Ce-valence state and hydrogen-induced volume effects in Ce-based intermetallic compounds and their hydrides

An average Ce-valence state (v) of two types of Ce-containing intermetallic compounds, equiatomic CeNiX (X= Al, Ga, Sn) and CeM3 (M= Ni, Co, Mn), and their hydrides was estimated from X-ray absorption spectroscopy (XAS) and analysed in parallel with hydrogen-induced volume changes on hydrogenation. The largest valence states in the initial compounds were found in CeM3(v=3.32−3.36) followed by CeNiAl1−xGax(v=3.25−3.27). This contrasts to CeNiSn, which is close to a pure trivalent state (v=3.07). On hydrogenation, a conversion from mixed-valent CeNiAl1−xGax, x=0.5, 1, to pure CeIII hydrides takes place (ΔV /V=19.6−20.0 %). In CeNiSnDz (z=1, 1.8, ΔV/V=3.0, 8.0%) the changes in the valence state towards CeIII are very small. The situation for the CeM3-hydrides is complex. For CeNi3D2.8, CeNi2.75Mn0.25D3.4 and CeCo3D3.4 (ΔV /V=24−32%) where rather similar electronic properties can be expected, a decrease in the contribution of CeIV for CeNi3D3 and CeNi2.75Mn0.25D3.4 (v=3.18 and 3.12, respectively) contrasts to the behaviour of CeCo3D3.4 where the hydrogen induced valence change is very small (v=3.32).