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

Article abstract of DOI:10.1107/S0108270103018869

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) A. The SiO₄ tetrahedron is remarkably regular, with Si-O bond lengths in the range 1.619 (2)-1.630 (2) A. 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.

Full text of DOI:10.1107/S0108270103018869

inorganic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
The present structure determination of Y Si O , (I),
2
2
7
con®rms 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 Si O pyrosilicate groups, which run parallel
ISSN 0108-2701
2
7
to the a axis. Fig. 1 shows an ellipsoid plot with the atomic
nomenclature of (I) and Fig. 2 shows a polyhedral repre-
sentation of the structure viewed along [001].
b-Y Si O , a new thortveitite-type
2
2
7
compound, determined at 100 and
80 K
All the YO octahedra in (I) share three edges with each
6
2
other and form a distorted honeycomb arrangement. The YÐ
Ê
O bond lengths are in the range 2.239 (2)±2.309 (2) A, with an
Ê
average value of 2.256 A. The octahedral site in (I) is signi®-
G uÈ nther J. Redhammer* and Georg Roth
cantly larger than that in Sc Si O (mean ScÐO bond length
2
2
7
Ê
of 2.123 A; Smolin et al., 1972), but comparable with that in
Institute of Crystallography, J aÈ gerstrasse 17/19, D-52056 Aachen, Germany
Correspondence e-mail: guenther.redhammer@aon.at
Ê
Yb Si O (mean YbÐO bond length of 2.240 A; Christensen,
2
2
7
1
lengths re¯ect the differences in ionic radii of Y (0.89 A),
994). These differences in the average octahedral bond
Ê
Received 17 June 2003
Accepted 26 August 2003
Online 30 September 2003
3+
3
+
3+
Ê
Ê
Yb (0.86 A) and Sc (0.72 A) in octahedral coordination
Shannon & Prewitt, 1969). The deviations of the individual
(
A new form of Y Si O (diyttrium heptaoxodisilicate) has
7
2
2
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
been synthesized which is isotypic with thortveitite, Sc Si O ,
2
2
7
and crystallizes in the centrosymmetric space group C2/m,
3
both at 100 and 280 K. The Y cation occupies a distorted
+
ꢀ
3+
high (OAV = 219.2 ), revealing that Y occupies a very much
distorted octahedral environment. Similar octahedral distor-
tion parameters are also found for Yb Si O (BLD = 0.44%
octahedral site, with YÐO bond lengths in the range
Ê
2
.239 (2)±2.309 (2) A. The SiO tetrahedron is remarkably
4
2
2
7
ꢀ
regular, with SiÐO bond lengths in the range 1.619 (2)±
and OAV = 215.2 ) and Sc Si O (BLD = 0.58% and OAV =
2
2
7
Ê
.630 (2) A. The bridging O atom of the Si O pyrosilicate
2 7
ꢀ
1
216.0 ).
The isolated Si O groups in (I) are packed in columns
group shows a large anisotropic displacement perpendicular to
the SiÐO 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 re®nement in the non-centrosymmetric space
group C2 are possible but do not yield a signi®cantly different
structure model. The SiÐOÐSi bond angle of the isolated
2
7
along the a axis. As mentioned above, the unusual feature of
the thortveitite structure is the existence of collinear SiÐO1Ð
Si bonds, as the bridging O1 atom is located at a centre of
1
2
symmetry at (0,0, ). Each terminal O atom (two O2 and one
O3) of the SiO tetrahedron is also part of two YO octahedra.
4
6
The triangular faces de®ned by the terminal O atoms in
adjacent SiO groups are oriented in opposite directions.
These tetrahedra are markedly regular in (I). The SiÐO bond
ꢀ
ꢀ
Si O groups is 179.2 (1) at 280 K in C2 and 180 per
2
7
4
symmetry in C2/m. The C2/m structure model is favoured.
