The incommensurate modulation of the structure of Sr2Nb2O7

Abstract

The incommensurately modulated structure of Sr_{2}Nb_{2}O_{7} at room temperature is reported, as determined by single-crystal X-ray diffraction. The crystal structure of Sr_{2}Nb_{2}O_{7} 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}Nb_{2}O_{7} crystallizes in the superspace group Cmc2_{1}(\alpha00)0s0, 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 \sigma (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 NbO_{6} octahedra and displacements of the Sr atoms.