No abstract

is a member of a large family of intermetallic compounds with the tetragonal crystal structure. It orders antiferromagnetically at with the propagation vector k = (0,0,1/2). We have performed magnetization and neutron scattering experiments on a single crystal that show that the uranium atoms order in the collinear irreducible representation of the magnetic group, with the moments aligned parallel to the unique tetragonal axis. The value of the ordered magnetic moment of the uranium atoms refined from the neutron scattering intensities is . This is considerably reduced from the free-ion value for both and configurations and suggests that the f electrons are strongly hybridized with the conduction band.

Single crystals of the heavy-electron compound have been grown by a modified mineralization technique. The x-ray structure refinement shows that single crystals form in the structure, instead of the structure reported for polycrystalline material. The polymorphism of is attributed to the experimental parameters, such as pressure and temperature, during the sample preparation process. The single-crystal susceptibility data reveal a weak maximum for at K, indicating the presence of short-range antiferromagnetic correlations, while has the tendency to diverge at low T (T > 2 K). The electrical resistivity of the single crystals (T > 0.3 K) is best described by with for and for . The magnetic and transport data show pronounced deviations from the standard Fermi-liquid picture, and lead to a classification of as a non-Fermi-liquid compound. As the origin of NFL behaviour in we propose the proximity to a quantum critical point or Kondo disorder.

Abstract. Members of the solid solution seriesZnl_xF%A1204 (x = 0.2, 0.4, 0.6 and 1.0) with spinel structure were synthesized by direct solid-state reaction of the simple metal oxides and metallic iron in evacuated silica ampoules at 1175 ~ C. Two aliquots of the single-phase spinels obtained for each composition were annealed under vacuum at 1075 ~ C and 725 ~ C for 48 hours and then quenched in liquid nitrogen.The cation distributions of all the quenched samples were determined by X-ray powder diffraction, using the Rietveld method of structural refinement. The degree of inversion increases with iron content and for spinels with the same chemical composition with quenching temperature. The relative areas estimated for the contributions to the M6ssbauer spectra of tetrahedrally-and octahedrally-coordinated Fe 2+ suggest that most of Zn 2+ cations remain at the tetrahedral site, as expected from the relative cation site preferences.Failure to quench the equilibrium cation distributions, suggested by deviations between the observed composition dependence of the cation distribution and that expected from the thermodynamic model of O'Neill and Navrotsky (1983, 1984), may be explained by an enhancement of cation diffusion rates in the Znl_xF%A1204 (0 < x < 1) spinels caused by the presence of cation vacancies. Fe3+/vacancy defects are easily formed in these spinels due to partial oxidation of Fe z+ at high temperature.