Synthesis, crystal structure and magnetic properties Of U2Co6Al19

The new compound U2Co6Al19 was prepared by reaction of the elemental components in an are-melting furnace followed by a heat treatment at 1050degreesC for 500h. Its chemical composition was checked by energy-dispersive X-ray analyses and its crystal structure was determined by single crystal X-ray diffraction experiments. It crystallizes with four formula units in the monoclinic space group C2/m in a unit cell of dimensions a = 17.4617(3) Angstrom, b = 12.0474(2) Angstrom, c = 8.2003(1) Angstrom, beta = 103.915(1)degrees. The crystal structure of U2Co6Al19 can be regarded as a superstructure of NdCo4-xGa9 structure type. This complex structure consists of a three-dimensional Co-Al framework delimiting tunnels where the U atoms reside. The shortest U-U distances are found in the c direction with alternating values of 3.98(1) and 4.22(1)Angstrom. Temperature-dependent magnetization shows a first peak at 12.5 K and a weak ferromagnetic character below the temperature T-C = 8 K. Magnetization at 1.9 K reaches almost saturation in 5 T with the moment of 0.36 mu(B)/U atom. The complex magnetic behavior of U2Co6Al19 may be ascribed to a canted spin structure resulting from an antiparallel arrangement of the magnetic moments not fully compensated at low temperature. At higher temperature, the compound displays simple paramagnetic behavior. (C) 2003 Published by Elsevier Science (USA).