Mechanical and electronic properties of van der Waals layered hcp PdH(2)

Mechanical and electronic properties of palladium dihydrides (PdH(2)) 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 PdH(2) crystal structures under high pressure. A hexagonal close packed (hcp) phase of PdH(2) with space group P6(3)mc 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 PdH(2) 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 PdH(2) is mechanically stable, and the Pd-H(2b) bond is the major factor that limits the mechanical stability. The elastic constant C(44) 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 PdH(2) 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 PdH(2) is dynamic stability under pressure. The results suggest that hcp PdH(2) can be metastable in van der Waals layered structure.