UCo_2Sn, UCo_4Sn and UCo_5Sn: a crystallographic and magnetic investigation

摘要

In our investigation of Co-rich alloys in the ternary U-Co-Sn system, we have identified three intermetallic compounds with composition UCo_2Sn, UCo_4Sn and UCo_5Sn, respectively. The existence and the crystal structure of the first compound, already known in the literature, have been confirmed, while the latter two compounds have been identified for the first time. The crystal structure of these compounds was determined by X-ray diffraction methods, performed both on powders (all samples) and single crystals (UCo_4Sn and UCo_5Sn). The crystal data are as follows (lattice constants from Guinier powder patterns): UCo_2Sn [UPd2Sn-type, orthorhombic, oP16-Pnma, a = 9.402(3), b=4.321(1), c = 6.615(2)A], UCo_4Sn [MgCu_4Sn-type, cubic, cF24-F4-bar 3m, a = 6.992(2)A] and UCo_5Sn [CeCu_(4.38)In_(1.62)-type, orthorhombic, oP56-Pnnm, a=10.250(l), b=16.012(2), c=4.837(l)A]. The physical properties of the compounds have been studied by electric transport (1.5-300 K), heat capacity (1.8-40 K) and magnetic measurements (1.8-300 K). The magnetisation data reveal weakly paramagnetic behaviour (with weak low temperature upturn due to parasitic impurity phases) in all the three alloys and absence of long-range magnetic ordering, despite the presence of uranium and a substantially high concentration of cobalt. The results for UCo_2Sn are in agreement with earlier reports in the literature. The magnitudes of the coefficients of the linear term in the heat capacity and the T~2 term in the low temperature resistivity track the room temperature magnetisation.