The incommensurate modulation of the structure of Sr2Nb2O7

Titelangaben

Daniels, Peter ; Tamazyan, Rafael ; Kuntscher, Christine A. ; Dressel, Martin ; Lichtenberg, Frank ; van Smaalen, Sander:
The incommensurate modulation of the structure of Sr2Nb2O7.
In: Acta Crystallographica Section B. Bd. 58 (2002) Heft 6 . - S. 970-976.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S010876810201741X

Volltext

Link zum Volltext (externe URL):

Abstract

The incommensurately modulated structure of Sr₂Nb₂O₇ at room temperature is reported, as determined by single-crystal X-ray diffraction. The crystal structure of Sr₂Nb₂O₇ 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, Sr₂Nb₂O₇ crystallizes in the superspace group \it Cmc2₁(α00)0\it s0, with lattice parameters a = 3.9544\,(7), b = 26.767\,(6) and c = 5.6961\,(8)Å. The modulation wave vector is determined as \bf q = 0.488\,(3)\,\bf a*. 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 σ (I\,) were obtained. Refinements using a single harmonic modulation wave converged at R = 0.057 (R = 0.051 for the main reflections and R = 0.121 for the satellite reflections). The modulated structure is interpreted in terms of rotations of NbO₆ octahedra and displacements of the Sr atoms.