Hydrogen absorption in CexGd1−x alloys

The effect of alloying on the thermodynamics of hydrogen absorption was studied for CexGd1−x alloys (0 ≤ x ≤ 1) at temperatures between 850 K and 1050 K in the 1–10−4 Torr pressure range. The temperature-dependent hydrogen solubilities and plateau pressures for hydride formation were obtained from hydrogen absorption isotherms. The terminal hydrogen solubility (THS) at a given temperature changes in a monotonic way as a function of x. It is approximately three times higher in Gd, than in Ce, throughout the investigated temperature range. This monotonic behavior is opposed to that of many other substitutional alloys, for which the hydrogen terminal solubility increases with increasing solute concentrations. The enthalpies, ΔHf, and the entropies, ΔSf, of the dihydride formation exhibit a pronounced and broad negative minimum starting at x ≈ 0.15, yielding the most negative ΔHf values ever found for metal hydrides. On the other hand, the enthalpies and entropies of ideal solution display a positive trend at x = 0.15 and x = 0.3. Both behaviors are considered in view of a reported distinct variation of the CexGd1−x hardness as a function of x. The particular compositional variations of the entropies of solution and formation as a function of x reflect most likely the vibrational properties of the hydrogen atoms in the metal matrices.

► CexGd1−x alloys exhibit the most negative heats of hydride formation ever found.
► Thermodynamics of H absorption in CexGd1−x correlates with the alloys hardness.
► The entropies of H solution and hydride formation reflect the hydrogen vibrations.
► Terminal hydrogen solubilities change in a monotonic way between Ce and Gd.