Controlling phase stability of superhydrides by combining pressure and electrochemistry

Recently progresses have been made on synthesizing superhydrides with a variety of metals at very high pressures. In this work, we explore the possibility of controlling phase stability of superhydrides by uniquely combining electrochemistry and applied pressure. We predict possible crystal structures of the superhydrides and calculate their energies over a broad range of pressures and electrode potentials, using density functional theory and particle swarm optimization calculations. Based on a thermodynamic analysis, we construct pressure-potential phase diagrams and provide an alternate synthesis concept, pressure-potential (P^2), to access novel phases having high hydrogen content. Palladium-hydrogen is a widely-studied material system with the highest hydride phase being Pd3H4. Most strikingly for this system, at potentials above hydrogen evolution and about 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH10). We predict the generalizability of this approach for La-H, Y-H and Mg-H with orders of magnitude reduction in required pressure for stabilizing phases. In addition, the P^2 strategy allows stabilizing new phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.