New Insights into the Crystal Structure of UTE0.25Ge2 = U4TGe8 (TE = Ru, Os) and Ferromagnetic Properties of the Os-bearing Uranium Germanide

A single crystal of the compound previously described as "URu0.25Ge2" was studied by means of transmission electron microscopy and synchrotron X-ray diffraction (XRD) measurements. The experiments revealed that the deficient ruthenium atoms order to a superstructure of the CeNiSi2 structure-type with space group P21/n (no. 14) and cell parameters a' = 5.7422(7) angstrom, b' = 15.931(2) angstrom, c' =11.4903(9) angstrom and beta' = 90.755(8)degrees, corresponding to the Tb4FeGe8-type structure. In addition, a new isostructural compound, U4OsGe8, was synthesized and charac-terized by powder X-ray diffraction, electron diffraction, DC magnetization, AC susceptibility, electrical resistivity and specific heat measurements. The experiments showed that it crystallizes with the monoclinic, ordered P21/n unit cell with lattice parameters a' = 5.7438(1) angstrom, b' = 15.9355(1) angstrom, c' = 11.4586(1) angstrom and beta' = 90.681(1)degrees, and orders ferromagnetically at about 54 K, which is clearly seen in all the physical properties studied. Moreover, its electrical resistivity was found to exhibit features characteristic of semimetals, and the electron contribution to the specific heat is moderately enhanced, as in all other known phases of the UTE1-xGe2 (with TE = transition element) family of compounds.