Spin-glass like behavior in a new ternary uranium cobalt alummide, U3CO4+xAl12-x with x=0.55(2)

The new compound U3Co4+xAl12-x, where x = 0.55(2), was prepared by arc-melting of the elemental components, followed by a prolonged annealing at elevated temperature. Scanning electron microscopy-energy-dispersive spectroscopy and powder X-ray diffraction were used to determine the deviation from the ideal stoichiometry. A small homogeneity range, that extends around the composition U3Co4+xAl12-x with 0.4(1) less than or equal to x less than or equal to 0.7(1), could be detected. Single-crystal diffraction experiments revealed that U3Co4.55Al11.45 crystallizes with the Gd3Ru4Al12 type-structure, (space group P6(3)/mmc, Z = 2) in a cell of dimensions at room temperature. a = 8.6518(2) Angstrom, c = 9.2620(2) Angstrom. The crystal structure can be viewed as an intergrowth of two distinct layers of Co and Al atoms. and U, Al and mixed Al/Co atoms that pile up along the hexagonal axis. The results of the DC magnetization suggest the occurrence of a spin glass state at low temperature (T-f = 8 K). The origin of freezing of the magnetic moments may arise from a topological frustration due to the location of the U atoms on the apexes of a distorted Kagome lattice. (C) 2004 Elsevier Inc. All rights reserved.