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

Nuclear-magnetic-resonance (NMR) experiments have been done on cerium hydride ($\mathrm{Ce}{\mathrm{H}}_x$) 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 ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ spin-lattice relaxation rate $R_1 (=\frac{1}{T_1})$ and some line shapes for $2.10\le x\le 2.92$ for temperatures from about 100 to 375 K. Although two ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ 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 $R_1=\frac{A}{T}+R$, where $\frac{A}{T}$ is attributed to direct dipolar coupling between protons and the electronic magnetic dipole moment of ${\mathrm{Ce}}^{3+}$, and $R$ is an essentially temperature-independent term attributed to indirect [Ruderman-Kittel-Kasuya-Yosida (RKKY)] coupling to the ${\mathrm{Ce}}^{3+}$ moment. The $\frac{A}{T}$ 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 $x\approx 2.65$ just below the regime $2.75<x<\mathrm{}2.85\mathrm{}$, where the metal-semiconductor transition occurs in $\mathrm{Ce}{\mathrm{H}}_x$. It is proposed that $R\propto N_d\left(E_F\right)$ and that the RKKY interaction includes coupling through the $d$-band density of states. The marked decreases in $R_1$ 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 $R_1$ is found. Results are consistent with a Mott transition model in which the electron donors are hydrogen vacancies.