NMR study of thorium hydride (Th4H15)

Two samples of thorium hydride, ${\mathrm{Th}}_4$ ${\mathrm{H}}_{15}$, have been examined with a variety of NMR techniques. A moment analysis of the rigid-lattice line shape provides confirming evidence for the proton locations hypothesized from x-ray measurements of the thorium positions. $T_1$ and $T_2$ measurements above 390°K furnish activation energies for proton motion of 16.3 ± 1.2 and 18.0 ± 3.0 kcal/mole, respectively, while below 350°K, $T_{1r}$ measurements indicate an activation energy of 10.9 ± 0.7 kcal/mole for proton motion. This large change in activation energy over a rather narrow temperature range indicates complex, or more than a single, mechanism for proton motion within thorium hydride. $T_1$ at room temperature and below is dominated by relaxation due to conduction electrons ($T_{1}T=180\mathrm{}\pm 10$°K sec), and by using multiple-pulse techniques to reduce the homonuclear dipolar broadening, a temperature-dependent line shift is observed. A time-temperature hysteresis was characterized in the measurements on one of the two samples and is strongly indicative of a phase change.