First-principles Study Of The Compositions,structures And Properties Of U-H Binary System

The uranium-hydrogen binary system is one of the hot spots in the field of nuclear materials because of its wide application prospects in the nuclear power technology,hydrogen corrosion of uranium and hydrogen storage materials.Recently,with the rapid improvement of computational speed and the maturity of crystal structure prediction algorithm,it has become one of the important means of material science research to discover new materials and structures through calculation.Through the combination of crystal prediction algorithm and first-principles calculations,we can quickly predict new materials and structures,thus greatly improving the speed of materials discovers.On this basis,this paper focuses on the uranium-hydrogen binary system.By combining the crystal structure prediction algorithms and the first principles calculations,we systematically investigate their composition,structure and properties under different pressures.The main research contents and conclusions are as follows:(1)By means of the variable-composition evolutionary algorithm combined with the density functional theory,the novel structures of uranium hydrides were systemically searched in the pressure of 0 GPa,20 GPa,50 GPa,100 GPa,150 GPa,200 GPa,250 GPa and 300 GPa,respectively.Twenty new structures of uranium hydrides,including UHx(x= 1,2,…,9,17),U2Hy(y=3,5,7,13,15,17),U3H8,U3H10,U4H5 and U4H9,are predicted at different pressure.According to the derived convex hull,it can be ensured the stable pressures ranges of uranium hydrides and also found that the UH4,U3H10,U4H9 and U2Hy(y = 3,5,7,15,17)are thermodynamically metastable structures,while others are thermodynamically stable structures.Among these twenty structures,we have found seven novel compounds that have never been reported in previous studies,namely,Cmcm-UH3,Fmmm-UH4,R3m-U2H7,Pm-U2H15,Cm-U3H8,C2-U4H9 and C2/m-U4H5.It also find that the structured phase change induced by pressure for UH,U2H5 and UH3.Above 65 GPa,the Cmcm-UH will be change into P63/mmc-UH.Cmcm-U2H5 is more stable than C2/c-U2H5 above 76 GPa.For UH3,between 0 and 6 GPa,the Pm3n-?-UH3 is most stable,from 6 GPa to 35 GPa the Pm3n-?-UH3 is the most stable,and above 35 GPa,the Cmcm-UH3 is most stable.(2)The mechanical stabilities of these 20 kinds of uranium hydrides were verified by calculating the elastic constants through the first principle calculations,and substituting the elastic constants into the corresponding mechanical stability criteria.Furthmore bulk modulus,shear modulus,Young's modulus,Poisson's ratio,Puch's modulus ratio,the estimated Vickers hardness and the universal elastic anisotropy index of these 20 kinds of uranium hydrides were calculated.The C2/m-U4H5,Ibam-U2H3,Cm-U3H8,C2/m-U3H10 are found to be ductile,whereas others are brittle.Note that the estimated Vickers hardness of P4m2-U2H17 and Cm-UH17 are 43.1 GPa and 44.6 GPa,respectively,which indicate that they are potential superhard properties.Through the study of electron structure,all these hydrides are metallic.By combining the electron local function and Bader charge analysis,it can be seen that covalent bonds between uranium hydrides exist.With increasing pressure,the covalent bond effects of most structures are enhanced,as accompanied with the increase hardness.