First Principles Study Of The Structural Stability And Superconductivity Of Hydrogen-rich Compound ErHn Under High Pressur

According to the BCS theory,metallic hydrogens and hydrogen-based metal hydrides are considered as good superconducting materials because of their high phonon frequencies,strong electron-phonon coupling and high density of electron states at the Fermi energy level.Therefore,the exploration of these materials is of great research significance and application value.Theoretical calculations predict that metallization of singlet hydrogen due to high pressure occurs at about 450 GPa.However,the realization of metallic hydrogen remains difficult due to the fact that this pressure is difficult to achieve under current experimental conditions.The physicist Ashcroft proposed the theory of"chemical precompression".This theory states that the metallization of hydrogen-rich compounds can be achieved at lower pressures due to the chemical precompression of non-hydrogen elements.Therefore,hydrogen-rich compounds may be the most ideal material to replace metallic hydrogen as a high-temperature superconductor.For example,both experimental and theoretical studies have recently shown that H₃S and LaH₁₀ can metallize and become high-temperature superconducting materials near room temperature at pressures below 200 GPa.These findings have given rise to a surge of research on hydrogen-rich compounds.In this paper,we investigate the crystal structure,thermodynamic and kinetic stability,electronic structure properties,and superconductivity of ErHn（n=1-6）by doping the hydrogen with the rare-earth metal erbium（Er）,using the H-Er system as the research object,through a crystal structure prediction technique based on a particle swarm optimization algorithm combined with a first-nature principle calculation method.The main research results are as follows:1)Several stable ratios of erbium hydride at different pressures and its structure and superconductivity have been successfully predicted.The stable ratios and structures obtained are:Fm-3m-ErH,P6/mmm-ErH₂,C2/m-ErH₃,I4-mmm-ErH₄,and Im-3m-ErH₆.among them,the stable structures of Fm-3m-ErH and P6/mmm-ErH₂ are in agreement with the previous results.I4-mmm-ErH₄ and Im-3m-ErH₆ are the two new stable ratios and structures we predicted.And,interestingly,both I4-mmm-ErH4 and Im-3m-ErH₆ form an inclusion complex.The H atoms in both structures form weak H-H covalent bonds,and the H and H are bound to each other and connected into cage-like structural hydrogen structures consisting of 24 H atoms and 18 H atoms,respectively.Their Er atoms are placed at the center of both caged hydrogens.This special structure and the weak H-H covalent bond leads to a strong electron-phonon coupling in the system and is therefore a potential superconducting material.Electronic structure calculations show that both I4-mmm-ErH₄ and Im-3m-ErH₆ exhibit metallic properties in the stable pressure range.In addition,the superconducting transition temperatures of I4-mmm-ErH₄ and Im-3m-ErH₆ reach the maximum at 150 GPa and300 GPa,80 K and 180 K,respectively,while comparing the superconducting temperatures of ErH₄ and ErH₆,the superconducting transition temperatures of metal hydrides are largely influenced by the percentage of H content.2)Meanwhile,we added a computational study of ErH₃ in the ferromagnetic and antiferromagnetic states to address the contradiction between the pre-experimental studies and theoretical predictions of ErH₃,and we found four stable structures of ErH₃,namely p-3c1,p6₃/mmc,Fm-3m and Cmcm phases.Their energies,phonon spectra and phonon density of states were calculated and compared with the nonferromagnetic states.The calculations showed that the ground state structure of the rare earth trihydride ErH₃ should belong to the antiferromagnetic structure,i.e.,the structure in the ferromagnetic or antiferromagnetic state is more stable.We calculated the electronic density of states for the antiferromagnetic state of p-3c1-ErH₃ at 0 GPa,and the structure has a DOS value of 0 at the Fermi energy level and exhibits insulator properties.This is in agreement with the laboratory results.Therefore,we believe that the failure to consider magnetism in the previous theoretical predictions may be the main reason for the contradiction between the previous experimental studies and the theoretical predictions.Our results can provide a lesson for later theoretical studies:the effect of magnetism on structural stability cannot be neglected in theoretical calculations of metal-rich hydrogen compounds.