Theoretical Study On Co-H And Rh-H Systems Of Transition Metal Hydrides Under High Pressure

The research of hydride under high pressure has always been a hot spot in the field of condensed matter.Since the atomic mass of the hydrogen atom in the hydride can provide higher Debye temperature,and the interaction of non-hydrogen and hydrogen will provide“chemical precompression”effect on the system.So it is believed that the metallized hydride will become an ideal superconductor with a high superconducting transition temperature within the pressure range achievable by the experiment.In addition,when hydrogen reacts with a metal element,the hydrogen element usually fills the octahedron or tetrahedral position of the closely arranged metal atom in the form of atoms.Therefore,metal hydrides are also considered as potential hydrogen storage materials.In recent years,the structures and properties of transition metal hydrides under high pressure have received more and more attention.In recent experiments,a hydrogen-rich transition metal compound FeH₅ was synthesized by laser heating at 130 GPa,in which hydrogen atoms forms an extended layered structure.The theory predicted that its superconducting temperature can reach46 K at 100 GPa.In addition,a new transition metal hydride RhH₂ with high volumetric hydrogen density has been synthesized under high pressure.Its volumetric hydrogen density can reach 163.7 g H₂/L under normal pressure,which is a potential hydrogen storage material.Fe,Co and Rh are all Group VIII elements with similar physical and chemical properties.And their monohydrides have the same Fm-3m structure.The discovery of new hydrogen-rich compounds and superconductors in Fe-H system under high pressure has aroused our interest in studying the phase diagram and properties of Co-H system and Rh-H system under high pressure.We systematically studied the stable stoichiometries,structure,electronic properties,superconductivity and hydrogen storage content of Co-H and Rh-H compounds in the pressure range of 1atm to 300 GPa by first-principles calculation.And the following innovative results were obtained:?1?For the Co-H system,in addition to the Fm-3m-CoH that has been synthesized in the experiment,two new polyhydrides CoH₂ and CoH₃ were found at10 GPa and 30 GPa,respectively.CoH₂ is stable with cubic phase Fm-3m and converted to an I4/mmm phase when the pressure reaches 42 GPa.CoH₃ is stable with Pm-3m structure in the pressure range of 30³00 GPa.It is calculated to have a volumetric hydrogen density of up to 425 g H₂/L and is a potential hydrogen storage material with high hydrogen capacity.The I4/mmm-CoH₂ and Pm-3m-CoH₃ are similar with I4/mmm-FeH₂ and Pm-3m-FeH₃,which was discovered by laser heating.Therefore,CoH₂ and CoH₃ could be synthesized at high pressure.In terms of magnetism,CoH is a ferromagnetic with a magnetic moment of 1.167?_B,while other cobalt hydrides CoH_n?n=2,3,4 and 5?are not magnetic.All the cobalt polyhydrides exhibit metallic and ionic characteristics,and transfer of electrons from Co to H.Furthermore,application of the Allen-Dynes-modified McMillan equation estimated no superconductivity for cobalt polyhydrides.?2?Rh and H₂ form stable compounds RhH and RhH₂ at 2 GPa.In the pressure range of 2³00 GPa,the RhH space group is always Fm-3m structure.RhH₂ phase transitions sequence is Fm-3m?I4/mmm at 72 GPa.The new phase I4/mmm-RhH₂have the same structure as FeH₂.When the pressure rises to 40 GPa,another numerous H content stoichiometry RhH₃ with Pnma phase emerged.Considering magnetic properties,RhH is a ferromagnetic with a magnetic moment of 0.45?_B,while other rhodium hydrides RhH_n?n=2 and 3?are not magnetic.All of the structures of the Rh-H system showed metallic and ionic properties.According to the McMillan equation modified by Allen-Dynes,it is found that the superconducting transition temperature?T_c?of RhH-Fm-3m structures 8.998 K at 5 GPa.