Effect of Hydrogenation on the Paramagnetic Resonance of Gd in Pd and Pd-Rich Alloys

Abstract

Paramagnetic resonance of Gd in Pd and rare-earth-doped Pd has been reported recently.1,2 It was shown that a strong interaction with a long range exists between the conduction electrons and the spin part of the localized rare-earth ions. This was exhibited via the g shift of the Gd in pure Pd, with respect to its ionic value, e.g., g = 1.992±0.001, and the additional shift of the resonance when additional rare-earth impurities were alloyed into the Pd. The interaction and its range are attributed to the high density of states of the unfilled d band of the Pd. In Pd-Ag alloys the Gd g value shifts continuously toward its ionic g value with the increase of the silver content. This shift has the same behavior as the decrease of susceptibility with the increase of Ag concentration.1 For 40% Pd 60% Ag the g shift, as well as the susceptibility, are almost zero. These results confirm the above statement. When Pd is hydrogenated the g value of Gd changes from 1.88 for the pure sample to 2.00 for a sample with H:Pd ratio of 50%. Thus hydrogenation shifts the g of Gd back to its ionic value. Alloying rare-earth impurities into Pd broadens the Gd line but produces no g shift in the hydrogenated samples. These results show that the interaction between the Gd localized moment and the conduction electrons is largely suppressed. This is expected from the fact that hydrogen reduces the anomalously high susceptibility of Pd. The Gd resonance in mixed α and β phases (H/Pd from 5% to 60%) shows two lines: one at g ≈ 2.00, the other at g ≈ 1.90 corresponding to the lines in the separated β and α phase, respectively. The line intensities are roughly proportional to the amount of each phase present. This result contradicts Mott's hypothesis that the linear decrease of the Pd susceptibility with the increase of the hydrogen concentration is caused by filling the holes in the Pd d band with the hydrogen electrons.3 If Mott's hypothesis were correct, a continuous shift of the Gd g value, as in the case of Pd-Ag alloys, would be expected. We, therefore, conclude that the linear decrease of the susceptibility with hydrogen content is due to the proportion of the α and β palladium. The additional increase of the linewidth in β palladium with increase of Gd or other rare-earth concentration is mostly accounted for by dipole-dipole broadening. Even with signal-to-noise ratios of 2000:1 only one line was observed. Despite the fact that powdered samples were used, the shape closely approximates a Lorenzian line. Therefore, we have assumed that all the seven ΔM = ±1 transition lines of the Gd 8S7/2 state are included in this line. The extrapolated linewidth for zero Gd concentration was 80 G. This gives an estimated upper limit for the zero field splitting of Gd in β palladium of 5 10−4 cm−1.