Neutron diffraction study of the Pd0.772Ag0.228Dν membrane for hydrogen separation

- Deuterium occupies octahedral sites in the FCC structure of the Pd77Ag23 alloy.
- D occupancy decreases with temperature and increases with pressure.
- D uptake at 1 bar is enhanced by previous sample treatment at higher pressure.
- Observed deviations from Sieverts' law are ascribed to D-D repulsion in the structure.

Experiments of deuterium absorption/desorption were performed on a foil of Pd77Ag23 (wt%) alloy in the 78-196 °C temperature and 1-4 bar pressure ranges under the neutron beam at ILL (Grenoble). Powder diffraction patterns were collected on the D1B diffractometer (λ = 1.2871 Å) on the Pd0.772Ag0.228Dν deuterated alloy, and the face-centred-cubic structure was Rietveld-refined locating the D atom in the octahedral site with a variable occupancy ν (=D/M ratio). The results of ν(T) curves at different p(D2) pressures are discussed in comparison with literature data on H and D absorption into PdAg alloys from thermodynamic measurements. A negative deviation from predictions of Sieverts' law is shown by the dependence of D occupancy on pressure. This effect relates the behaviour of the activity coefficient γD to D-D and D-M interactions in the solid solution phase.