H1NMR and magnetic susceptibility study ofThNiAlHxandUNiAlHx

A continuous-wave (cw) and pulsed-proton nuclear-magnetic-resonance (NMR) study was conducted on two ${\mathrm{Fe}}_2$P-type compounds, $\mathrm{ThNiAl}{\mathrm{H}}_x$ and $\mathrm{UNiAl}{\mathrm{H}}_x$, where $x=2.5\mathrm{} \mathrm{and} 1.9$ for the thorium and uranium compounds, respectively. The proton cw NMR spectra of the $\mathrm{ThNiAl}{\mathrm{H}}_x$ compound show a broad line (∼ 9 Oe) below 175 K, a narrow line (∼ 1 Oe) above 196 K, and a superposition of a broad and a narrow line between these two temperatures. The simultaneous appearance of two lines may be explained in terms of either the coexistence of two crystallographic phases or the hydrogens occupying two different crystallographic sites of a single-phase compound in which the mobility of the hydrogen in one of the phases, or sites, is higher than that in the other phase above 175 K. The activation energy for hydrogen diffusion in $\mathrm{ThNiAl}{\mathrm{H}}_x$ was deduced from the spinlattice relaxation time $T_1$ to be 11 KJ/mole based on the Bloembergen, Purcell, and Pound model. Spin-lattice relaxation time $T_1$ and the magnetic-susceptibility results indicate the existence of two antiferromagnetic transition temperatures, one at 122 K and the other at 34 K, in $\mathrm{UNiAl}{\mathrm{H}}_x$. It is most likely that the $\mathrm{UNiAl}{\mathrm{H}}_x$ contains two antiferromagnetic phases with $T_N=122\mathrm{} \mathrm{and} 34$ K.