Modulated structure of La2Co1.7 from neutron and X-ray diffraction data

An La2Co1.7 crystal was investigated by single-crystal neutron and X-ray diffraction. The neutron measurement was performed with a Laue white-beam technique at 15 K and room temperature, using a large position-sensitive detector. The X-ray measurements were obtained at room temperature from a CCD detector. The average structure of La2Co1.7 is hexagonal with cell parameters a = 4.885 (1), c = 4.273 (2) Angstrom and space group P6(3)/mmc. The satellites are located at the vertices of small hexagons perpendicular to the c axis. The modulated crystal was indexed assuming a sixfold twinned 3 + 1 dimensional structure with q = (alpha, 0, gamma). The structure was solved in the pseudoorthorhombic cell, with a = 8.461 (1), b = 4.885 (1), c = 4.273 (2) Angstrom, in the superspace group C2/m(alpha, 0, gamma). Owing to space requirements, the Co atoms cannot rt precisely into the octahedral sites of the La h.c.p. (hexagonal close packing). Instead, the Co atoms adopt a different periodicity, which is not commensurate with the periodicity of the La atoms. Two structure models have been refined in order to describe this behaviour, one using the sawtooth function for the positional modulation of cobalt and the other describing the structure as a composite system. The chemical composition calculated from the composite model is La2Co1.8(1) with the estimated standard deviation arising from the variation of q for different samples. In both models lanthanum is incommensurately modulated, while the position of cobalt seems not to be affected by any relative periodic displacement. [References: 13] 13

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Acta Crystallographica - Section B, Structural Science, 56, Part 6, 959-971
English Article Current Contents(R)/Physical, Chemical & Earth Sciences. Reprint available from: Dusek M Univ Lausanne, Inst Cristallog, BSP Dorigny CH-1015 Lausanne Switzerland Univ Lausanne, Inst Cristallog, BSP Dorigny CH-1015 Lausanne Switzerland Acad Sci Czech Republ, Inst Phys Prague 18221 8 Czech Republic ETH Zurich, Lab Kristallog CH-8092 Zurich Switzerland Univ London Kings Coll, Dept Phys London WC2R 2LS England Inst Max Von Laue Paul Langevin F-38042 Grenoble 9 France ITIMS Hanoi Vietnam Univ Amsterdam, Van der Waals Zeeman Inst NL-1018 XE Amsterdam Netherlands 0006 Acta Crystallogr. Sect. B-Struct. Sci

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