Uranium And Uranium Dioxide Optical Properties Of First Principles Calculation

The crystal structure,elastic constants,bulk modulus,band structure and optical properties of uranium and uranium dioxide were investigated using the plane-wave pseudopotential technique and the local density approximation(or generalized gradient approximation) to exchange and correlation in density -functional theory.Uranium at low temperature is stable in a face centered orthorhombic(63-Cmcm) structure. We used the GGA to perform the geometry optimization to obtain a stable structure of polymorph.The calculated crystal lattice parameters compare favorable well with experiment data.We calculated the elastic constants of uranium and then obtain the Bulk modulus,Shear modulus and the Young modulus from it.The calculated elastic constants compare well with the experimental data from E.S.Fisher and the fist principle calculation result in FP-LMTO method by P.Soderlind.The calculated bulk modulus is 133.9GPa,shear modulus is 6.02 GPa, the rate is 1.37,which indicates that the uranium is brittleness from Pugh's experience judgement.There are three major band structures in the valence band of uranium,the 5f and 6d orbital take part in chemical banding near the fermi energy.From the single particle approximation and direct transition rule,we calculated the matrix elements and then obtain the dilecctric function,and then derive other optical properties of uranium like absorption function, reflective index,electron energy loss function et al.The product of uranium metal reacted with pure oxygen under not very high temperature was UO₂,which exhibits the fluorite structure,space group 225- Fm3m.We have tried several methods to perform the geometry optimization of uranium dioxide,the calculation results indicate that the GGA give better rusult than LDA.The crystal structure and elastic constants of uranium dioxide were calculated using the spin-polarized generalized gradient approximation(PW91),the results compare well with experiment data.We obtain a metallic state for UO₂ while it is experimentally an insulator.It is indeed known that density of states calculation made in the LDA(GGA)Iead to a metallic state for UO₂.Us we obtained a metallic state we can only make qualitative analysis of the dielectric function of UO₂,and we got the 6d orbital split energy of 2.8 eV which in consistent with the experiment.