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

Abstract

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 $2.10\ensuremath{\le}x\ensuremath{\le}2.92$ 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 $x\ensuremath{\approx}2.65$ just below the regime $2.75<x<2.85$, where the metal-semiconductor transition occurs in $\mathrm{Ce}{\mathrm{H}}_{x}$. It is proposed that $R\ensuremath{\propto}{N}_{d}({E}_{F})$ 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.