Comment
Thortveitite, Sc Si O , is one of the few scandium minerals
2
2
7
representing silicates with isolated Si O groups (sorosili-
2
7
cates). A unique feature of the thortveitite structure is the
ꢀ
unusual SiÐOÐSi angle, which is reported to be 180 (Kimata
et al., 1998; Bianchi et al., 1988; Smolin et al., 1973; Cruick-
shank et al., 1962). There has been considerable discussion
about the correctness of this structure model, as the value of
ꢀ
1
sponding angles are usually much smaller (130±140 ; Liebau,
80 is rather unusual for sorosilicates, where the corre-
ꢀ
1
986). This discussion was 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
ꢀ
Figure 1
Aview of the structure of (I) at 280 K, with 95% probability displacement
and angles, in spite of an SiÐOÐSi 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
ellipsoids [symmetry codes: (i) x, � y, z; (ii) � x, y, 1 � z; (iii) � x, � y,
1
2
1
2
1
2
1
2
1
2
1
2
1 1
2 2
1
� z; (iv) � x, � y, � z; (v) � x, + y, � z; (vi) x � , + y, z � 1;
1 1
2 2
ꢀ
SiÐOÐSi angles of about 165 .
(vii) x � , � y, z; (viii) � x, y, � z].
Acta Cryst. (2003). C59, i103±i106
DOI: 10.1107/S0108270103018869
# 2003 International Union of Crystallography i103

inorganic compounds
Ê
lengths vary between 1.619 (2) and 1.630 (2) A, with an
Ê
average value of 1.627 A, which is slightly larger than the
(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 displace-
ments 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 brid-
ging O1 atom and its large anisotropic 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 Y Si O ,
Ê
average SiÐO bond length in Sc Si O (1.624 A; Smolin et al.,
2
2
7
1
(
973). The tetrahedral bond-length distortion in (I) is small
BLD = 0.24%) and about half the value calculated for
Sc Si O (BLD = 0.55%; Smolin et al., 1973). Contrary to
2
2
7
other thortveitite-type compounds, the SiÐO1 bond in (I)
bridging the two pyrosilicate groups) is not the shortest; the
shortest is the SiÐO2 bond. The SiÐO1 bond length is about
(
Ê
0
Sc Si O (SiÐO1 = 1.606 A; Smolin et al., 1972) or in In Si O
.010±0.015 A longer in (I) than, for instance, in synthetic
Ê
2
2
7
2
2
7
2
2
7
Ê
SiÐO1 = 1.608 A; Patzke et al., 2000). It is the longer SiÐO1
(
where atom O1 is saturated [bond-valence sum (Æs) = 1.98],
atom O2 is slightly overbonded (Æs = 2.16) and atom O3
appears to be underbonded (Æs = 1.83). [Bond-valence cal-
culations were performed using the parameters of Brese &
O'Keeffe (1991) and Brown & Altermatt (1985).]
bond which reduces the BLD in (I) compared with the other
compounds investigated to date. In addition to the BLD, the
SiO tetrahedra in (I) are also more regular in terms of the
4
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
O2ÐSiÐO3 angle in (I) and a larger (more ideal) O1ÐSiÐ
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 displacement parameters of all atoms decreased
signi®cantly (20±50%) between 280 and 100 K. The most
pronounced reduction in anisotropy occurs for the bridging
O1 atom of the Si O group, for which U decreases from
ꢀ
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 thortveitites (Smolin et al., 1973).
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 SiÐO1
bond, re¯ecting some displacement with respect to the linkage
2
7
22
2
2
Ê
Ê
0.0286 (19) A at 280 K to 0.0126 (15) A at 100 K, and the
U /U ratio correspondingly drops by almost a factor of 2.
2
2
11
Since the ®rst structure determination of Sc Si O by
2
2
7
of the individual SiO tetrahedra. For their natural thortveitite
4
Zachariasen (1930), several silicate compounds belonging to
the thortveitite structure type have been synthesized and their
complete structural data reported. These include Yb Si O
sample, i.e. (Sc1.693Y0.181Yb0.095Fe0.031)Si O , Kimata et al.
