Hydrogen permeation in V₇₅Fe₂₅ alloy investigated by in situ neutron diffraction

In situ neutron diffraction measurements were carried out to investigate the hydrogen permeation path in V₇₅Fe₂₅ alloy. During heating under D₂, replacing H₂ to enhance neutron scattering length, the atmosphere at 3 bar, deuterium atoms began to permeate into V₇₅Fe₂₅ alloy at around 350 °C signified by the increased diffuse background signal. While the scattering feature of vanadium-iron solid solution in a BCC structure exhibits just a thermal expansion, a new Bragg peak corresponding to the α-V phase, in which deuterium atoms occupy tetrahedral interstitial sites of V randomly, appeared indicating that deuterium atoms are permeated to segregated vanadium clusters in V₇₅Fe₂₅ alloy. At room temperature, the hydrogen ordering in the vanadium clusters occurred, which is evidenced by a phase transition from α-V (cubic, solid solution) to β-V (monoclinic, hydride). Interestingly, we could conclude that additional hydrogen atoms occupy randomly octahedral sites surrounding ordered hydrogen atoms in the β-V of V₇₅Fe₂₅ alloy.