ON THE MODULATED STRUCTURE OF La2CO1.7 : A NEUTRON LAUE DIFFRACTION STUDY
P.Schobinger-Papamantellos1,
C. Wilkinson2,
D. Mylesb2, K.H.J. Buschowc3,
and L.D.Tungc3
1Laboratorium fuer
Kristallographie, ETHZ CH-8092 Zuerich Switzerland.
2European
Molecular Biology Laboratory (EMBL), Avenue des Martyrs, BP156, 38042, Grenoble,
Cedex 9, France.
3Van der
Waals-Zeeman Institute, University of Amsterdam Valckenierstr. 65
1018 XE Amsterdam, The Netherlands.
wilkinson@ill.fr
Keywords: Neutron diffraction, rare earth intermetallic compounds, La2Co1.7, magnetic ordering.
In previous studies [1,2] the hexagonal compound La2Co1.7 (P63/m, space group, a=4.885 A, c=4.275 A, Z=1, La at 2c: (1/3,2/3,1/4)) has been reported to display an incommensurately modulated structure associated with the Co atom distribution along the hexagonal axis. However, no exact structural determination has yet been made concerning the nature of the modulation. Laue neutron diffraction patterns from a single crystal of La2Co1.7 at 155K and 15K have been recorded on the LADI neutron sensitive image plate diffractometer [3]. The diffraction diagrams display a large number of satellites, distributed around the reciprocal lattice positions of the main reflections H. They could be indexed as satellites of the main reflections Q=H±q with a two dimensional wave vector q=(qx,0,qz) with temperature independent length (qx=0.109 r.l.u. qz=0.213 r.l.u.). These values agree with the refined neutron powder pattern obtained for the same temperatures and the incommensurate character of the chemical structure. A structure determination is under way. The low temperature data confirm the triangular character of the Co magnetic structure described in reference [2].