2
7
2
2
7
7
(
(
Christensen, 1994), Pr Si O (Felsche, 1971) and In Si O
2 2 7 2 2
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 Y Si O shows four
2
2
7
polymorphic forms (ꢁ, ꢀ, ꢂ and ꢃ) with increasing tempera-
ture, following the sequence ꢁ ! ꢀ (1498 K), ꢀ ! ꢂ (1718 K)
and ꢂ ! ꢃ (1808 K). The ꢁ, ꢀ, ꢂ and ꢃ polymorphs correspond
to the B, C, D and E types de®ned by Christensen (1994). The
ꢂ phase is monoclinic, space group P2 /m (Batalieva &
1
Pyatenko, 1971). The ꢃ phase is orthorhombic, with Pnam
(Diaz et al., 1990) or Pna2 symmetry (Smolin & Shepelev,
1
1
970; Christensen, 1994). Diaz et al. (1990) noted that the ꢂ
phase transforms to an `ꢁ' phase with C2/m symmetry below
718 K. This `ꢁ'-Y Si O was described earlier by Batalieva et
1
2
2
7
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 identi®ed as the ꢁ phase.
Instead, this polymorph corresponds to the ꢀ phase, as shown
by the excellent match of its lattice parameters with those
determined by Ito & Johnson (1968) for the ꢀ 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 ꢀ-Y Si O . On the other
2
2
7
hand, ꢁ-Y Si O is triclinic and, although no structural data
2
2
7
have been reported to date, it is probably isotypic with triclinic
-Ho Si O (Felsche, 1972) and Dy Si O (Fleet & Liu, 2000).
In contrast with the thortveitite structure, the structure of
Figure 2
The structure of (I) at 280 K viewed down the [001] direction, showing the
distorted honeycomb arrangement of the YO octahedra.
ꢁ
2
2
7
2
2
7
6
ꢁ
i104 Redhammer and Roth
2
ꢀ-Y Si
O
2 7
at 100 and 280 K
Acta Cryst. (2003). C59, i103±i106

inorganic compounds
Dy Si O contains linear triple tetrahedral [Si O ] groups and
3
temperature solution (¯ux) growth methods. Na
were mixed in the proportions corresponding to the chemical
composition of NaYSi . The carefully ground powders were mixed
with Na MoO as a high-temperature ¯ux (nutrient±¯ux ratio of 1:10)
2 3 2 3 2
CO , Y O and SiO
2
2
7
10
3
isolated [SiO ] tetrahedra, which are crosslinked by Dy in
+
4
2
O
6
one sixfold- and three eightfold-coordinated positions (Fleet
&
2
4
Liu, 2000). Therefore, the structural topologies of the ꢁ and
phases of Y Si O seem to be quite distinct.
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
ꢀ
2
2
7
As mentioned before, ꢀ-Y Si O (C2/m) transforms to ꢂ-
� 1
2
2
7
cooled (2 K h ) to 673 K. As synthesis experiments have shown,
Y Si O (P2 /m) and the structural topologies of these poly-
7
2
2
1
2 6
the clinopyroxene phase NaYSi O is not stable under these
morphs are rather different from each other. In ꢂ-Y Si O , the
2
2
7
experimental conditions and, after dissolving the molybdate ¯ux in
boiling water, the product consisted of colourless cuboid-shaped
crystals of (I), large thin platelets of Na Si O and colourless cuboid
layers of YO octahedra are still present, but the honeycomb
6
arrangement observed in (I) is broken up. Instead, chains of
cis-connected and edge-sharing YO octahedra are formed
2
2
5
6
crystals of a yttrium orthosilicate oxyapatite which will be described
elsewhere.
