BEYOND-PAIR INTERACTIONS IN THE PD-H-2 SYSTEM

By using a mean-field approximation we give a semianalytical method fo r reducing data on metal-hydrogen systems, which allows for hydrogen-h ydrogen (H-H) interactions beyond the level of pair interactions. Inte ractions up to the triplet level are explicitly included here, althoug h the method allows for interactions up to any finite order. The model fits low-concentration data at one temperature and then predicts pres sures, lattice expansion and various energies as functions of concentr ations for a range of temperatures. A Monte Carlo simulation is used t o lend support to the approximate validity of the mean-field approxima tion for calculating the entropy in the regime of low concentrations. We find an increasingly negative average triplet interaction energy as the H concentration is increased. Although the mean-field approach is not an adequate description of clustering phenomena leading to the al pha-beta phase transition, nevertheless our finding of a more negative triplet energy with increased concentration may indicate the importan ce of beyond-pair interactions in the phase transition.