Study On Mechanical And Electronic Properties Of Hydrogen Storage Material Palladium Dihydride

Palladium has a good property of absorbing a large amount of hydrogen reversibly to form palladium hydride.Therefore,palladium hydrogen（Pd-H）system as a hydrogen storage material has been concerned.In addition,Pd-H system is that as the first metal hydride with superconductivity,its superconductivity transition temperature T_c increases with the increase of hydrogen concentration.Previous studies focused on PdH_x（x≤1）with low hydrogen concentration.Palladium dihydrogen（PdH₂）with high hydrogen concentration is a potential vehicle hydrogen storage and high temperature superconductor material.However,its stable structure and performance are still an open question.Based on the first-principles theory of density functional theory,this paper studies the potential stable structures of three-dimensional,two-dimensional and one-dimensional PdH₂,discusses the stability of the materials.The mechanical properties and electronic properties are further studied.It provides a theoretical reference for the potential application of palladium hydride in hydrogen storage,electronic devices and flexible materials.The ab initio random structure searching technique was employed for screening potential PdH₂ crystal structures under pressure.A hexagonal close packed（hcp）phase of PdH₂ with space group P6₃mc was reported.The results show that there is an electronic topology transition of hcp PdH₂ at 15 GPa.When the c/a ratio is between 1.765 and 1.875,the hcp PdH₂ is metastable.The calculation of elastic constants shows that C₄₄ is the first one that cannot satisfy the mechanical stability criteria under pressure and Pd-H_2b bond is the main factor limiting the mechanical stability.The elastic properties of the structure,such as bulk modulus,Young’s modulus,shear modulus and Poisson’s ratio,are further studied.In addition,the study of electronic structure shows that hcp PdH₂ is metallic.A monolayer Pd H₂ was argued by using a universal structure prediction based on an evolutionary algorithm and cutting along the face（001）of hcp PdH₂.The structures obtained by these two methods are the same with the space group P3m1.The calculations of elastic constants and phonon spectra show that the monolayer PdH₂ is dynamic and mechanical stable.The mechanical and electronic properties of PdH_2-x（0.11≤x≤0.44）structure with hydrogen vacancy were further studied.The increase of hydrogen vacancy does not affect the mechanical stability of the structure.The increase of hydrogen vacancy leads to the decrease of Young’s modulus and Poisson’s ratio,but the shear modulus is almost the same.Furthermore,the electronic properties of PdH_2-x are calculated,and the monolayer PdH_2-x shows metal properties.The quantum fluctuations ensue a quantum phase transition of PdH_1.89 and PdH_1.56.According to the monolayer PdH₂ structure,the structure of PdH₂ nanotubes（PdNTs）was established.The stability,elastic properties and electronic properties of zigzage（n×0）and armchair（n×n）nanotubes were studied.It is found that the zigzag form is more stable than the armchair form.The Young’s modulus of PdNTs will converge to the monolayer limit when the diameter reaches 20?.The electronic calculations revealed that both of zigzag and armchair PdNTs present a metallic property.In addition,the electronic density of states point the narrow states at around Fermi energy level for the（10×0）,（14×0）,（22×0）and（8×8）nanotubes which predicts the appearance of quantum states.