X‐ray spectra of hydrogenated 3d and 4d metals

Abstract

IntroductionThere are two models commonly used to explain the changes in the different physical properties of the 3d and 4d transition metals subjected to either electrolytic or pressure-induced hydrogen diffusion. The protonic model presumes that the diffused hydrogen donates its electron to the unfilled part of the conduction band or the valence band [l] while the proton remains in the lattice at the sites indicated by neutron diffraction [2]. This model could explain the changes in the magnetic properties of the metals. The model, however, failed to account for the changes in the electronic specific heat of metals on hydrogen diffusion. The X-ray photoelectron emission [3] from hydrogenated palladium and nickel showed the existence of induced states [4] lying just below the Fermi level. These models, however, do not account for the trapped hydrogen [5] that resides at defect lattice sites and is retained by the lattice unless special efforts are made to remove it.The X-ray absorption and emission spectra of a few metals have been recorded both with and without hydrogen diffusion. The present note discusses which of the above two models is suitable to explain the extra features seen in the spectra of hydrogenated metals. The conclusion is that neither of the two models is sufficient to account for all the observed features. The two models represent two competitive processes and it is the mode of hydrogen diffusion that decides the dominance of a particular mechanism [24]. X-ray absorption spectraThe first reference in the literature is due to Hanawalt [6] who studied the effect of hydrogen diffusion on the L,,, edge of palladium. After hydrogen diffusion the edge shifted by 0.6 eV toward the higher-energy side with respect to the edge in pure metal. The extended structure showed slight expansion of the lattice.The next reference is due to Zhurakovskii and Vainshtein [7[ on the K edge of vanadium metal containing varying amounts of hydrogen in the range of 0.16 to 1.25%. The method of hydrogen diffusion has not been described but the observations were explained on the basis of the protonic model. The pure metal shows a step K, on the slope of the edge (Fig. 1) which vanishes on hydrogen diffusion. This step-like structure is interpreted as Is + 3d transition allowed by the perturbed symmety in the metal. The electron from the diffused hydrogen is supposed to fill the 3d-band and the structure K, vanishes.Lewis [8] recorded the K edge of nickel during continuous diffusion of hydrogen at 350 "C. The changes in the absorption curve though real were of very small magnitude. Janko [9] gave a fresh impetus to the work on hydrogen diffusion. He obscrved that if electrolytic hydrogen diffusion is done in the presence of an inhibitor like thiourea, the lattice parameters of nickel show a change by 6%. This method was orginally due to Baranowski and Smialowski [lo]. Faessler and Schmid [ll] adopted this method of