Hydrogen spectroscopy of Pd 0.9 Ag 0.1 -H alloys on NMR scales

Simultaneous realisation of hydrogen charging or discharging process and in situ measurements of different NMR characteristic quantities give outstanding chances for the investigation and understanding of local physical properties of and hydrogen diffusion in dis- ordered metal-hydrogen systems, including both equilibrium and non-equilibrium states. A few selected preliminary experimental results are presented in this letter for Pd0.9Ag0.1-H al- loy on in situ hydrogen concentration, proton NMR line-shift and magnetic susceptibility, 1 H NMR spectrum, spin-spin relaxation time and hydrogen diffusion activation energy and cor- relation time measurements. The motional averaged spectrum is asymmetric and the possible decompositions relate to the non-single-site residence of hydrogen in this alloy. Introduction. - Feenstra et al. (1) recently proposed to consider hydrogen embedded in a metallic lattice as a "local probe" for the investigation of local properties of hydrogen containing disordered alloys. In this concept Solomons and co-workers (2) used thermodynamic data of hydrogen absorption for Pd-Ag-H alloys and a multi-site lattice-gas model for the determination of site energies of hydrogen located in the centre of Pd1−xAgx octahedron, and the terminology of "hydrogen spectroscopy" was introduced by them. The Pd-Ag alloys are probably the simplest and consequently the best representatives of the chemically disordered alloys, they were subjects of widespread experimental and the- oretical investigations (see, e.g., (3-5) and references therein). The atomic size of the two alloying elements are nearly the same in this system, and Pd and Ag form homogenous solid solution of face-centred-cubic (fcc) structure in the whole concentration range. The hydrogen solubilities for the pure metals differ by many orders of magnitude, so the local distribution of constituents (probably binomial) could strongly affect the microscopic physical properties, too. The magnetic susceptibility of pure Pd is reduced by both silver and hydrogen addition demonstrating the change of the electron structure. The other properties, which are impor- tant in this context, i.e. hydrogen solubility, hydrogen diffusivity, nuclear magnetic relaxation rates, also depend on the composition. NMR —as is well known— is one of the most widely applied local methods, and hydrogen nuclei serve a priori as local probes for the investigation of different microscopic properties