Solubility of protium, deuterium, and tritium in the $\alpha$ phase of palladium

The first solubility determinations of tritium in bulk palladium are presented for the pressure range $0.016<~p<~1.33$ bars, temperature range $60<~T<~400°\mathrm{C}$, and concentration range $0.0015<~x<~0.02$ (for $x$ in $\mathrm{Pd}{\mathrm{T}}_x$). In addition, the first comparative study of the pressure-composition-temperature relationship of all hydrogen isotopes in Pd at low concentration is described using the same sample under identical experimental conditions. The solubility of tritium in Pd behaves as expected from the stable hydrogen isotopes. The partial molar enthalpy $\Delta {\overline{H}}_{{\mathrm{T}}_2}^{\infty }$ and the nonconfigurational entropy $\Delta {\overline{S}}_{{\mathrm{T}}_2}^{\infty ,\mathrm{nc}}$ at infinite dilution measured during desorption decrease with increasing temperature. Furthermore, $\Delta {\overline{H}}_{X_2}^{\infty }$ ($X=\mathrm{H},\mathrm{D},\mathrm{T}$) shows a strong isotopic dependence, whereas $\Delta {\overline{S}}_{X_2}^{\infty ,\mathrm{nc}}$ shows a weak one. The following inequalities are valid: $\Delta {\overline{H}}_{{\mathrm{T}}_2}^{\infty }<\Delta {\overline{H}}_{{\mathrm{D}}_2}^{\infty }<{\overline{H}}_{{\mathrm{H}}_2}^{\infty }$ and $\Delta {\overline{S}}_{{\mathrm{T}}_2}^{\infty ,\mathrm{nc}}<~\Delta {\overline{S}}_{{\mathrm{D}}_2}^{\infty ,\mathrm{nc}}<~{\overline{S}}_{{\mathrm{H}}_2}^{\infty ,\mathrm{nc}}$. The different behavior of the solubility for H and D or H and T can be related well to the different vibrational energies of H, D, and T in the measured temperature range if one uses the vibrational energies of the H and D atoms in Pd determined by inelastic neutron scattering and extrapolates for T in Pd.