Thermodynamics and isotope effects of the vanadium–hydrogen system using differential heat conduction calorimetry

The V–H (D) systems have been investigated using reaction calorimetry at 298 and 323 K over hydrogen contents from H (D)/V=0→≊1.8. Relative partial molar enthalpies have been determined by absorption of small increments of hydrogen in the single phase regions; for regions where two solid phases coexist, integral or plateau enthalpies are obtained by absorption (desorption) of either large or small amounts of hydrogen. The following calorimetric results were obtained for the two solid phase plateau regions: ‖ΔH‖=40.6 kJ/mol H, 40.8 kJ/mol D for (α + β) and ‖ΔH‖=19.1 kJ/mol H and 23 kJ/mol D for (β′+γ) and (α′+ε), respectively. Calorimetric data, relative partial molar enthalpies are also given for the single phase regions. Several phase boundaries are intersected as the hydrogen contents increase in this temperature range and the behavior of the enthalpies at the phase boundaries has been determined and is discussed in relation to the nature of the coexisting phases.