A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd–Au–H

Due to their ability reversibly absorb/desorb hydrogen without hysteresis, Pd--Au nanoalloys have been proposed as materials for sensing. For sensing, it is important that absorption/desorption isotherms are reproducible and stable over time. A few studies pointed the influence of short long range chemical order on these isotherms, but many aspects impact remained unexplored. Here, we use alloy cluster expansions describe thermodynamics in a wide concentration range. We investigate how different orderings, corresponding annealing at temperatures well external pressures hydrogen, behavior material with focus its isotherms. In particular, find long-range ordered L1$_2$ phase expected form if \ce{H2} pressure sufficiently high. Furthermore, construct diagram from \unit[250]{K} \unit[500]{K}, showing full equilibrium reached presence separation can often be occur, stark contrast para-equilibrium. Our results explain experimental observation time, also reveal pitfalls when this may not case.