[en] The γ-γ Perturbed Angular Correlation technique was used to study the hyperfine interaction of ¹⁸¹Ta at the Hf site(s) in UFe₄Al₈ at room temperature and 12 K. The data at room temperature are well described by two electric field gradients, while at low temperature two combined hyperfine interactions have to be considered, one with the magnetic hyperfine field collinear with the c-axis and another with the magnetic hyperfine field in the basal plane. The results are compared with previous Moessbauer and neutron diffraction experiments and the lattice site of Hf is discussed.

[en] Phase equilibria in the system U-Pd-B were established at 850 deg. C by light optical microscopy (LOM) and x-ray powder and single crystal diffraction. Whereas in as-cast alloys only one ternary compound, τ₁-U_2+xPd_21-xB₆, was found to form at x ∼ 0.5, a further compound τ₂ with hitherto unknown structure was observed in alloys annealed at 850 deg. C. Due to the formation of suitable single crystals, the crystal structures of two binary compounds, UB₁₂ and UPd₃ have been redetermined from high precision x-ray data. Similarly, the crystal structure of τ₁-U_2.5Pd_20.5B₆ was investigated by single crystal x-ray diffraction (XRD) revealing isotypism with the Cr₂₃C₆-type, (space group Fm3-barm; a = 1.1687(5) nm; R_F² = Σ|F₀² - F_c²|/ΣF₀² = 0.021). τ₁-U_2+xPd_21-xB₆ is a partially ordered compound 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 was found at 850 deg. C to be below 1.0 at.%, a large homogeneity region of fcc-Pd(U, B) extends into the ternary system. U_2.5Pd_20.5B₆ has metallic behavior; the ground state properties are determined from a balance of the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, revealing long range antiferromagnetic ordering below 6 K. An extraordinarily large Sommerfeld value (γ > 500 mJ mol^-1 K^-2) groups U_2.5Pd_20.5B₆ among heavy fermion materials.

[en] Novel ternary compounds, M₂Pd_14+xB_5-y (M = La, Ce, Pr, Nd, Sm, Eu, Gd, Lu, Th; x∼0.9, y∼0.1), have been synthesized by arc melting. The crystal structures of Nd₂Pd_14+xB_5-y and Th₂Pd_14+xB_5-y were determined from x-ray single-crystal data and both are closely related to the structure type of Sc₄Ni₂₉B₁₀. All compounds were characterized by Rietveld analyses and found to be isotypic with the Nd₂Pd_14+xB_5-y type. Measurements of the temperature dependent susceptibility and specific heat as well as the temperature and field dependent resistivity were employed to derive basic information on bulk properties of these compounds. The electrical resistivity of M₂Pd_14+xB_5-y, in general, is characterized by small RRR (residual resistance ratio) values originating from defects inherent to the crystal structure. Whereas the compounds based on Ce, Nd, Sm and Gd exhibit magnetic order, those based on Pr and Eu seem to be non-magnetic, at least down to 400 mK. While the non-magnetic ground state of the Pr based compound is a consequence of crystalline electric field effects in the context of the non-Kramers ion Pr, the lack of magnetic order in the case of the Eu based compound results from an intermediate valence state of the Eu ion.

[en] The ternary inter-metallic compound, UFe₅Al₇, crystallize in a tetragonal ThMn₁₂ type structure. In the as-cast samples a residual phase of FeAl (∼2% wt) was identified in the grain boundaries. The amount of the residual cubic phase of FeAl was determined by Rietveld analysis and reduced by the annealing process. UFe₅Al₇ maintains the tetragonal symmetry as a function of pressure, while FeAl keeps the cubic structure as was determined by the Rietveld analysis. The volume-pressure curve calculated from the x-ray analysis is V/V₀ = 0.87 for UFe₅Al₇ at 26.0 GPa

[en] This contribution focuses on the structural and physical properties of U-based Laves phases. It starts with the structural description of the different type of Laves phases, followed by a brief description of the factors that affect their stability. The majority of the uranium Laves phases show a weakly paramagnetic behaviour. The reason is the compact structure of the phases that leads to small a U-U spacing as well as very high coordination numbers, regarding both the uranium and the ligands sublattices, which brings a strong hybridization with non-f states. However, there are some exceptions of uranium Laves phases that do order magnetically (UFe₂, UNi₂ and the recently discovered U₂Fe₃Ge compound). These exceptions are discussed in more detail in the present manuscript.

[en] Millimetre size UZn₁₂ single crystals were grown by the high temperature solution growth method using zinc as the solvent. Single-crystal x-ray diffraction data confirm that this compound crystallizes in the hexagonal high temperature form of SmZn₁₂ (S.G. P6/mmm) and points to a U_1.01(1)Zn_11.7(1) stoichiometry for the crystals, with ∼ 4% of the U atoms being located at the 2c site due to the partial substitution of 4h Zn pairs. UZn₁₂ orders antiferromagnetically at T_N = 5.0(2) K, and the magnetization and resistivity measurements suggest that the magnetic moments are confined within the a-b plane. The Sommerfeld coefficient, derived from the paramagnetic region by the standard method, is γ_p∼200 mJ (mol K²)^-1, which definitely classifies UZn₁₂ as a moderate heavy-fermion system. The heavy-fermion character of UZn₁₂ is also manifested in the overall shape of temperature-dependent electrical resistivity that is dominated by a single-ion Kondo effect at high temperatures and coherent Kondo scattering at low temperatures. The paramagnetic magnetoresistivity isotherms can be fairly well superimposed onto each other using Schlottmann's scaling for the single-ion Kondo model, as expected for a Kondo system.

[en] The compound NpFe₄Al₈ was prepared by direct arc melting of the constituent elements, followed by annealing. It crystallizes in the ThMn₁₂-type structure (space group I4/mmm, a = 8.7480(5) A, c = 5.0372(4) A), with the iron atoms completely and only occupying the 8f positions. Magnetization measurements (T = 2-300 K, B = 0-7 T) show a ferromagnetic-type transition at T_C = 135(2) K and a second anomaly at 118(3) K. The low temperature magnetization cycle is characterized by a hysteresis with a step similar to that previously observed for UFe₄Al₈ single crystals. First-principles density functional theory calculations of the NpFe₄Al₈ band structure point to a magnetic structure similar to that of UFe₄Al₈, in agreement with the observed magnetization cycle. The calculations indicate that the neptunium moment is aligned along one of the a or b axes, and the iron moments form a noncollinear structure in the a-b plane, with the antiferromagnetic and ferromagnetic contributions perpendicular and antiparallel to the neptunium spin moment, respectively