Magnetic Susceptibility and Equilibrium Diagram of PdHn

Abstract The limits n αmax and n βmin of the two‐phase region of PdH n can only roughly be estimated from the shape of the equilibrium isotherms p H2 ( n ). Other methods applied so far do not yield more accurate results. More precise values can be obtained, however, from measurements of the magnetic susceptibility x as a function of the hydrogen content n at various temperatures. Such x( n ) isotherms have been measured at temperatures between 20 and 300°C and H 2 pressures up to 140 atm (0 · n · 0.8), using samples of Pd wire (1 mm) and Pd foil (33 μm). In the homogeneity range isotherms for adsorption and desorption were identical, in the two‐phase region, however, hysteresis was always observed. Here the desorption curve was taken as the equilibrium isotherm, and was applied to determine the values of n αmax and n βmin by extrapolation. Measurements on Pd black in the same region of pressure and temperature showed a number of peculiarities, for instance smaller values of susceptibility and smaller hysteresis loops as compared with bulk Pd. These can be attributed to the large specific surface area of Pd black as well as to its strongly distorted lattice structure. By means of the measurements on bulk Pd the position of the critical point of the palladium‐hydrogen system could be redetermined with rather high precision: T c = 291 ± 2°C; n c = 0.250 ± 0.005 mol H/mol Pd; P c = 19.7 ± 0.2 atm H 2 . The measurements on Pd‐black yielded within the limits of error the same values for the critical temperature and the critical pressure, whereas the value of the H/Pd ratio, properly corrected, was found to be a bit higher, namely n c = 0.260 ± 0.005.