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) A. The modulation wave vector is determined as q = 0.488 (3) 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 > 3sigma(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.
A number of lanthanide compounds LnX 2 (Ln Sc, Y, La-Lu; X S, Se, Te), LnY 2 (Y P, As, Sb), and LnZ 2 (Z Si, Ge) are known to crystallize with a structure derivable from only one aristotype, the ZrSSi type. This structure type contains square layers of silicon atoms (in general: square layers of the element of medium electronegativity). The structural deviations observed within the different compounds are usually small and principally affect this square layer. The space groups of the derivative structures under consideration are subgroups of that of the aristotype and the group-subgroup relations are outlined. Distortions of the square layers caused by electronic reasons can mostly be understood by applying simple electron counting rules. A number of compounds with chalcogen deficiency LnX 2± ±d is noted, too, in which the occurrence of vacancies in the square layers often leads to even larger superstructures.
The structure of samarium disulfide SmS 1.9, space group P4 2 /n; lattice constants a 19.717(2) A; c 15.928(1) A has been determined and refined by LS method to R 0.0345; R w 0.0419. The structure is a tenfold superstructure from the structure type ZrSSi.[S 2 ] 2± ± dumbbells with interatomic distances 2.146(1) A between S atoms were observed in the structure. Fully ordered vacancies and orientation of the [S 2 ] 2± ± dumbbells are responsible for the formation of the superstructure. The investigated sample was twinned by the reticular merohedral law, the twin symmetry is 4/mm H m H , the twinning symmetry elements are (310) and (120) mirror planes and the twin index is n 5.
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