Optical vibrations of hydrogen in disordered palladium-gold alloys

TL;DR

This study investigates the optical vibrations of hydrogen in disordered Pd-Au alloys, revealing a consistent spectral shoulder across concentrations, which impacts understanding of hydrogen behavior and isotopic effects in palladium-based materials.

Contribution

It provides new insights into hydrogen vibrational spectra in Pd-Au alloys, showing a persistent shoulder feature regardless of hydrogen concentration, challenging previous assumptions about dilute hydrogen vibrations.

Findings

Optical hydrogen band shows a sharp peak with a consistent broad shoulder.

Shoulder intensity remains unchanged across different hydrogen concentrations.

Implications for isotopic effects in hydrogen solubility in Pd are discussed.

Abstract

In the fcc lattice of Pd, hydrogen occupies octahedral interstitial positions with cubic symmetry. One could therefore expect that in dilute solid H solutions in Pd the fundamental band of optical H vibrations should be reduced to a narrow line of 3-fold degenerate, non-interacting isotropic local oscillators. A few attempts to test this assumption by inelastic neutron scattering gave ambiguous results due to the very small limiting hydrogen concentration, x<0.001, of the PdH$_x$ solutions at low temperatures. We increased maximum x by alloying Pd with 20 at% Au and studied disordered Pd$_{0.8}$Au$_{0.2}$H$_{x}$ solutions with concentrations varying from x = 0.03 to 0.74. According to reported $^{197}$Au Mossbauer studies, hydrogen atoms in the Pd$_{0.8}$Au$_{0.2}$H$_{x}$ solutions with x < (0.8)^{6} = 0.26 can only occupy interstices having no Au neighbours. This abates the effect of H-Au interactions in the solutions with lower H concentrations and effectively prevents hydrogen clustering at low temperatures. Our INS investigation showed that the fundamental optical H band in both concentrated and dilute Pd$_{0.8}$Au$_{0.2}$H$_{x}$ solutions consists of a sharp peak with a broad shoulder towards higher energies. The shoulder shows no tendency to vanish with decreasing H concentration and amounts to approximately 0.44 integral intensity of the main peak in all studied samples. The optical band in infinitely diluted H solutions in Pd should therefore consist of a peak with a pronounced shoulder too. Implications for the isotopic effects in the hydrogen solubility in Pd are considered in the present paper.