Hydrogen behavior in Pd0.75Ag0.25H x studied by PAC and PAS

Abstract

Hydrogen behavior in Pd0.75Ag0.25Hx has been investigated by both perturbed angular correlation (PAC) and positron annihilation lifetime spectroscopy (PAS) techniques. The PAC measurements with the 111Ag/111Cd probe nuclei were conducted at room temperature as a function of the hydrogen concentration x up to 0.35. At x = 0 the probe nuclei experienced no quadrupole perturbation, while a distinct perturbation was observed after hydrogen charging. The quadrupole interaction frequency ω1 and its distribution width σ1 and relative fraction f1 all decrease with increasing hydrogen concentration, indicating size growth and concentration reduction of the hydrogen induced vacancy clusters. The measured frequency distribution width σ1 reveals hydrogen diffusion. The PAS measurements were also performed at room temperature. The obtained lifetimes and their variation with hydrogen concentration strongly support the PAC results. The experimental results can be explained by the appearance of the β-phase in the Pd0.75Ag0.25Hx samples. The α-phase is the major phase in the Pd0.75Ag0.25Hx samples. Charging of the hydrogen leads to the formation of the β-phase and the fraction of the β-phase increases with increasing hydrogen concentration. The present PAC and PAS results show that as the fraction of the β-phase increases, hydrogen induced vacancy clusters are formed and become larger and larger, while the cluster density reduces due to the combination of the vacancy clusters into larger size vacancy clusters.