 0.046 (3)

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.

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

	x	y	z	Uiso*/Ueq
Y	0.0	0.30629 (3)	0.0	0.00438 (12)
Si	0.21793 (12)	0.0	0.41334 (17)	0.00385 (17)
O1	0.0	0.0	0.5	0.0105 (6)
O2	0.3796 (3)	0.0	0.7155 (5)	0.0063 (4)
O3	0.2357 (2)	0.14947 (19)	0.2223 (3)	0.0065 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Y	0.00449 (15)	0.00374 (15)	0.00486 (16)	0.0	0.00081 (9)	0.0
Si	0.0044 (3)	0.0037 (3)	0.0034 (3)	0.0	0.0007 (2)	0.0
O1	0.0069 (13)	0.0126 (15)	0.0118 (15)	0.0	0.0016 (11)	0.0
O2	0.0066 (9)	0.0068 (9)	0.0048 (8)	0.0	−0.0002 (7)	0.0
O3	0.0075 (6)	0.0052 (7)	0.0068 (7)	0.0018 (6)	0.0017 (5)	0.0010 (5)

Geometric parameters (Å, º)

Y—O3^i	2.2369 (16)	Si—O2	1.619 (2)
Y—O3	2.2369 (16)	Si—O1	1.6294 (9)
Y—O2^ii	2.2461 (15)	Si—O3^viii	1.6325 (18)
Y—O2^iii	2.2461 (15)	Si—O3	1.6325 (18)
Y—O3^iv	2.3070 (18)	O1—Si^ix	1.6294 (9)
Y—O3^v	2.3070 (18)	O2—Y^ii	2.2461 (15)
Y—Y^vi	3.4709 (10)	O2—Y^x	2.2461 (15)
Y—Y^vii	3.5784 (8)	O3—Y^v	2.3070 (18)
Y—Y^v	3.5784 (8)
O3^i—Y—O3	102.19 (9)	O3^v—Y—Y^vii	153.09 (4)
O3^i—Y—O2^ii	155.34 (8)	Y^vi—Y—Y^vii	106.371 (10)
O3—Y—O2^ii	93.54 (6)	O3^i—Y—Y^v	115.21 (5)
O3^i—Y—O2^iii	93.54 (6)	O3—Y—Y^v	38.74 (4)
O3—Y—O2^iii	155.34 (8)	O2^ii—Y—Y^v	88.91 (6)
O2^ii—Y—O2^iii	78.82 (9)	O2^iii—Y—Y^v	117.04 (6)
O3^i—Y—O3^iv	76.10 (7)	O3^iv—Y—Y^v	153.09 (4)
O3—Y—O3^iv	117.13 (7)	O3^v—Y—Y^v	37.36 (4)
O2^ii—Y—O3^iv	79.86 (8)	Y^vi—Y—Y^v	106.371 (10)
O2^iii—Y—O3^iv	84.87 (8)	Y^vii—Y—Y^v	147.26 (2)
O3^i—Y—O3^v	117.13 (7)	O2—Si—O1	106.23 (10)
O3—Y—O3^v	76.10 (7)	O2—Si—O3^viii	111.79 (8)
O2^ii—Y—O3^v	84.87 (8)	O1—Si—O3^viii	108.31 (7)
O2^iii—Y—O3^v	79.86 (8)	O2—Si—O3	111.79 (8)
O3^iv—Y—O3^v	160.21 (8)	O1—Si—O3	108.31 (6)
O3^i—Y—Y^vi	128.91 (4)	O3^viii—Si—O3	110.22 (13)
O3—Y—Y^vi	128.91 (4)	Si^ix—O1—Si	180
O3^iv—Y—Y^vi	80.11 (4)	Si—O2—Y^ii	129.00 (5)
O3^v—Y—Y^vi	80.11 (4)	Si—O2—Y^x	129.00 (5)
O3^i—Y—Y^vii	38.74 (4)	Y^ii—O2—Y^x	101.18 (9)
O3—Y—Y^vii	115.21 (5)	Si—O3—Y	130.21 (10)
O2^ii—Y—Y^vii	117.04 (6)	Si—O3—Y^v	123.09 (9)
O2^iii—Y—Y^vii	88.91 (6)	Y—O3—Y^v	103.90 (7)
O3^iv—Y—Y^vii	37.36 (4)

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