H-1-NMR STUDY OF CENIALHX AND CECUALHX

H-1 cw and pulsed NMR studies on the hydrides CeNiAlH(x), with hydrogen concentration 0.09 less-than-or-equal-to x less-than-or-equal-to 2.85 and CeCuAlH(x) with x = 0.64 and 1.51 have been performed in the temperature range 100-400 K. In all samples, the H-1 spectra appear as a broad resonance line at low temperatures, a narrow resonance line at high temperatures, and a superposition of a narrow component and a broad component at intermediate temperatures. The temperature dependence of the linewidth of the narrow component indicates the presence of mobile hydrogen. The activation energy of hydrogen motion in CeNiAlH2.85 has been estimated to be E(a) = 1.7 kcal/mol. Measurement of the spin-lattice relaxation time T1 of protons reveals the coexistence of two types of H-1 nuclear magnetizations at all hydrogen concentrations. In one type (in the case of CeNiAlH(x)), which makes the dominant contribution to the total magnetization, the relaxation mechanism is dominated by the influence of localized magnetic moments, whereas the other type present in small proportion is characterized mainly by a Korringa-type relaxation mechanism. The results thus suggest that hydrogenation of CeNiAl leads to the formation of a two-phase compound. In one phase, i.e., the hydride phase, hydrogenation results in the transformation of the Ce4+ state to the Ce3+ state. This hydride phase contains two types of hydrogen sites; hydrogen atoms in one of the sites are mobile resulting in a narrow line. The second phase contains a cerium ion in the Ce4+ state, as in the parent compound CeNiAl; this phase may be the solid-solution phase of CeNiAl and hydrogen. In the case of CeCuAl, there is no hydrogen-induced valence transformation, and cerium is in the Ce3+ state in CeCuAl as well as in its hydrides. It is found that in CeCuAlH(x) there are also two types of H-1 nuclear magnetization, but in this case the relaxation behavior of both magnetizations are influenced by localized magnetic moments. These results have been analyzed by considering contributions from different mechanisms to the relaxation process, e.g., (a) a contact interaction between the conduction electrons and the proton, (b) the Ruderman-Kittel-Kasuya-Yosida interaction between the proton and the localized f electron through the conduction electrons, and (c) a dipolar interaction between,a proton and Ce3+ localized magnetic moments.