Hydride phases, structure, and magnetic properties of the UNiAlHy system -art. no. 094414

The hydriding behavior of the heavy fermion compound UNiAl has been investi gated. A range of hydride compositions with orthorhombic structure (space g roup Pnma) has been stabilized. As a result, a different nomenclature for t he UNiAlHy hydride phases is proposed. X-ray diffraction and magnetic studi es are reported on three compositions in this range of hydrides, viz., y = 0.06, 0.14, and 0.58. All these compositions do not show magnetic ordering down to 2 K, though weak ferromagnetic correlations are apparent in their m agnetic studies. Low effective moment values on uranium atoms in these case s, appreciably below the value for itinerant antiferromagnetic UNiAl, indic ate increased itinerant character of U 5f electrons. We also report magneti zation studies on hexagonal UNiAlHy (y = 0.7). It shows ferromagnetic order ing below 87 K. This means that in the hexagonal hydride phases, the nature of magnetic ordering changes from antiferromagnetic (y = 0) to ferromagnet ic (y = 0.7) and again back to antiferromagnetic for y = 2.3. It is shown t hat the moment on the uranium atoms in the hexagonal hydride phases (y cong ruent to 0.7 and 2.0-2.5) is close to the free-ion local moment value.