Research at the Laboratory of Crystallography
Incommensurate structure of Sr2Nb2O7
[Daniels, Tamazyan, Kuntscher, Dressel, Lichtenberg, van Smaalen, Acta Cryst. (2002) B 58, 970-976]
The incommensurately modulated structure of Sr2Nb2O7 at room temperature has been determined by single-crystal x-ray diffraction . The crystal structure of Sr2Nb2O7 comprises slabs with a perovskite-type structure, that are seperated by planes of additional oxygen. At room temperature Sr2Nb2O7 crystallizes in the superspace group Cmc21 (α00)0s0, with lattice parameters a = 3.9544(7)Å, b = 26.767(6)Å and c = 5.6961(8)Å. The modulation wave vector is determined as q = 0.488(3)a*.
X-ray diffraction data were collected at the synchrotron using a CCD area detector. A total of 3626 unique main reflection and 1262 unique first-order satellites with I > 3π(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. The image sequence shows the deviation of the atoms from their average positions relative to each other. All amplitudes have been multiplied by five to enhance visibility. The modulation ocurring in space is displayed as a time dependent movement here.
 P.Daniels, R.Tamazyan, C.Kuntscher, M.Dressel, F. Lichtenberg, S. van Smaalen, Acta Crystallogr. B 58, 970 - 976 (2003).