A model calculation for the α-α' phase transition in metal-hydrogen systems

The 'gas-liquid' (alpha-alpha prime) part of the phase diagram of hydrogen dissolved in metals is calculated and compared with experiments for H in Nb. The underlying lattice model includes a hard-core repulsion between protons and an elastic interaction mediated by the host metal lattice. The long-range part of this interaction is treated in mean-field approximation. Low-density cluster expansions and Monte Carlo calculations are investigated for the hard-core repulsion and the microscopic part of the elastic interaction. Reasonable agreement with experimental data is obtained, if the microscopic contribution of the elastic interaction is properly taken into account.