High-Pressure Synthesis of Metastable Superhydride PdH₃ 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 PdH₃ 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_1.3, is obtained, which possesses the highest hydrogen ratio compared to the previously reported ambient-stable Pd hydrides. In contrast, Pd₃H₅ is obtained using crystalline Pd NPs with a conventional face-centered cubic (fcc) phase as the starting material under ∼2000 K and ∼33.5 GPa, which transforms to PdH_0.706 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 PdH₃. This work provides insights into the preparation of metastable metal hydrides with a high hydrogen ratio for promising applications, such as superconductivity.