Hydrogen absorption in Ce _xGd _1-x alloys

The effect of alloying on the thermodynamics of hydrogen absorption was studied for Ce _xGd _1-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, ΔH _f, and the entropies, ΔS _f, of the dihydride formation exhibit a pronounced and broad negative minimum starting at x ≈ 0.15, yielding the most negative ΔH _f 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 Ce _xGd _1-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.