Proton spin-lattice relaxation mechanisms and the metal-insulator transition in cerium hydrides

Nuclear-magnetic-resonance (NMR) experiments have been done on cerium hydride (CeHₓ) samples to search for correlations between NMR properties and known electrical conductivity changes as a function of hydrogen concentration and temperature. Data are presented for the ¹H spin-lattice relaxation rate R₁ (=1T₁) and some line shapes for 2.10≤x≤2.92 for temperatures from about 100 to 375 K. Although two ¹H resonances are observed at some temperatures, proton spin-lattice relaxation is characterized by a single relaxation time at each x and T. To a good approximation R₁=A/T+R, where A/T is attributed to direct dipolar coupling between protons and the electronic magnetic dipole moment of Ce³⁺, and R is an essentially temperature-independent term attributed to indirect [Ruderman-Kittel-Kasuya-Yosida (RKKY)] coupling to the Ce³⁺ moment. The A/T term is so large that for most experiments the proton-proton dipolar and proton---conduction-electron couplings are negligible. The x dependence of the constant A is consistent with the dipolar coupling. The constant R decreases in a steep manner as x is increased above x≈2.65 just below the regime $2.75<x<2.85$, where the metal-semiconductor transition occurs in CeHₓ. It is proposed that R∝Nd(EF) and that the RKKY interaction includes coupling through the d-band density of states. The marked decreases in R₁ and in the electrical conductivity that are associated with the concentration-dependent transition are thus attributed to the vanishing electron density of states at the Fermi surface. No temperature-dependent transition in R₁ is found. Results are consistent with a Mott transition model in which the electron donors are hydrogen vacancies.