Hydrogen in Niobium-Molybdenum Alloys: A Realistic Example of a Random-Field Ising Model

A theory of metal-hydrogen systems is described which predicts how the phase diagram will change when low concentrations of substitutional impurities are added to the system. Realistic models for ${\mathrm{Nb}}_{0.95}$ ${\mathrm{Mo}}_{0.05}$-H and ${\mathrm{Nb}}_{0.85}$ ${\mathrm{Mo}}_{0.15}$-H systems are developed which are lattice-gas analogs of the random-field Ising model. By Monte Carlo simulation the phase diagrams are obtained and compared with recent experiments.