Metal-semiconductor transitions in the superstoichiometric dihydrides YH2+x

In this paper after a brief review of the structural and electronic properties of the non-stoichiometric rare-earth hydrides, RH2+x, the authors discuss metal-semiconductor transitions observed in several β-phase RH2+x-systems in the temperature range 200 to 300 K for x-values close to the phase boundary to the trihydride γ-phase. The special case of YH2+x is treated in detail, showing the evolution from a purely metallic case with x = 0.085 to a clear M-S transition for x = 0.10 (at the β-phase limit), passing through a fluctuating situation for x = 0.095. The essential role played by an order - disorder transformation occurring in the octahedral (x) hydrogen sublattice as the driving mechanism for the M-S transition is emphasized. A second M-S transition observed at low temperatures (approx-lt 80 K) is attributed to weak carrier localization due to x-hydrogen disorder