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 ꢂ-Y Si O structure contains two
Compound (I) at 280 K
2
2
7
different Y sites (within and between the octahedral chains),
which are distinct in terms of polyhedral distortion (for the Y1
Crystal data
�
3
2
Y Si
O
2 7
D
x
= 4.042 Mg m
ꢀ
site, BLD = 2.1% and OAV = 352.4 ; for the Y2 site, BLD =
ꢀ
M
r
= 345.98
Mo Kꢁ radiation
Cell parameters from 2329
re¯ections
0
.5% and OAV = 262.3 ) and which are both signi®cantly
Monoclinic, C2=m
Ê
a = 6.8691 (16) A
underbonded (Æs = 2.65 and 2.46 for Y1 and Y2, respectively).
Another striking difference between the ꢀ and ꢂ polymorphs
of Y Si O is the arrangement of the pyrosilicate groups.
Ê
b = 8.960 (2) A
ꢀ
ꢄ = 3.8±32.1
ꢅ = 20.72 mm
T = 280 (1) K
Ê
ꢀ
� 1
c = 4.7168 (11) A
ꢀ = 101.730 (18)
V = 284.26 (12) AÊ
Z = 2
2
2
7
3
ꢀ
Cuboid, pale yellow
0.17 Â 0.15 Â 0.14 mm
Whereas the SiÐOÐSi angle is 180 in the ꢀ phase, it is only
ꢀ
1
contains two different tetrahedral sites, which are more
34 in the ꢂ phase (Batalieva & Pyatenko, 1971). The latter
Data collection
distorted than in the ꢀ phase (for the Si1 site, BLD = 1.2% and
Stoe IPDS-2 diffractometer
Rotation scans
Absorption correction: numerical
via equivalents (X-SHAPE and
X-RED; Stoe & Cie, 1996)
520 independent re¯ections
497 re¯ections with I > 2ꢆ(I)
ꢀ
ꢀ
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 ꢂ phase is saturated (Æs = 2.02),
with adequate bond-valence sums around atoms Si1 (Æs =
Rint = 0.039
ꢀ
ꢄ
max = 32.2
h = � 10 ! 9
k = � 13 ! 11
l = � 6 ! 6
Tmin = 0.054, Tmax = 0.109
672 measured re¯ections
1
3
.91) and Si2 (Æs = 4.11). [It has been noted that the Inorganic
Re®nement
Crystal Structure Database (2003) entry for ꢂ-Y Si O (28004)
2
2
7
2
contains a typing error for the z coordinate of atom O5, which
should read 0.736 instead of 0.786.]
Re®nement on F
R(F) = 0.019
(Á/ꢆ)max < 0.001
Áꢇmax = 0.58 e A
Ê
� 3
Ê
� 3
2
wR(F ) = 0.045
Áꢇmin = � 0.71 e A
At about 1808 K, ꢂ-Y Si O transforms to orthorhombic
2
2
7
S = 1.17
520 re¯ections
Extinction correction: SHELXL97
Extinction coef®cient: 0.068 (3)
(
Pnam) ꢃ-Y Si O , which is also structurally distinct from both
2
2
7
3
2 parameters
2 2
the ꢂ and ꢀ phases (Diaz et al., 1990). The ꢃ-Y Si O structure
2
2
2
7
w = 1/[ꢆ (F
o
) + (0.0226P)
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 YO polyhedra.
+ 0.3813P]
where P = (F
2
o
2
c
+ 2F
)/3
7
The ꢃ phase also contains two symmetry-non-equivalent
Si sites with similar tetrahedral angular distortions (for Si1,
Table 1
Selected geometric parameters (A, ) for (I) at 280 K.
