Acta Crystallographica Section B

Structural Science

Volume 56, Part 6 (December 2000)

Acta Cryst. (2000). B56, 959-971    [doi:10.1107/S0108768100011332]

Modulated structure of La₂Co_1.7 from neutron and X-ray diffraction data

M. Dusek, G. Chapuis, P. Schobinger-Papamantellos, C. Wilkinson, V. Petricek, L. D. Tung and K. H. J. Buschow

Abstract: An La₂Co_1.7 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 La₂Co_1.7 is hexagonal with cell parameters a = 4.885  (1), c = 4.273  (2)  Å and space group P6₃/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 = (, 0, ). The structure was solved in the pseudoorthorhombic cell, with a = 8.461  (1), b = 4.885  (1), c = 4.273  (2)  Å, in the superspace group C2/m(, 0, ). Owing to space requirements, the Co atoms cannot fit 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 La₂Co_{1.8 (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.

Online 1 December 2000

	Structure factor file (CIF format) (87.9 kbytes) Contains datablock ltnc
	Structure factor file (CIF format) (87.9 kbytes) Contains datablock ltns
	Structure factor file (CIF format) (78.3 kbytes) Contains datablock rtnc
	Structure factor file (CIF format) (78.3 kbytes) Contains datablock rtns

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