Anomalous resistivity peaks, localization transitions, and the electronic structure of substoichiometric lanthanum trihydrides

The resistivity ρ(T) of ${\mathrm{LaH}}_{\mathrm{x}}$ (2.80≤x≤2.90) was determined from Q-factor measurements at 100 K<T<700 K. Its temperature coefficient α>0 is small at T≤200 K, and ρ(T) peaks at some T between 220 and 260 K. The peak narrows with increasing x and is extremely sharp for x≥2.86. At higher temperatures the behavior of ρ(T) is consistent with variable-range hopping. It is suggested that a delocalized band of states at $E_F$, associated with the superlattice of octahedral vacancies, becomes localized and its mobility is quenched when the superlattice collapses above ∼240±20 K.