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The incommensurately modulated structure of Sr
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
Nb
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
O
![_{7}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi3.gif)
at room temperature is reported, as determined by single-crystal X-ray diffraction. The crystal structure of Sr
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
Nb
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
O
![_{7}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi3.gif)
comprises slabs with a perovskite-type structure that are separated by planes of additional O atoms. The driving force for the modulation is shown to be internal strain around the Sr atoms that lie at the interface between the slabs. At room temperature, Sr
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
Nb
![_{2}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi1.gif)
O
![_{7}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi3.gif)
crystallizes in the superspace group
Cmc2
![_{1}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi13.gif)
(
![\alpha](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi14.gif)
00)0
s0, with lattice parameters
![a = 3.9544\,(7)](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi15.gif)
,
![b](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi16.gif)
=
![26.767\,(6)](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi17.gif)
and
![c = 5.6961\,(8)](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi18.gif)
Å. The modulation wave vector is determined as
![{\bf q} = 0.488\,(3)\,{\bf a}^{*}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi19.gif)
. X-ray diffraction data were collected at a synchrotron using a CCD area detector. A total of 3626 unique main reflections and 1262 unique first-order satellites with
![I \,\gt \,3 \sigma (I\,)](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi20.gif)
were obtained. Refinements using a single harmonic modulation wave converged at
![R = 0.057](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi21.gif)
(
![R = 0.051](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi22.gif)
for the main reflections and
![R = 0.121](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi23.gif)
for the satellite reflections). The modulated structure is interpreted in terms of rotations of NbO
![_{6}](//journals.iucr.org/b/issues/2002/06/00/ck0015//teximages/ck0015fi24.gif)
octahedra and displacements of the Sr atoms.
Supporting information
Program(s) used to refine structure: (Jana2000; Petricek and Dusek, 2000); software used to prepare material for publication: (Jana2000; Petricek and Dusek, 2000).
Crystal data top
Nb2O7Sr2 | c = 5.6961 (8) Å |
Mr = 473 | V = 602.92 (19) Å3 |
Orthorhombic, Cmc21(αlpha00)0s† | Z = 4 |
q = 0.48800a* | Dx = 5.21 Mg m−3 |
a = 3.9544 (7) Å | X-ray radiation, λ = 0.5 Å |
b = 26.767 (6) Å | × × mm |
† Symmetry operations: (1) x1, x2, x3, x4; (2) x1, −x2, 1/2+x3, 1/2+x4; (3) −x1, −x2, 1/2+x3, −x4; (4) −x1, x2, x3, 1/2−x4; (5) 1/2+x1, 1/2+x2, x3, x4; (6) 1/2+x1, 1/2−x2, 1/2+x3, 1/2+x4; (7) 1/2−x1, 1/2−x2, 1/2+x3, −x4; (8) 1/2−x1, 1/2+x2, x3, 1/2−x4.
|
Data collection top
IPDS Stoe diffractometer | k = ?→? |
??integration method?? scans | l = ?→? |
h = ?→? | |
Crystal data top
Nb2O7Sr2 | c = 5.6961 (8) Å |
Mr = 473 | V = 602.92 (19) Å3 |
Orthorhombic, Cmc21(αlpha00)0s† | Z = 4 |
q = 0.48800a* | X-ray radiation, λ = 0.5 Å |
a = 3.9544 (7) Å | × × mm |
b = 26.767 (6) Å | |
† Symmetry operations: (1) x1, x2, x3, x4; (2) x1, −x2, 1/2+x3, 1/2+x4; (3) −x1, −x2, 1/2+x3, −x4; (4) −x1, x2, x3, 1/2−x4; (5) 1/2+x1, 1/2+x2, x3, x4; (6) 1/2+x1, 1/2−x2, 1/2+x3, 1/2+x4; (7) 1/2−x1, 1/2−x2, 1/2+x3, −x4; (8) 1/2−x1, 1/2+x2, x3, 1/2−x4.
|
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Sr1 | 0.5 | 0.20587 (3) | 0.02977 (17) | 0.01165 (13) | |
Sr2 | 0.5 | 0.051847 (16) | 0.46660 (13) | 0.00598 (7) | |
Nb1 | 0 | 0.162002 (12) | 0.5 | 0.00319 (6) | |
Nb2 | 0.5 | 0.442230 (11) | 0.47045 (10) | 0.00224 (5) | |
O1 | 0 | 0.00701 (14) | 0.2222 (8) | 0.0081 (8) | |
O2 | 0 | 0.10986 (18) | 0.1770 (11) | 0.0144 (12) | |
O3 | 0.5 | 0.15089 (15) | 0.4729 (11) | 0.0103 (10) | |
O4 | 0 | 0.21409 (16) | 0.2852 (9) | 0.0077 (7) | |
O5 | 0.5 | 0.30054 (19) | 0.2639 (10) | 0.0101 (9) | |
O6 | 0.5 | 0.40683 (15) | 0.1709 (8) | 0.0076 (6) | |
O7 | 0 | 0.45098 (16) | 0.4512 (11) | 0.0090 (8) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Sr1 | 0.00385 (12) | 0.0221 (3) | 0.0090 (2) | 0 | 0 | 0.0054 (2) |
Sr2 | 0.00394 (10) | 0.00762 (12) | 0.00636 (16) | 0 | 0 | −0.00140 (13) |
Nb1 | 0.00205 (8) | 0.00393 (9) | 0.00359 (11) | 0 | 0 | 0.00035 (7) |
Nb2 | 0.00124 (8) | 0.00352 (8) | 0.00197 (11) | 0 | 0 | −0.00015 (8) |
O1 | 0.0108 (16) | 0.0058 (10) | 0.0078 (14) | 0 | 0 | 0.0026 (10) |
O2 | 0.025 (3) | 0.0081 (12) | 0.0102 (17) | 0 | 0 | −0.0030 (13) |
O3 | 0.0020 (7) | 0.0098 (10) | 0.019 (3) | 0 | 0 | −0.0014 (16) |
O4 | 0.0082 (13) | 0.0080 (11) | 0.0071 (12) | 0 | 0 | 0.0037 (10) |
O5 | 0.0109 (17) | 0.0100 (12) | 0.0094 (15) | 0 | 0 | 0.0036 (11) |
O6 | 0.0096 (13) | 0.0073 (9) | 0.0059 (11) | 0 | 0 | −0.0016 (9) |
O7 | 0.0021 (8) | 0.0118 (12) | 0.013 (2) | 0 | 0 | 0.0011 (15) |
Experimental details
Crystal data |
Chemical formula | Nb2O7Sr2 |
Mr | 473 |
Crystal system, space group | Orthorhombic, Cmc21(αlpha00)0s† |
Temperature (K) | ? |
Wave vectors | q = 0.48800a* |
a, b, c (Å) | 3.9544 (7), 26.767 (6), 5.6961 (8) |
V (Å3) | 602.92 (19) |
Z | 4 |
Radiation type | X-ray, λ = 0.5 Å |
µ (mm−1) | ? |
Crystal size (mm) | × × |
|
Data collection |
Diffractometer | IPDS Stoe diffractometer |
Absorption correction | – |
No. of measured, independent and observed (?) reflections | ?, ?, ? |
Rint | ? |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | ?, ?, ? |
No. of reflections | ? |
No. of parameters | ? |
No. of restraints | ? |
Δρmax, Δρmin (e Å−3) | ?, ? |
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