Trapping by Dislocations in α’-PdD_x Observed by NMR

Abstract

Spin-spin relaxation of ²D in α’-PdD_0.64 measured by a CPMG sequence exhibits two coexisting relaxation times, T_2b < T_2a both much less than the spin-lattice relaxation time T₁ at 20°C, when the sample is loaded from the gas phase so that it passes through the α-α∃ two-phase region. T₁ is explained by a fluctuating quadrupolar interaction with electric field gradients due to the motion of neighbouring deuterons. T_2a must be predominantly due to some other mechanism, identified as interaction with field gradients due to long-range strain fields of dislocations, with a much longer correlation time than that characterizing the T₁ mechanism. T_2b is tentatively ascribed to deuterons trapped in dislocation cores, since this decay is absent in a sample loaded under high pressure so as to avoid the mixed-phase region and thus greatly reduce the dislocation density. The signal amplitude associated with T_2b is temperature dependent with an activation energy ~ 40 meV which would represent the deuterium-dislocation interaction energy. A difficulty with this interpretation is the large apparent fraction of trapped deuterium.