Diffusion-induced reduction of the57Fe Mossbauer fraction in βPdHx

Abstract

The temperature dependence of the ⁵⁷Fe Mossbauer f factor, center shift and linewidth of ⁵⁷Co:PdH_x sources with x=0 and 0.67 and of ⁵⁷Fe:PdH_x absorbers with x=0.55 and 0.71 were measured between 100 and 300K. In all hydrides an anomalous decrease of the f factor was observed between 200 and 260K. This can be attributed to the transient strains produced by the hydrogen atoms diffusing in the Pd lattice. In the samples with x=0.67 and 0.71, the temperature dependence of the centre shift deviates from the mere second-order Doppler behavior. This reveals changes in the mean population of the hydrogen sites near the Mossbauer probe. The temperature dependence of the linewidth can be understood qualitatively as a superposition of two effects, namely broadening due to the phase incoherence that arises from the fluctuating strains and motional averaging of the hyperfine interactions, which cause an inhomogeneous line broadening at low temperatures.