High pressure investigation of the heavy-fermion antiferromagnet U_3Ni_5Al_19

Measurements of magnetic susceptibility, specific heat, and electrical resistivity at applied pressures up to 55 kbar have been carried out on single crystals of the heavy-fermion antiferromagnet U_3Ni_5Al_19, which crystallizes in the Gd_3Ni_5Al_19 orthorhombic structure with two inequivalent U sites. At ambient pressure, a logarithmic T-dependence of the specific heat and T-linear electrical resistivity below 5 K indicates non-Fermi liquid (NFL) behavior in the presence of bulk antiferromagnetic order at T_N=23 K. Electrical resistivity measurements reveal a crossover from non-Fermi liquid to Fermi liquid behavior at intermediate pressures between 46 kbar and 51 kbar, followed by a return to NFL T^{3/2} behavior at higher pressures. These results provide evidence for an ambient pressure quantum critical point and an additional antiferromagnetic instability at P_c=60 kbar.Comment: 12 pages, 5 figure

EuTM2Ga8EuTM_{2}Ga_{8} (TM= Co, Rh, Ir) - A Contribution to the Chemistry of the CeFe2Al8CeFe_{2}Al_{8} -type Compounds

The isostructural compounds EuTM(2)Ga(8) (TM = Co, Rh, Ir) were prepared by direct reaction of the elements by high-frequency thermal treatment. All three phases are isotypic with CeFe(2)Al(8) (space group Pbam, Pearson symbol oP44, Z = 4). The crystal structure was established from single-crystal X-ray diffraction data: a = 12.4322(7) A, b = 14.3814(9) A, and c = 4.0378(2) A for EuCo(2)Ga(8); a = 12.6001(6) A, b = 14.6757(7) A, and c = 4.1172(2) A for EuRh(2)Ga(8); and a = 12.6237(7) A, b = 14.6978(8) A, and c = 4.1486(2) A for EuIr(2)Ga(8), respectively. Analysis of the chemical bonding in EuRh(2)Ga(8) with the electron localizability tools reveals formation of the 3D [Rh(2)Ga(8)] polyanion build by polar covalent bonds. Europium interacts in two ways with the polyanion: mainly as a cation by charge transfer and additionally covalently by means of the electrons of the inner shells. Magnetic susceptibility measurements show Curie-Weiss paramagnetic behavior above 40 K with effective magnetic moments of 7.81, 8.05, and 8.27 micro(B)/f.u. for EuTM(2)Ga(8) (TM = Co, Rh, Ir). Antiferromagnetic ordering of Eu moments is observed in all three compounds below 20 K. Independently on the chemical composition of the coordination sphere, magnetic behavior and, especially, X-ray absorption spectra indicate predominantly the 4f(7) electronic configuration of europium with small admixture of the 4f(6) state