Physical properties of new ternary U compounds U$_{3}$Bi$_{4}$M$_{3}$ (M = Rh, Ni)

A common belief has been that only Sb and Sn can form the U$_{3}$X$_{4}$M$_{3}$ structure, but here we show that also Bi can stabilize the structure and two new U ternary compounds U$_{3}$Bi$_{4}$Ni$_{3}$ and U$_{3}$Bi$_{4}$Rh$_{3}$ can be grown as single crystals out of Bi flux. Both materials form in the same crystal structure as Ce$_{3}$Pt$_{3}$Bi$_{4}$, the well known Kondo insulator. Resistivity and photoemission spectroscopy measurements on U$_{3}$Bi$_{4}$Ni$_{3}$ indicate the presence of a charge gap, suggesting that it might be a Kondo insulator. However, the nonmagnetic reference compound Th$_{3}$Bi$_{4}$Ni$_{3}$ is a semiconductor, and, consequently, U$_{3}$Bi$_{4}$Ni$_{3}$ is rather a band insulator, similar to U$_{3}$Sb$_{4}$Ni$_{3}$. On the other hand, replacing Ni with Rh, which has one less electron than Ni, to form U$_{3}$Bi$_{4}$Rh$_{3}$ produces a metallic resistivity and modestly large electronic specific heat coefficient ($\gamma $(0.4K)=200 mJ/mol-U K$^{2}))$, characteristic of a Kondo-lattice system. Details of specific heat, magnetic susceptibility and electrical resistivity measurements on these single crystals will be discussed.