Hydrogen ordering and metal-semiconductor transitions in superstoichiometric rare earth dihydrides

Abstract

Temperature-dependent metal-semiconductor (MS) transitions in RH2 + x systems were first noted 20 years ago by Libowitz in CeH2 + x, in the range 0.7 ≲ x ≲ 0.8, and later by Shinar and coworkers in LaH2 + x, for 0.8 ≲ x ≲ 0.9. They were associated with the breakdown (with increasing T) of a band near the Fermi level related to a superlattice of octahedral vacancies, VO, forming below room temperature in those rather substoichiometric trihydrides.We have observed similar MS transitions at TMS = 250–300K in the superstoichiometric dihydrides RH2 + x (R = Y, Gd, Ho, Er), with x near the β phase limit xmaxβ = 0.1–0.3, depending on R. The mechanism responsible for the MS transition is, this time, associated with the breakdown of a direct superlattice of octahedral hydrogen atoms, HO, forming at low concentrations x. Necessary conditions for the occurrence of MS transitions are formulated and a general relationship between TMS and the sample resistivity ρMS at this temperature is established. An additional MS transition is observed in these systems with decreasing T at low temperatures which is attributed to carrier localization due to atomic disorder.