Nuclear Magnetic Resonance Studies of Metal-Hydrogen Systems

Pulsed nuclear magnetic resonance methods have been used to investigate the properties of various metal -hydrogen systems including ZrH_{rm x}, ScH_{rm x} , NbH_{rm x}, VH_{rm x} etc. For pure dihydride-phase samples of ZrH_{ rm x}, the spin-lattice relaxation time T_1 has been measured as a function of temperature in the temperature range from 10 K to 1300 K at 12.2 MHz and 40 MHz. These measurements show that the activation energy E_{rm a } for hydrogen diffusion first increases smoothly from 0.57 eV/atom at x = 1.58 to 0.63 eV/atom at x = 1.93 and then increases quite rapidly near the stoichiometric limit, reaching 1.06 eV/atom at x = 1.98. T_1 measurements at temperatures below 300 K reveal that the d-band electronic states are split due to the Jahn-Teller effect. For ZrH_ {rm x} samples doped with paramagnetic impurity ions (Mn, Cr, Fe), an additional spin-lattice relaxation rate R_{1rm p} was observed. The rate R_{1 rm p} increases in the sequence Fe-Cr -Mn and also increases sharply with increasing hydrogen concentration in each case. The former result is consistent with the observation that the tendency towards localized -moment formation in Zr increases in the same sequence, whereas the latter may be accounted for by the anti-trapping behavior of these impurities. For dilute solid solution alpha -phase Sch_{rm x} , we measured T_1 of both ^1H and ^{45 }Sc as a function of temperature to investigate hydrogen diffusion. The activation energy E_ {rm a} = 0.54 eV/atom and attempt frequency nu_{rm o} = 1. times 10^ {14} Hz were obtained. The absence of a prefactor anomaly in this dilute system is consistent with the hypothesis that such anomalies in other systems may result from repulsive particle-particle interactions at the saddle point. We have also observed anomalous behavior of the proton spin-lattice relaxation time T_1 at high temperatures (up to 1300 K) for hydrogen in faced-centered-cubic (fcc) dihydride phases of ZrH _{rm x}, TiH_ {rm x}, YH_{ rm x} and LaH_{rm x} and in the body-centered cubic (bcc) solid solution phases of NbH_{rm x }, VH_{rm x} and their alloys. In addition to the usual T _1 minimum, T_1 decreases sharply at higher temperatures, contrary to the expectation that T_1 would return to the value T_{1rm e} determined by the conduction electron contribution to the total relaxation rate. This decrease in T_1 may have its origin in highly correlated hydrogen motion at high temperature. ftn*DOE Report IS-T-1359. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.