EXPLANATION OF THE HIGH-TEMPERATURE RELAXATION ANOMALY IN A METAL-HYDROGEN SYSTEM

The source of high-temperature anomalous spin relaxation in a specific metal-hydrogen system has been investigated. Proton spin-lattice rela xation rates R1 from 290 to 1025 K were compared for Nb0.5V0.5H0.36 an d its 90% deuterided counterpart, Nb0.5V0.5(H0.1D0.9)0.36. A substanti al reduction in the anomalous proton relaxation rate R1 is observed fo r the deuterided metal. Thus, the proton relaxation rates R1 are deter mined by proton-proton (or proton-deuteron) dipolar interactions in th e anomalous regime, suggesting that H-2 (or HD) molecules are the sour ce of the relaxation. Relaxation studies with overpressures of argon a nd xenon gas indicate that the anomalous relaxation of protons in the metal occurs via fast exchange with rapidly relaxing gas phase H-2 mol ecules. A small fraction of protons in the gas phase, consistent with the H-2 vapor pressure of the metal hydride, is adequate to explain ou r results. A model, based on experiment, is presented along with measu rements of the proton R1 for pure H-2 and HD gas and gaseous mixtures of H-2 with Ar and H-2 with D2. The model quantitatively accounts for the isotope and rare-gas effects as well as the temperature dependence . Thus, in Nb0.5V0.5H0.36, the relaxation anomaly is proven not to be an intrinsic effect.