Mechanical and electronic properties of van der Waals layered hcp PdH₂

Mechanical and electronic properties of palladium dihydrides (PdH₂) as a function of pressure were studied by ab initio calculations based on density functional theory (DFT). The ab initio random structure searching technique was employed for screening potential PdH₂ crystal structures under high pressure. A hexagonal close packed (hcp) phase of PdH₂ with space group P6₃mc was reported. The structure geometry and elastic constants were calculated as a function of pressure. It was found that H atoms are in the interstitial position of Pd atoms layer at 0 GPa. There is an electronic topology transition of hcp PdH₂ at 15 GPa. When pressure exceeds above 15 GPa, one hydrogen atom occupies the tetrahedral site and another hydrogen atom locates in the interstitial position. When the c/a ratio is between 1.765 to 1.875, the hcp PdH₂ is mechanically stable, and the Pd-H_2b bond is the major factor that limits the mechanical stability. The elastic constant C₄₄ is the first one that cannot satisfy the mechanical stability criteria under pressure. The anisotropy parameters are far from 1(one) shows that the hcp PdH₂ is a highly anisotropic structure. The electronic structure study indicates that the bonding force between Pd and H atoms along the z-axis direction increases with the increasing pressure. Also, the phonon dispersion study shows that PdH₂ is dynamic stability under pressure. The results suggest that hcp PdH₂ can be metastable in van der Waals layered structure.