Abstract
After a brief review of the structural and electronic properties of the non-stoichiometric rare-earth hydrides, RH 2+x, we discuss metal-semiconductor transitions observed in several β-phase RH 2+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 YH 2+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 (≲ 80 K ) is attributed to weak carrier localisation due to x-hydrogen disorder.
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