A SINGLE-CRYSTAL X-RAY AND HRTEM STUDY OF THE HEAVY-FERMION COMPOUND YBCU4.5

The title compound crystallizes with monoclinic symmetry. Its structur e derives from the cubic AuBe5-type structure via the introduction of planar defects parallel to {hhh} which lead to a nearly orthogonal app roximate 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) Angstro m, beta = 91.24(1)degrees. G four-dimensional structural analysis reve als a quasi-linear modulation of the atomic positions and occupancies of ytterbium and copper along c(s). On the basis of this result and th e electron microscopy images, a three-dimensional model of the superst ructure was constructed in space group C2. The model contains 7448 ato ms per unit cell which are distributed over 350 Yb sites and 1519 Cu s ites. It has the overall composition YbCu4.43, contains short Yb-Yb di stances, and is consistent with the chemical and physical properties.