Tuning The Thermodynamic Stability of High Hydrogen-To-Metal Ratio Superconducting Hydrides

Superconducting superhydrides are typically formed under high pressures by densely packing hydrogen atoms in unique clathrate structures with strong electron-phonon coupling. The hydrogen-metal interactions in these clathrate networks lead to diverse stoichiometries and exceptionally rich, hypervalent compounds (e.g. CaH₆, YH₈, LaH₁₀). Typically, such compounds are thermodynamically stabilized in bulk at ultra-high pressures, of the order of tens and hundreds of GPa. Here we show that reducing the particle size has a remarkable effect on phase diagram of and thermodynamic stability of such clathrate structures. Spacial confinement of Ca, Yb, and La in the presence of ammonia borane leads to significant changes in the thermodynamic stability, and in selected cases allows stabilization of metastable superhydride phases at lower pressures compared to bulk. Examples of successful interplay between theory and experiment to identify new promising sueprhydride materials and provide fundamental insights into their stability will be also presented.