NMR study of the plutonium hydride system

An NMR study was conducted on protons in the nonstoichiometric plutonium hydride system, $\mathrm{Pu}{\mathrm{H}}_x$. The following compositions were studied: $x=1.78, 2.35, 2.65, \mathrm{and} 2.78$. The line shapes, Knight shifts ($K$), spin-spin relaxation times ($T_2$), and spin-lattice relaxation times ($T_1$) were measured in the temperature range of 77-300 K. The results indicate the existence of paramagnetic phases at high temperatures, with localized $5f$ moments on the Pu ions, and possible magnetically ordered phases at low temperatures. The predicted transition temperatures and type of magnetic order were found to depend strongly on the hydrogen concentration. Different line shapes and shifts are found for the tetrahedral and octahedral protons, which indicate a different bound state for the hydrogen ions. The hydrogen self-diffusion constants were derived from $T_2$ measurements in the motionally narrowed regime, and were found to be quite similar to those found in the $\mathrm{La}{\mathrm{H}}_x$ system. $T_{1}T$ as well as ${(K-K_0)}^{-1\mathrm{}}$ were found to be linear with temperature. The proportionality constants were found to be composition independent. The possible magnetic couplings between the protons and the paramagnetic moments are discussed.