Nuclear magnetic resonance study of hydrogen in niobium-vanadium alloys

Proton spin relaxation rates have been measured across the random alloy system Nb/sub y/V/sub 1-y/H/sub x/ at temperatures from 10 to 800 K and various frequencies from 4.45 to 127.5 MHz. Enhanced spin-lattice relaxation rates found below 100 K in all samples with Nb, including NbH/sub 0.2/, at frequencies of 40 MHz and below have been explained by a process of dipolar cross relaxation between proton Zeeman energy levels and the combined Zeeman-quadrupole levels of 93Nb. A semi-quantitative model, applicable in metal-hydrogen as well as other multi-spin systems, has been developed to describe the cross relaxation mechanism. Unusual features found experimentally and interpreted via this model include: (1) cross relaxation in unrotated polycrystalline samples, (2) cross relaxation to spins with pure quadrupole frequencies substantially less than the proton Larmor frequency, and (3) both temperature-dependent and temperature-independent contributions which can vary with frequency. Effects of Zeeman-quadrupole splittings on motional relaxation rates at higher temperatures were also estimated. 143 refs., 44 figs., 5 tabs