The incommensurate modulation of the structure of Sr₂Nb₂O₇

The incommensurately modulated structure of SrNbO at room temperature is reported, as determined by single-crystal X-ray diffraction. The crystal structure of SrNbO 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, SrNbO crystallizes in the superspace group Cmc2(00)0s0, with lattice parameters , = and  Å. The modulation wave vector is determined as . 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 were obtained. Refinements using a single harmonic modulation wave converged at ( for the main reflections and for the satellite reflections). The modulated structure is interpreted in terms of rotations of NbO octahedra and displacements of the Sr atoms.

Computing details

Program(s) used to refine structure: (Jana2000; Petricek and Dusek, 2000); software used to prepare material for publication: (Jana2000; Petricek and Dusek, 2000).

Figures

(I)

Crystal data

Nb2O7Sr2	c = 5.6961 (8) Å
Mr = 473	V = 602.92 (19) Å^3
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

IPDS Stoe diffractometer	k = ?→?
??integration method?? scans	l = ?→?
h = ?→?

Refinement

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

	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 (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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)