Inhomogeneous Strain Drives Formation of Single-Crystal PdH toward Quenching the Metastable Phase via High Pressure Sintering

In rational precursor design, nanostructuring strategies could reduce the synthesis pressure and quench metastable phases under ambient conditions, but the mechanisms are unclear. Here, we employed in situ high pressure angle dispersive X-ray diffraction and transmission electron microscopy to trace the distinct hydrogenatison pathways of Pd icosahedral (Pd_ico) and octahedral (Pd_oct) nanoparticles. Our findings reveal that inhomogeneous strain significantly influences nanoparticle detwinning and aggregation under high pressure. Pd_icoH develops a texture at 30 GPa and could be sintered into a two-dimensional single-crystal PdH with a (111) oriented plane. In contrast, Pd_octH nucleates individually during high-pressure sintering, forming polycrystalline PdH_0.706 with (111) and (100) planes. Computational simulations further show that the (111) plane of Pd has a high energy barrier for hydrogen release. Thus, regulating PdH nanoparticles with inhomogeneous strain through pressure and temperature to achieve a (111)-dominated structure is crucial for quenching metastable PdH at ambient pressure.