The iron-palladium-hydrogen alloy system. (Mossbauer studies)

Abstract

Using the results of previous workers a phase diagram has been constructed for the AgPdH alloy system showing the hydrogen to metal atomic ratio as a function of Ag concentration. From the results of X-ray and Mossbauer studies reported earlier by the authors, an analogous phase diagram has been obtained for the FePdH system. In each case, the limits of stability of the alpha (low hydrogen content) and beta (high hydrogen content) phases are shown. Further analysis of Mossbauer spectra has determined the variation with temperature of hyperfine field at Fe⁵⁷ nuclei for various concentrations of iron in alpha and beta phase hydrides. The saturation fields for alpha _max and beta _min iron-palladium hydrides are identical to within +or-5 kG at a given iron concentration. The observed Fe⁵⁷ isomer shift difference between alpha and beta phase hydrides is shown to result entirely from changes in lattice parameter. An extension of the model for Fe impurities in a Pd lattice is proposed to interpret these effects. On introducing an iron atom into palladium, rearrangement of band structure occurs which localizes 4d(e_g) type magnetized d states on nearest and next nearest Pd atoms. Polarization of the band t_2g d states of Pd occurs to large distances via a RKKY mechanism. In the hydrides, the t_2g moment is quenched but the localized e_g moment is unchanged except at hydrogen concentrations beyond the normal solubility limit.