Ê
ꢀ
ꢀ
ꢀ
TAV = 68.4 ; for Si2, TAV = 63.7 ) but different bond-length
distortions (BLD = 2.1 and 0.5%, respectively). Within the
i
v
YÐO3
YÐO3
YÐO2
YÐO2
YÐO3
2.2386 (15)
2.2386 (15)
2.2479 (14)
2.2479 (14)
2.3091 (17)
YÐO3
2.3091 (17)
1.619 (2)
1.6280 (9)
1.6301 (17)
1.6301 (17)
ꢀ
SiÐO2
SiÐO1
SiÐO3
SiÐO3
pyrosilicate groups, the SiÐOÐSi angle is 157.3 and the
ii
bridging O atom is saturated (Æs = 2.06). The bond-valence
sum for the bridging O atom seems to increase slightly from
the ꢀ to the ꢂ and ꢃ polymorphs, perhaps corresponding to the
fact that this atom in ꢃ-Y Si O is not only bonded to the two
iii
iv
vi
i
O3 ÐYÐO3
v
102.13 (9)
155.39 (7)
93.54 (6)
93.54 (6)
155.39 (7)
78.84 (8)
117.13 (7)
76.19 (6)
84.82 (7)
79.81 (7)
76.19 (6)
O3ÐYÐO3
v
117.13 (7)
79.81 (7)
84.82 (7)
2
2
7
i
O3 ÐYÐO2
O3ÐYÐO2
ii
ii
O2 ÐYÐO3
iii v
O2 ÐYÐO3
O3 ÐYÐO3
Si atoms but also to the Y atom. Overall, among the three
ii
i
O3 ÐYÐO2
O3ÐYÐO2
iii
iv
v
polymorphs of Y Si O , the thortveitite-type ꢀ phase contains
160.08 (8)
106.25 (10)
111.70 (7)
108.40 (6)
111.70 (7)
108.40 (6)
110.21 (12)
180.000 (13)
2
2
7
iii
O2ÐSiÐO1
the most regular YO and SiO polyhedra.
6
4
ii
O2 ÐYÐO2
iii
vi
vi
O2ÐSiÐO3
O1ÐSiÐO3
O2ÐSiÐO3
i
O3 ÐYÐO3
O3ÐYÐO3
iv
iv
ii
O2 ÐYÐO3
iv
O1ÐSiÐO3
iii
O2 ÐYÐO3
iv
vi
O3 ÐSiÐO3
SiÐO1ÐSi
Experimental
i
O3 ÐYÐO3
v
vii
Single crystals of Y
2
Si
2
O
7
, (I), were obtained while attempting to
O
1
2
1
2
1 1
2
Symmetry codes: (i) � x; y; � z; (ii) � x; � y; 1 � z; (iii) x � 2
;
‡ y; z � 1; (iv)
1
2
1
2
1 1
synthesize the clinopyroxene compound NaYSi
2
6
using high-
� x; � y; � z; (v) x �
;
2 2
� y; z; (vi) x; � y; z; (vii) � x; � y; 1 � z.
ꢁ
Acta Cryst. (2003). C59, i103±i106
Redhammer and Roth
2 2
ꢀ-Y Si O
7
at 100 and 280 K i105

inorganic compounds
Compound (I) at 100 K
Y1 and Y2. The Y1ÐO bond lengths vary between 2.231 (7) and
Ê Ê
.341 (7) A (mean 2.274 A) and the Y2ÐO bond lengths vary
2
between 2.244 (7) and 2.285 (9) A (mean 2.260 A). The SiO
hedron in the C2 structure is more distorted, with bond lengths
Crystal data
Ê
Ê
4
tetra-
�
3
Y
2
Si O
2 7
D
x
= 4.042 Mg m
M
r
= 345.98
Mo Kꢁ radiation
Cell parameters from 2397
re¯ections
Ê
Ê
ranging between 1.591 (9) and 1.659 (9) A (mean 1.625 A). In
Monoclinic, C2=m
Ê
a = 6.8667 (16) A
contrast with the C2 model reported by Cruickshank et al. (1962), our
ꢀ
Ê
b = 8.959 (2) A
ꢀ
C2 model contains an SiÐOÐSi angle of 179.2 (1) , and as in C2/m,
the bridging O atom shows a large anisotropic displacement
perpendicular to the SiÐO bond.
