A 2D NMR study of deuterium trapping by dislocations in palladium

Abstract

Deuteron spin-lattice and spin-spin relaxation rates have been measured in annealed and cold-rolled foil samples of palladium deuteride PdDapproximately=0.6 in the temperature range 100-390 K using pulsed NMR at 14.8 MHz. The CPMG sequence spin-spin decay in samples containing different densities confirms the coexistence of two relaxation rates, R2a<R2b ( approximately=11 and approximately=170 s-1 at room temperature for instance), both of which are much larger than R1 (<or=0.42 s-1). R2a describes relaxation of deuterons in normal interstitial sites whereas R2b is thought to originate from deuterons trapped in dislocation core regions. For T<or=160 K, R1 and R2a are attributed to a diffusion-modulated, deuterium-vacancy-induced, quadrupolar interaction; for T>or=160 K, R2a is dominated by quadrupolar interactions of diffusing deuterons with long-range electric field gradients associated with dislocation strain fields. The temperature dependence of R2b yields an activation energy of approximately=0.024 eV for deuterium diffusion within the core regions where a quadrupolar coupling constant of at least 4 kHz is measured. In spite of this low activation energy, diffusion in the cores is relatively slow since nearly all sites in core regions are filled. It is estimated from the signal amplitudes that trapping can cause almost all dilated deuterium sites to be filled out to a radius of 30 AA around dislocation cores for a dislocation density approximately 1012 cm-2.