The system uranium–palladium–boron withU2.5Pd20.5B6, a new heavy fermion compound

Abstract

Phase equilibria in the system U–Pd–B were established at850 °C by light optical microscopy (LOM) and x-ray powder and single crystaldiffraction. Whereas in as-cast alloys only one ternary compound,τ1-U2 + xPd21 − xB6, was found to format x ∼ 0.5, a furthercompound τ2 with hitherto unknown structure was observed in alloys annealed at850 °C.Due to the formation of suitable single crystals, the crystal structures of two binary compounds,UB12 andUPd3 have been redetermined from high precision x-ray data. Similarly, the crystal structure ofτ1-U2.5Pd20.5B6 was investigated by single crystal x-ray diffraction (XRD) revealing isotypism with theCr23C6-type, (space group ; a = 1.1687(5) nm; RF2 = Σ|F02 − Fc2|/ΣF02 = 0.021). τ1-U2 + xPd21 − xB6 is a partially orderedcompound where 0.37(1)U + 0.63Pd atoms randomly share the 4a site in (0, 0, 0). Whereas mutual solubility of U-borides and Pd-borides wasfound at 850 °C to be below 1.0 at.%, a large homogeneity region of fcc-Pd(U, B) extends into the ternary system.U2.5Pd20.5B6 has metallic behavior; the ground state properties are determined from a balance of theKondo effect and the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, revealing longrange antiferromagnetic ordering below 6 K. An extraordinarily large Sommerfeld value (γ > 500 mJ mol − 1 K − 2) groupsU2.5Pd20.5B6 among heavy fermion materials.