HYDROGEN ORDERING AND METAL-SEMICONDUCTOR TRANSITIONS IN THE SYSTEM YH2+X

The electrical resistivity of beta-YH2+x specimens, measured between 1.5 and 330 K, exhibits, for x greater than or similar to 0.05, a break in d-rho/dT centered at 155 K, which is attributed to short-range ordering of the excess-hydrogen atoms on octahedral sites. This anomaly is, for x greater-than-or-equal-to 0.085, superimposed by a first-order transition in the region 200-240 K, probably due to long-range ordering in the H-0 sublattice as suggested by the analysis of quenching experiments. Moreover, it appears to be the driving mechanism for a metal-semiconductor (M-S) transition, at 235 and 255 K for x = 0.10 in the cooling and in the heating regime, respectively, and at 280 K for x = 0.095 when warming up after a quench. At the same time, a resistivity minimum shows up at low temperatures, whose depth and position grow with x increasing from 0.085 to 0.10: from 0.01 to 7.5-mu-OMEGA cm and from 10 to 79 K. The origin of the M-S transition is ascribed to the collapse of a delocalized band near the Fermi energy which forms below the transition temperature due to H-0-atom ordering; that of the low-T transition is tentatively attributed to carrier localization due to hydrogen disorder. We have also determined, by x-ray lattice-parameter measurements, the boundary of the pure beta-phase to x(beta)max = 0.10. This shows that the insulating gamma-phase coexisting just above x = x(beta)max may also participate in the driving mechanism for the M-S transitions, e.g., as percolating micrograins present at, and just below, x(beta)max.