Mechanical and electronic properties of van der Waals layered hcp PdH2

Abstract

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. Metal hydride (M-H) systems have attracted a lot of attention because of their properties, such as high hydrogen-storage capacity, fast hydrogen absorption/desorption, long-term cycle life and low toxicity 1. Palladium-hydrogen (Pd-H) is used to be a system to understand the hydrogen atoms bonding with the metal host lattice in the M-H systems. The phase diagram and electronic properties of Pd-H have been used as a prototype in other M-H systems 2. Some applications of Pd-H system have been investigated. The palladium can absorb hydrogen at ambient conditions and the dissociative adsorption of H 2 molecules occurs with little or no activation energy barrier on the palladium surface. The reversible hydrogen absorption property can be used for hydrogen storage 3. Increasing the hydrogen concentration will cause volume expansion of PdH x. Based on this mechanism, a Pd based H 2 sensor was used to measure the hydrogen concentration 4. Due to the dissociative properties of H, Pd-H can be used for hydrogen-related catalytic reactions 5. The hydrogen concentration affects the superconductivity of Pd-H system. PdH x is a superconductor and the transition temperature Tc increased with the increase of H concentration x 6. For the H/Pd is 0.81, a Tc = 1.3 K was observed. At the highest concentration ratio of about 1.0, the Tc to the superconducting state higher than 8.0 K 7. Recently Syed et al. found that by rapidly cooling the hydride after loading with hydrogen at elevated temperature, the Tc has a remarkable increase to 54 K when the H/Pd is about 1 8...

Advances in highly hydrided palladium

“…Higher hydrides are in principle possible through occupation of the tetrahedral other than the octahedral sites. Despite the lack of credible experimental evidence, there are several theoretical studies predicting the stability and structures of highly hydrided palladium such as stoichiometric PdH, PdH 2 , and PdH 3 (Houari et al, 2014;Yang et al, 2017;Long et al, 2018;Liu et al, 2020). The monohydride PdH is considered in either NaCl-typed fcc structure with hydrogen atoms filling all of the octahedral positions, or CaF 2 -typed fcc structure with hydrogen atoms filling half of the tetrahedral positions.…”