Solubility of hydrogen in Pd2Er5, Pd2Er3, PdEr, Pd4Er3, and Pd3Er: Thermodynamics of Pd —Er —H Systems

Abstract

AbstractThe solubility of hydrogen in the intermetallic compounds Pd2Er5, Pd2Er3, PdEr, Pd4Er3, and Pd3Er has been measured by the pressure reduction method in the ranges 1 ≤ P/mbar ≤ 1000 and 473 ≤ T/K ≤ 1148. The partial molar enthalpy of solution at infinite dilution (ΔH0H) and the partial molar excess entropy of solution at infinite dilution (ΔSE0H) have been determined from the α‐phase solubility isotherms. |H0H| is found to decrease with increase of palladium content in the following order: Pd2Er5(‐71.8 kJ ‐ mol−1), Pd2Er3 (–61.0 kJ ‐ mol−1), PdEr (‐18.9 kJ · mol−1), and Pd4Er3 (‐9.2 kJ · mol−1). In Pd3Er, however, ΔH0H is −45.1 kJ · mol−1. The extrapolation of ΔH0H, =f(xEr) in solid solution of erbium in palladium to xEr = 0.25 gives ΔH0H = –37.3 kJ · mol−1, whereas, ΔH0H = –45.1 kJ · mol−1 is observed in Pd3Er, indicating that the formation of the ordered structure results in an additional increase of exothermicity of hydrogen absorption reaction. The increase of exothermicity of hydrogen absorption in Pd3Er when compared to PdEr may be attributed to the smaller octahedral interstitial hole size. ΔSE0H is found to lie between −59.0 and −64.0 J · K−1 · mol−1. Pd2Er5, Pd2Er3, and PdEr show decomposition reactions by which erbium hydride and the next intermetallic compound in the Pd–Er phase diagram with less content of erbium are formed. The decomposition pressure at any temperature is found to increase with increase of palladium content, suggesting that the stability of the compounds increases with increase of palladium content at least till PdEr. The heats of formation of Pd2Er3 and Pd4Er3 have been obtained from the present work and compared with the calculated ones.