High-Pressure Synthesis of Metastable Superhydride PdH by Using Amorphous Pd as a Starting Material.

Pressure has been considered as a versatile and promising means in the discovery of metal superhydrides. However, although a series of metastable metal hydrides with excellent superconducting properties have been predicted through theoretical calculations, it is still challenging to obtain metal hydrides with metastable phases via a high-pressure synthetic route. Herein, we have successfully fabricated a metastable PdH superhydride using amorphous Pd nanoparticles (NPs) as a starting material at ∼32.2 GPa and ∼2000 K. Intriguingly, after unloading the pressure and decreasing the temperature to ambient conditions, another metal hydride, i.e., PdH, is obtained, which possesses the highest hydrogen ratio compared to the previously reported ambient-stable Pd hydrides. In contrast, PdH is obtained using crystalline Pd NPs with a conventional face-centered cubic () phase as the starting material under ∼2000 K and ∼33.5 GPa, which transforms to PdH after quenching to ambient conditions. The experimental results and theoretical calculations reveal that the disordered atomic arrangement and high entropy of amorphous Pd NPs play a critical role in the generation of metastable PdH. This work provides insights into the preparation of metastable metal hydrides with a high hydrogen ratio for promising applications, such as superconductivity.