NMR-STUDIES OF HYDROGEN DIFFUSION IN ZRBE2H1.4

The diffusivity of hydrogen in the metal-hydride ZrBe2H1.4 has been st udied by different nuclear magnetic resonance (NMR) techniques. The pr oton spin-lattice relaxation rate Gamma(1) has been measured at 27.7 M Hz and 67.7 MHz in the temperature range from 175 K to 500 K. At four distinct temperatures the frequency dependence of Gamma(1) has been in vestigated between 0.31 MHz and 29.2 MHz using the field-cycling (FC) NMR technique. These studies indicate that the hydrogen diffusion is r estricted to two dimensions within each particle of the powdered sampl e. The structure of ZrBe2H1.4 suggests that. the hydrogen diffusion ta kes place in the xy planes formed by the Zr atoms. The variation of th e echo amplitude with the applied field gradient in pulsed-held-gradie nt (PFG) spin-echo experiments confirmed the two-dimensional character of the hydrogen diffusion. The PFG data taken between 272 K and 478 K yielded directly the diffusivities in the xy plane D-xy. The temperat ure dependence of D-xy is well represented by an Arrhenius law with an activation enthalpy H-a=(0.25+/-0.02) eV. This activation enthalpy is in very good agreement with values deduced from both the frequency an d the temperature dependence of Gamma(1). (C) 1998 Elsevier Science S. A.