YbCu4.5 - A GIANT STRUCTURE DETERMINED BY SINGLE-CRYSTAL X-RAY DIFFRACTION AND HRTEM. R. Cerny, Laboratoire de Cristallographie, University of Geneva, 24, quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland

YbCu4.5 crystallizes in one of the largest structures among intermetallic phases, having 7448 atoms per unit cell. It has monoclinic symmetry and its structure derives from the cubic AuBe5 type by introducing anti-phase boundaries and Cu-deficient shear planes parallel to {hhh} which lead to a nearly orthogonal 7 x 7 x 6.5 supercell having cell parameters as=48.961(20), bs=48.994(4), cs=45.643(4) , [[beta]]=91.24(1). A 4-dimensional structure analysis reveals a quasi-linear modulation of the atomic positions and occupancies of Yb and Cu along cs. Based on this result and the high resolution transmission electron microscopy images a 3-dimensional model of the superstructure was constructed in space group C2 [1]. Yb was distributed on 336 (4c) sites, 7 (2b) sites and 7 (2a) sites, while Cu was distributed on 1519 (4c) sites. Three different types of Yb co-ordination occur in the structure with their relative abundancies 2:1:1 which is consistent with observed Mssbauer spectra. The Yb sites near the shear planes show Yb - Yb distances of the order of 3.6 which are presumably related to the formation of shear planes, and might be of relevance for the interpretation of the magnetic properties. The composition and calculated density of the model are consistent with their observed values. The calculated and observed X-ray structure factors agreed to R = 12 %. The structure of a related compound YbCu3.5 can be predicted as having the same topology of plane defects as YbCu4.5 but with a higher density of occurrence.

[1] R. Cerny, M. Franois, D. Jaccard, E. Walker, K. Yvon,V. Petricek, I. Cisarova, H.-U. Nissen and R.Wessicken; submitted to J. Physics: Cond. Matter