The Preparation And High-Pressure Studies Of Rhodium,Tantalum And Niobium Based Hydrides

In recent years,the excessive use of traditional petrochemical energy has brought unprecedented pollution.It is urgent to explore sustainable new energy sources to replace fossil fuels.Hydrogen is a chemical combustion source with high reserves and clean renewable energy carriers.It is the ideal substitute for fossil energy.However,the storage and transportation of hydrogen are the main factors limiting its widespread adoption and application.There are many drawbacks in the storage and transportation of compressed hydrogen,such as the technical difficulty is high,and the cost is high.Compared with the traditional gas compression methed,storing hydrogen in the solid form in the hydride,which not only possesses high hydrogen content,but also safe and convenient storage process,has become a hot issue for scientists.As the most important member of the hydride family,metal hydrides not only have strong hydrogen storage capacity,but hydrogen-rich metal hydrides are also ideal candidates for high temperature superconductivity.The reaearch on superconductivity is mainly due to the prediction of Metallic hydrogen.The metallic hydrogen under high pressure is a room temperature superconductor.Therefore,metallic hydrogen is considered to be one of the important physical problems in the 21st century.It is the holy grail of high-pressure physics,but there has been a lot of controversy in experiments.In addition,scientists believe that hydrogen in hydride can achieve hydrogen metallization at lower pressure due to chemical preloading.Therefore,the study of metal hydride properties under high pressure has important scientific significance.At the same time,the preparation of metal hydrides is also one of the obstacles affecting its application and promotion.As a hydrogen storage material,how to improve the absorption and desorption efficiency of hydrogen by metal hydride,and increase the hydrogen content in hydride is a hot issue in research.With the development of science and technology,the nanoscience has provided a new path for the development of materials science.Metal nanomaterials have the advantages of large specific surface area,large number of surface mismatched atoms,and abundant active sites.They not only optimize the thermodynamic and kinetic properties of metal hydrides,but also improve the hydrogen storage capacity of metal materials.Therefore,we combine nanotechnology with high-pressure technology to systematically study the effects of nanostructures on the hydrogenation process of metals under high pressure,and exploring whether high-hydrogen metal hydrides can be obtained under mild conditions.Further study the new phase structure and phase stability under high pressure.In this paper,we selected Rh nanoparticles with different intrinsic structures as the research object,and study the effect of nanostructures on the formation of hydrides under high pressure.At the same time,we successfully prepared niobium hydrides（NbH and NbH₂）and tantalum hydride（TaH）by using the hydrogen embrittlement effect,systematically carry out research on physical properties under high pressure.This paper has studied the above system and obtained the following results:（1）The research on the influence of the inherent structure of Rh nanocrystals on hydrogenation under high pressure was carried out systematically.The results show that the pressure of the icosahedral Rh nanocrystals,cubic Rh nanocrystals and small-sized Rh nanocrystals convert to RhH is lower than that of the bulk material Rh.The icosahedral Rh nanocrystals form RhH at 3.5 GPa,and the similar size of the cube Rh nanocrystals is 4.4 GPa,which is attributed to the fact that the icosahedron is a five-fold twin structure with 30 twin interfaces.The crystal interfaces will generate structural internal stress to provide additional energy to the system,thereby reducing the formation of RhH and further reducing the RhH transition pressure.The small-sized Rh nanocrystals are partially converted to RhH at the initial pressure of 0.2 GPa,mainly because the number of mismatched atoms on the surface increases with the decrease of crystal size,resulting in an increase in structural defects.Meanwhile,small-sized nanocrystals shorten the diffusion path of hydrogen atoms.The combination of two factors reduces the RhH conversion pressure.In summary,the introduction of nanostructure defects in the nanocrystals can effectively reduce the formation enthalpy of metal hydrides,thereby reducing the metal hydride reaction conditions.This will provide new ideas and ways for the preparation of hydrides.（2）Preparation of Niobium and Tantalum based hydrides by hydrogen embrittlement effect and the study of their high-pressure properties.In this system,we skillfully use hydrogen embrittlement effect to synthesize high-melting point and stable metal hydrides.In the experiments,niobium and tantalum was used as the precursor.Water or alcohol was used as the reaction solvent and hydrogen source.The pure phase NbH,NbH₂ and TaH were successfully synthesized by the hydrogen embrittlement effect.This method is different from the traditional direct hydrogen reaction metyhod.The reaction conditions of this method are relatively mild,and the reaction can be completed at about 200℃.We systematically studied the structure and stability of NbH under high pressure.The results show that the phase transition of NbH₂ occurs at a pressure of 50.2 GPa,and the phase transition is completed at 68.9 GPa.After the structural refinement,it is confirmed that the high-pressure phase structure is the orthogonal structure with space group Pnma,and the result is in good agreement with the theoretical prediction.This is the first time to obtain the high-pressure phase structure Pnma of NbH₂.The hydrogen embrittlement effect will provide new ideas and insights for the rational design of metal hydrides.