Q-factor measurements of the bulk resistivity and nonmetal-metal transitions inLaHxandCeHx(x≥2.70)

The resistivity ρ(T) of ${\mathrm{CeH}}_{\mathrm{x}}$, x=2.70, 2.78, and ${\mathrm{LaH}}_{\mathrm{x}}$, x=2.70, 2.80 2.90, and 2.93, was measured in the temperature range 100≤T≤500 K, using an indirect Q-meter technique. The measured values of ${\mathrm{CeH}}_{2.78}$ are in excellent agreement with the previous direct-contact measurements of Libowitz, Pack, and Binnie. In all cases except ${\mathrm{LaH}}_{2.93}$, ρ(T) peaks at 240≤T≤270 K, in rough agreement with the onset of the slight tetragonal distortion of the fcc phase and/or the formation of an ordered hydrogen superlattice. Thus, the temperature coefficient of the resistivity α is positive below the peak but negative above. In ${\mathrm{LaH}}_{2.90}$ the peak is very sharp and the behavior semiconductinglike above 250 K. In ${\mathrm{LaH}}_{2.93}$ the behavior is semiconductinglike throughout the measured range. The room-temperature resistivity of ${\mathrm{LaH}}_{\mathrm{x}}$ rises sharply with x, from around 7×${10}^{\mathrm{-}4}$ Ω cm at x=2.70 to around 0.5 Ω cm at x=2.93. These results are discussed in light of previous experimental results and band-structure calculations of ${\mathrm{LaH}}_2$ and ${\mathrm{LaH}}_3$. It is suggested that the peak in ρ(T) is induced by an order-disorder transition, and the density of states at the Fermi level rapidly decreases to zero and a gap is consequently opened up as the concentration x approaches the trihydride stoichiometric limit.