ELECTRICAL BEHAVIOR OF NDH2+X WITH REGARD TO STRUCTURAL ORDERING, MAGNETISM AND APPROACH TO THE METAL-INSULATOR-TRANSITION

Electrical resistivity measurements of NdH2+x show the effects of a structural order-disorder transition, related to the excess H concentration x, for which we give information on the ordering temperature, T(s), and the stabilizing effects of quenching on the disordered H-sublattice. A second important feature concerns the magnetic ordering: for x less-than-or-equal-to 0.32, the ferromagnetic ordering gives rise to a strong decrease of the spin-disorder resistivity, which is not much influenced by the structural order-disorder transition, but for x greater-than-or-equal-to 0.6, a sharp magnetic transition occurs essentially for the ordered structural state. For high x, a minimum of the resistivity is often observed when lowering the temperature towards the magnetic transition, which indicates either a localization of the conduction electrons or the occurrence of an incommensurate antiferromagnetic ordering with the opening of a gap. The fact that for x greater-than-or-equal-to 0.6 the resistivity over the whole temperature range is very high (attaining 6 m-OMEGA cm for x = 0.65) is a sign that the gamma-phase (trihydride) has nearly insulating behaviour.