A single-crystal x-ray and HRTEM study of the heavy-fermion compound YbCu4.5

The title compound crystallizes with monoclinic symmetry. Its structure derives from the cubic AuBe5-type structure via the introduction of planar defects parallel to {hhh} which lead to a nearly orthogonal approximate to 7 x 7 x 6.5 supercell having the cell parameters a(s) = 48.961(20) Angstrom, b(s) = 48.994(4) Angstrom, c(s) = 45.643(4) Angstrom, beta = 91.24(1)degrees. G four-dimensional structural analysis reveals a quasi-linear modulation of the atomic positions and occupancies of ytterbium and copper along c(s). On the basis of this result and the electron microscopy images, a three-dimensional model of the superstructure was constructed in space group C2. The model contains 7448 atoms per unit cell which are distributed over 350 Yb sites and 1519 Cu sites. It has the overall composition YbCu4.43, contains short Yb-Yb distances, and is consistent with the chemical and physical properties.