Peculiar Properties of UMB4 (M = V, Cr, Mo, W) Uranium Borides

UMB4 (M = V, Cr, Fe, Co, Mo, W, Re, Os) materials prepared by arc-melting have UMB4 as the main phase, with (V, Cr, Fe, Co) crystallising in the YCrB4 structure type, while (Mo, W, Re, Os) adopt the ThMoB4-type. For M = V, Cr, Mo, W the magnetic susceptibility is low and only weakly temperature dependent, but higher than alpha-U, which can be ascribed to a higher density of states at the Fermi level. Heat capacity results point to a very high Debye temperature for both structures, which is associated with very high lattice stiffness of the boron network. However, for W and Mo the boron vibrations are accompanied by low-energy Einstein modes coming from vibrations of the atoms inside the boron cages that could lead to superconductivity. The electronic structure of UMB4 is influenced by a strong 5f-d hybridization and by the B-2p states, which explains the observed weak Pauli paramagnetism.