ꢄ = 3.8±32.1
ꢅ = 20.72 mm
T = 100 (1) K
Ê
ꢀ
Ê
� 1
c = 4.7167 (11) A
ꢀ
= 101.724 (18)
3
V = 284.11 (12) A
Z = 2
Cuboid, pale yellow
0.17 Â 0.15 Â 0.14 mm
For compound (I) at both 100 and 280 K, data collection: X-AREA
(
Stoe & Cie, 2002); cell re®nement: X-AREA; data reduction: X-RED
Stoe & Cie, 1996); structure solution: SHELXS97 (Sheldrick, 1997);
Data collection
(
Stoe IPDS-2 diffractometer
Rotation scans
Absorption correction: numerical
via equivalents (X-SHAPE and
X-RED; Stoe & Cie, 1996)
518 independent re¯ections
499 re¯ections with I > 2ꢆ(I)
structure re®nement: SHELXL97 (Sheldrick, 1997). For (I) at 280 K,
molecular graphics: DIAMOND (Brandenburg & Berndt, 1999);
preparation of publication material: WinGX (Farrugia, 1999).
Rint = 0.042
ꢀ
ꢄ
max = 32.2
h = � 9 ! 10
k = � 13 ! 10
l = � 6 ! 6
GJR acknowledges ®nancial support from the Austrian
Academy of Science via an APART (Austrian Program for
Advanced Research and Technology) scholarship. The authors
also thank J. Barbier for constructive and useful comments.
Tmin = 0.057, Tmax = 0.111
1
660 measured re¯ections
Re®nement
2
2
2
2
Re®nement on F
R(F) = 0.021
o
w = 1/[ꢆ (F ) + (0.0237P)
+ 0.6763P]
where P = (F
2
wR(F ) = 0.048
S = 1.12
2
2
c
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: BC1023). Services for accessing these data are
described at the back of the journal.
o
+ 2F
)/3
(Á/ꢆ)max < 0.001
Áꢇmax = 0.75 e A
Ê
� 3
5
3
18 re¯ections
2 parameters
Ê
� 3
Áꢇmin = � 0.81 e A
Extinction correction: SHELXL97
Extinction coef®cient: 0.046 (3)
References
Batalieva, N. G., Bondar, I. A., Sidorenko, G. A. & Toropov, N. A. (1967).
Dokl. Akad. Nauk SSSR Ser. Chem. 173, 339±341.
Batalieva, N. G. & Pyatenko, Y. A. (1971). Kristallogra®ya, 16, 905±910.
Bianchi, R., Pilati, T., Diella, V., Gramaccioli, C. M. & Mannucci, G. (1988).
Am. Mineral. 73, 601±607.
Table 2
Selected geometric parameters (A, ) for (I) at 100 K.
Ê
ꢀ
i
v
YÐO3
YÐO3
YÐO2
YÐO2
YÐO3
2.2369 (16)
2.2369 (16)
2.2461 (15)
2.2461 (15)
2.3070 (18)
YÐO3
2.3070 (18)
1.619 (2)
1.6294 (9)
1.6325 (18)
1.6325 (18)
SiÐO2
SiÐO1
SiÐO3
SiÐO3
ii
Brandenburg, K. & Berndt, M. (1999). DIAMOND. Release 2.1b. Crystal
Impact GbR, Bonn, Germany.
iii
iv
vi
Brese, N. E. & O'Keeffe, M. (1991). Acta Cryst. B47, 192±197.
Brown, I. D. & Altermatt, D. (1985). Acta Cryst. B41, 244±247.
Christensen, A. N. (1994). Z. Kristallogr. 209, 7±13.
Cruickshank, D. W. J., Lynton, H. & Barclay, G. A. (1962). Acta Cryst. 15, 491±
498.
Diaz, H. W., Glasser, F. P., Gunwardane, R. P. & Howie, R. A. (1990). Z.
Kristallogr. 191, 117±123.
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837±838.
Felsche, J. (1971). Z. Kristallogr. 133, 365±385.
Felsche, J. (1972). Naturwissenschaften, 59, 35±36.
Fleet, M. E. & Liu, X. (2000). Acta Cryst. B56, 940±946.
