The hydriding behaviour of U(Fe1-xNix)Al system (0≤x≤0.75) and magnetic studies on U(Fe1-xNix)AlH0.8

The hydriding behaviour of the solid-solution series of compounds, U(Fe1-xNix)Al, has been investigated. In the parent pseudoternary compounds, as Ni is gradually substituted for Fe, the magnetic correlations grow. As a result, the magnetic properties change from exchange enhanced Pauli-paramagnetic and spin fluctuating (x=0), via weakly magnetic with no long range magnetic ordering down to 4 K (0<x less than or equal to 0.3) to ferromagnetic (0.35 less than or equal to x less than or equal to 0.75) and eventually to antiferromagnetic for x greater than or equal to 0.9. The present hydriding report covers compositions up to the ferromagnetic regime. It is found that hydrogen absorption does not take place for x below congruent to 0.7. The lowest nickel containing composition, which forms a well defined hydride phase, viz., U(Fe0.3Ni0.7)Al, absorbs y=0.8 H atoms per formula unit. This matches exactly with the lower hydride phase of pure UNiAl. No other hydride phase is formed either above or below y=0.8. The magnetization studies show that U(Fe0.3Ni0.7)AIH(0.8) has much higher values of the magnetic ordering temperature (T-C), paramagnetic Curie temperature (theta(p)) and the uranium magnetic moment, relative to the unhydrided composition. These findings are in consonance with the large increase in the Fe-57 Mossbauer isomer-shift value, on hydriding. It is suggested that the electron charge transfer from H to the Fe 3d band (inferred from the Mossbauer studies) weakens the 5f-3d hybridization, thus enhancing the ferromagnetic correlations. The observed large increase in the a-axis cell parameter also implies a reduction in U-U hybridization, further justifying the observed increase in T-C, theta(p) and U moment values. (C) 2000 Published by Elsevier Science S.A. All rights reserved.