Hydrogen ordering and metal-semiconductor transitions in the systemYH2+x

Abstract

The electrical resistivity of β-${\mathrm{YH}}_{2+\mathit{x}}$ specimens, measured between 1.5 and 330 K, exhibits, for x≳0.05, a break in dρ/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≥0.085, superimposed by a first-order transition in the region 200–240 K, probably due to long-range ordering in the ${\mathrm{H}}_{\mathit{O}}$ 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 μΩ 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 ${\mathrm{H}}_{\mathit{O}}$-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 β phase to ${\mathit{x}}_{\mathrm{\beta }}^{\max }$=0.10. This shows that the insulating γ phase coexisting just above x=${\mathit{x}}_{\mathrm{\beta }}^{\max }$ may also participate in the driving mechanism for the M-S transitions, e.g., as percolating micrograins present at, and just below, ${\mathit{x}}_{\mathrm{\beta }}^{\max }$.