Gaewdang, T., Chaminade, J. P., Gravereau, P., Garcia, A., Fouassier, C.,
Pouchard, M. & Hagenmueller, P. (1994). Z. Anorg. Allg. Chem. 620, 1965±
i
O3 ÐYÐO3
v
102.19 (9)
155.34 (8)
93.54 (6)
93.54 (6)
155.34 (8)
78.82 (9)
76.10 (7)
117.13 (7)
79.86 (8)
84.87 (8)
117.13 (7)
O3ÐYÐO3
v
76.10 (7)
84.87 (8)
79.86 (8)
160.21 (8)
106.23 (10)
111.79 (8)
108.31 (7)
111.79 (8)
108.31 (6)
110.22 (13)
180
i
O3 ÐYÐO2
O3ÐYÐO2
ii
ii
O2 ÐYÐO3
iii v
O2 ÐYÐO3
ii
i
O3 ÐYÐO2
O3ÐYÐO2
iii
iv
v
O3 ÐYÐO3
O2ÐSiÐO1
iii
ii
O2 ÐYÐO2
iii
vi
O2ÐSiÐO3
O1ÐSiÐO3
O2ÐSiÐO3
i
O3 ÐYÐO3
O3ÐYÐO3
iv
vi
iv
ii
O2 ÐYÐO3
iv
O1ÐSiÐO3
vi
O3 ÐSiÐO3
iii
O2 ÐYÐO3
iv
i
O3 ÐYÐO3
v
vii
Si ÐO1ÐSi
1970.
1
2
1
2
1
1
ICSD (2003). Inorganic Crystal Structure Database. FIZ Karlsruhe, Germany.
(URL: www.®z-informationsdienste.de/en/DB/icsd/index.html.)
Ito, J. & Johnson, H. (1968). Am. Mineral. 53, 1940±1952.
Kimata, M., Saito, S., Matsui, T., Shimizu, M. & Nishida, N. (1998). Neues
Jahrb. Mineral. Monatsh. pp. 361±372.
Liebau, F. (1986). Structural Chemistry of Silicates. Berlin: Springer-Verlag.
Patzke, G. R., Wartchow, R. & Binnewies, M. (2000). Z. Kristallogr. New Cryst.
Struct. 215, 15±16.
Reid, A. F., Li, C. & Ringwood, A. E. (1977). J. Solid State Chem. 20, 219±226.
Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567±570.
Shannon, R. D. & Prewitt, C. T. (1969). Acta Cryst. B25, 925±946.
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of
G oÈ ttingen, Germany.
Smolin, Yu. I. & Shepelev, Yu. F. (1970). Acta Cryst. B26, 484±492.
Smolin, Yu. I., Shepelev, Yu. F. & Butikova, I. K. (1972). Sov. Phys. Crystal-
logr. 16, 790±795.
Symmetry codes: (i) � x; y; � z; (ii) � x; � y; 1 � z; (iii) x �
;
2 2
‡ y; z � 1; (iv)
1
2
1
2
1
2
1
2
x �
;
� y; z; (v) � x; � y; � z; (vi) x; � y; z; (vii) � x; � y; 1 � z.
Structure solution using the 280 K data was initially performed in
space group C2, as E statistics suggested a non-centrosymmetric
2
space group (E � 1 = 0.745). The non-centrosymmetric structural
model found with the Patterson method could be re®ned 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 de®nite.
Validation tests of the ®nal 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
Smolin, Yu. I., Shepelev, Yu. F. & Titiov, A. P. (1973). Sov. Phys. Crystallogr.
17, 749±750.
®
nal residual values (with fewer re®ned parameters) and positive
Spek, A. L. (2003). J. Appl. Cryst. 36, 7±13.
de®nite 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,
Stoe & Cie (1996). X-SHAPE and X-RED. Stoe & Cie, Darmstadt, Germany.
Stoe & Cie (2002). X-AREA. Stoe & Cie, Darmstadt, Germany.
Zachariasen, W. H. (1930). Z. Kristallogr. 73, 1±6.
ꢁ
i106 Redhammer and Roth
2
ꢀ-Y Si
O
2 7
at 100 and 280 K
Acta Cryst. (2003). C59, i103±i106