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Abstract

A novel ternary solid solution U(1-y)square(y)(Ni1-xGax)(5) has been observed, whose homogeneous region at 800 degrees C has been determined to range from 0.20 less than or equal to x less than or equal to 0.60 and 0 less than or equal to y less than or equal to 0.17. Employing quantitative room-temperature, Rietveld- powder analyses, the crystal structures of the novel solution range have been evaluated to be isotypic with the CaCu5-type for practically the entire homogeneity region. An enclave of alloys with an orthorhombically distorted unit cell has been observed in samples quenched from 800 degrees C for x = 0.45-0.55 and y similar to 0.09. There are monotonic increases in the lattice parameters and in the volumes for nickel-rich alloys when nickel atoms at a constant U level are randomly replaced by gallium atoms in the 3g sites, whereas on further Ni/Ga replacement the nickel atoms preferentially occupy the prism centres at 2c, causing a maximum for the c-parameter at UNi3Ga2. The magnetic measurements for the gallium-rich compounds (x > 0.5) show the typical broad maximum in the susceptibility at T similar to 60 K that is commonly observed in heavy-fermion systems. Furthermore, two samples exhibit an additional kink at T similar to 26 K (T similar to 23 K) possibly due to an antiferromagnetic coupling of the uranium spins. Anomalies in the specific heat versus temperature observed at 30.5 K for U16.5Ni40Ga43.5 confirm the indications of the onset of AF magnetic order. The enhanced y-value of similar to 100 mJ/mol K-2 suggests that this is a strongly correlated electron system.