The dift'usion of hydrogen through the hexagonal metal hydride ZrBe&H& 4 has been studied by measuring the H nuclear spin-lattice relaxation rate T~a s a function of frequency and temperature. At high temperatures, where the hopping rate of the H interstitials is faster than the nuclear magnetic resonance (NMR) frequency, the relaxation rate has a frequency dependence much larger than expected for random diffusive motion in three dimensions. A previous NMR study of ZrBe2H& 4 did not account for the anomalous frequency dependence. Our data more fully define the frequency dependence, and we find that it is consistent with a model in which the motion of the hydrogen is restricted to two dimensions. Two-dimensional motion allows a self-consistent analysis of the Td ata for all the measured frequencies and provides an improved measure of the activation energy, E = 0.27 + 0.02 eV. The frequency dependence of T~c an be used to identify two-dimensional motion in a randomly oriented powder sample.

No abstract